Sn (MeV) SP (MeV)
'"OSn
Sn
Sn
0.34 1.41 1.71 3.42 7.40 17.65 2.80
1.87 5.19 2.25 9.30 3.30 9.10 10.70
2.92 8.01 2.64 13.57 2.50 3.90 16.2
'"Pb
'"'Pb
Pb
1.39 3.44 2.20 7.10 5.90 11.75 1.30
3.50 7.87 2.61 13.98 3.10 7.40 8.00
3.80 8.50 2.73 15.03 2.90 5.10 8.80
p+Pb
< n >, En < 2MeV < n >, 2MeV < En < 20MeV < n >, En > 20MeV Sn (MeV) SP (MeV)
There is, of course, a symmetric effect on the proton multiplicities. It is less dramatic in absolute values, but more dramatic in proportion. 3.3. Residue production cross-section
The residue mass spectra for pinduced reactions on Sn targets are given in Fig. 1. The abscissa x is equal to 120 minus the mass loss. It is appropriate to compare the so-called fragmentation peaks, corresponding to the residues which are created by evaporation (x 2 70). As expected, the fragmentation peak is broader for 134Sn,mainly because starting from the neutron-rich side of the valley of stability, the system can evaporate more nucleons before reaching the evaporation corridor. The fission contribution is very small in this case. It corresponds t o the x 5 70 part of the spectrum. It is only significant for '"Sn, which has the largest fissility parameter.
443 p( lGeV)+Sn 1. e 4 5
1. e 4 4
1. e 4 3 VI I
C
s 1. e 4 2
1. e 4 1
I. e 4 O 20
40
60
80
100
120
140
A+( 120-A,)
Figure 1. Residue mass spectra for proton-induced reactions on three S n targets at 1 GeV. Note that the horizontal scale has been chosen such that the isobars corresponding to the same mass loss appear at the same place for the three systems. p( 1GeV)+Pb 1. e 4 5
1. e
44
I. e 4 3
I. e 4 2
1. e+OO
50
Figure 2.
100
150
Same as Fig. 1 for three Pb targets.
The residue mass spectra for three isotopes of Pb are given in Fig. 2. In this case, the fragmentation peaks have roughly the same width. In fact,
444
they are practically identical for '08Pb and "'Pb. The shape is different for 18'Pb, but this is mainly linked with the most striking result of Fig. 2, namely the large fission cross-section for this target. This, of course, is due to the large fissility parameter of the remnants and leads to a large depopulation of the mass spectrum around mms loss x 5-6. As for the other targets, a peak exists for mass loss of one mass unit. This peak arises because peripheral collisions lead to small excitation energy; for smaller impact parameters, more excitation energy is left in the target remnant and leads, either to important evaporation or, as in this particular case of 182Pb,t o fission. 4. Conclusion
We have presented here exploratory calculations for spallation reactions on neutron-rich and neutron-poor nuclei.Neutron and proton multiplicities can be substantially changed. The shape of the residue mass spectra is not drastically different from the ordinary target case, except for fission. References 1. I. Tanihata, On the possible use of secondary radioactive beams, in Treatise on Heavy-Ion Science, vol. 8 , ed. by D.A. Bromley, Plenum Press, (1989). 2. T. K. Gaiser, Cosmic Rays and Particle Physics, Cambridge University Press, (1992). 3. M. Longair, High Energy Astrophysics, vol. 1 and 2, Cambridge University Press, (1997). 4. W. Gudowski, Nucl. Phys. A654,436c (1999). 5. A. Koning et al, HINDAS, A European Nuclear Data Program for AcceleratrDriven Systems, Tsukuba Conference, 2002; J.-P. Meulders et al, HINDAS final report, to be published in EU publications 2004. 6. A. Boudard, J. Cugnon, S. Leray and C. Volant, Phys. Rev. C66, 044615 (2002). 7. J. Cugnon and P. Henrotte, Eur. Phys. J . A 16,393 (2003). 8. J.-J. Gaimard and K.-H. Schmidt, Nucl. Phys., A531 709 (1991). 9. A. R. Junghans, M. de Jong, H. G. Clerc, A. V. Ignatyuk, G. A. Kudyaev and K.-H. Schmidt, Nucl. Phys. A629,635 (1998). 10. Th. Aoust and J. Cugnon, to be published in Eur. Phys. J . A (2004).
445
SPIN MODES IN NUCLEI AND NEUTRINO NUCLEUS REACTIONS
TOSHIO SUZUKI Department of Physics, College of Humanities and Sciences, Nzhon University, Sakurajosui 3-25-40, Setagaya-ku, Tokyo 156-8550, Japan E-mail: suzukiQchs. nihon-u. ac.jp Spin structure of nucleus and neutrino nucleus reactions at low energy are investigated. We focus on charge exchange reactions 12C(ue,e-)12N and l 2 C ( u P p-)"N , induced by DAR (decay a t rest) and DIF (decay in flight) accelerator u's, respectively, as well as 208Pb(ve,e-)208Bi induced by DAR and supernova u's. Exclusive reactions 1 2 C ( u e ,e - ) l a N and ' 2 C ( u p , p - ) ' 2 N are found to be well described by the shell models by including the Gamow-Teller (GT) and spin-dipole transition strengths. Inclusive reaction cross sections are found to be rather sensitive to the radial wave functions. Effects of spreading and quenching in the GT transition strength are found to be important in zosPb ( u e ,e-)208Bi, especially in the reaction induced by supernova u's. Accurate evaluation of the reaction cross section is pointed out to be essential in obtaining the evidence and rate of the neutrino oscillations of supernova up's and ur's into ye's.
1. Introduction
Neutrino nucleus reactions induced by low energy neutrinos of E, = 10-100 MeV are investigated. The reactions are mainly induced by spin-dependent transitions, that is, Gamow-Teller (GT) and spin-dipole transitions. Charge exchange reactions induced by v's produced in accelarator experiments as well as v's from supernovae are studied. In the former case, we are interested in the spin structure of the nucleus. In case of v's from supernovae, we can get information on the oscillation properties of v's. We investigate how the extraction of this information can be affected by the nuclear structure problems. 2. Spin Modes in Nuclei and Charge Exchange Reactions
Induced by Accelerator v's Charge exchange reactions, "C(v,, e-)12N and '08Pb(v,, e-)'08Bi, induced by DAR (decay of a pion at rest) v,'s are studied as well as "C(v,, pL-)12N
446
reaction induced by DIF (decay of a pion in flight) up's. In case of DAR experiments, average energy of u, is about 35 MeV and momentum transfer to the nucleus is around q = 0.1-0.2 fm-', while for the DIF case the neutrino energy and the momentum transfer involved are larger: E, = 123-300 1 fm-'. In case of DIF, therefore, higher multipoles than MeV and q J=2 become non-negligible, and we need to take into account transition strength at high excitation energies. We first show results of the " C ( Y , , e-)"N reaction for the DAR case. The calculated and experimental B(GT) values and cross sections are shown in Fig. 1. Shell model calculations explain well the observed B(GT) value of the "C + ''Ng,s,(l+) transition. The Warburton- Brown interaction (WBT') and Millener-Kurath interaction (PSDMK22>3)give smaller B(GT) values than the observed one by 15%, while the improved interaction ( S F 0 4 ) gives a little bit larger value about by 10%. The improved Hamiltonian (SFO) was obtained by enhancing the neutron-proton spin-flip interaction and the energy gap between the Opllz and 0p3l2 orbits. Remarkable improvements are found in spin dependent observables in the p-shell such as G T transitions and magnetic moments4. In particular, considerable systematic improvement is obtained for the magnetic moments.
-
1.5
F
2 m
.
(a)
I
.
.
.
I+
1.0
0.5
0.0 SM SM SM SM SM'CRPANCI (HO) (HF) (WS)[SFOI IWBTI IHTI
Figure 1. (a) B(GT) values calculated by using shell models (WBT1, MK2 and S F 0 4 ) and the observed value5 for I2C -+ "N (1Ls,). (b) Cross sections for 12C(u,,e-)12N (1LS.) induced by DAR u,'s obtained by using the shell models and CRPA'. Values denoted by [WBT], [HT] and NCSM are taken from Refs. [7], [8]and [lo], respectively. Experimental upper and lower values are denoted by straight linesg.
Calculated exclusive cross sections obtained by the shell models are consistent with the observed value. Here, the configurations including up to 2-3 hw excitations are taken into account. Though the SFO interaction a little bit overestimates the experimental value, the one obtained with the
447
use of a renormalized axial vector coupling constant g>** / g A = 0.95, which reproduces the observed B( GT) value, falls within the allowed experimental regiong. The calculated value obtained by a no-core shell modello with twoand three-body interactions is also found t o be close to the experimental one. RPA type calculations give values that overshoot the experimental value. A continuum RPA (CRPA) calculation6 deviates from the observation by about 50% as shown in Fig. l b . Cross sections for excited states are shown in Fig. 2. Calculated shell model values show rather sensitive dependence on radial wave functions7. The one with the use of Woods-Saxon wave functions' and the value by the CRPA calculation6 are close to the observed value, while the shell model values obtained by the harmonic oscillator or Hartree-Fock wave functions are larger than the experimental oneg by about twice7. 20,
4. . 5
I
I
,
,
,
excited states
,
,
(
I
101
Figure 2. Cross sections for 1 2 C ( v , , e-)12N induced by DAR v,'s leading to the excited states of "N. Those obtained by using the shell models and CRPA' are denoted by points while experimental upper and lower bounds are shown by straight linesg. Values denoted by [WBT] and [HT] are taken from Refs. [7] and [S],respectively.
In the DIF case, exclusive ( v p , p - ) cross section is well explained by the shell models7~*.The inclusive cross section depends on the radial wave functions similar to the DAR case. The shell model calculation obtained by the Woods-Saxon wave functions is consistent with the observation', while those with the harmonic oscillator or HF wave functions overestimate the experiment7. There remains a possibility for further quenching of the axial vector coupling constant. We show results of cross sections for 208Pb(v,, e-)"'Bi reaction for
448
the DAR v,'s. The G T and spin-dipole transitions are taken into account. RPA and TDA with the SKI11 interaction are employed to get the transition strengths11i12. The model can explain G T and spin-dipole strength distributions observed13J4 rather well". The effects of the 2p-2h configurations are also taken into acount in the G T transition15. We find appreciable quenching and spreading of the GT strength due t o the coupling to the 2p-2h configurations as shown in Fig. 3 . The GT strength beyond E, = 25 MeV is non-negligible. Total cross sections as well as each contribution from the G T and SD transitions are also shown in Fig. 3 . The calculated result with the inclusion of the spreading effects can be simulated in the H F t R P A model by using an effective axial vector coupling constant, g eAf f / g A = 0.8-0.9.
-
-
10
wg
"Pb(v,e')-Bi
*g
6
5
h
...... 0-+1-+21' Dang
T
0
;.,
0
23
v
4
2
2
4
\...,.. .: :: _. \< '..
1
I
, ~ . 10 20
30
0
40
HF+RPA W + W A
EAMeV)
GT (Dange l al.)
g:ff+.8gA
Figure 3. Calculated cross sections for 20sPb(v,,e-)208Bi induced by the DAR y e ' s obtained by H F RPA (TDA) model and the method which takes into account the effects of the spreading in the GT transition strength [15].
+
3. "'Pb(v,,
e-)208Bi Induced by Supernova v's
Now, we discuss on reactions induced by supernovae u's. As neutral current reactions do not depend on neutrino flavors, they are free from neutrino mixing and oscillations. If up and u, with an average energy of 25 MeV from supernovae change into v,, the cross section of the charge exchange reaction 208Pb(ue,e-)'08Bi gets larger as the average energy of u, produced in supernovae is as low as 10 MeV. If we can evaluate the magnitude of the charge exchange cross section accurately, we would be able to get valuable information on the neutrino oscillations from supernovae. One of the characteristics of the neutrino spectra from supernovae is that it has a tail in the high energy region. For example, energy spectrum of up and u, described by Fermi distributions
f(E) =
E2/T3
1
+ ezp(E/T - a )
449
with ( T , a )= (8 MeV, 0) or (6.26 MeV, 3) has a long tail beyond E = 50 MeV. N in Eq. (1) is a normalization factor. Now, we study the effects of the spreading of the G T strength in 208Pb(v,, e-)208Bi for v,’s with ( E ) = 25 MeV produced by oscillations of up and v, from supernovae. Calculated cross sections are shown in Fig. 4 for the HF+RPA model case and the case in which the spreading effects are taken into account in the G T transition. When the neutrino spectrum with ( T , a ) = (6.26, 3) is used, the total G T cross section is enhanced by about 50% for the strength with the spreading effects compared with the HF+RPA model with g L f f / g ~= 0.8. This is due to the considerable spreading of the G T strength beyond E, = 25 MeV. When the spectrum with (??,a)= (8,O) is used, the enhancement factor becomes as large as 2.6. In this case, the contributions to the cross section remains in the high energy region even beyond E = 100 MeV. The effects of the spreading of the G T strength due t o the coupling to the 2p-2h configurations are found to play an essential role. In the spectrum with ( T , a )= (6.26, 3), the high energy tail is suppressed compared to the case with ( T ,a ) = ( 8 , 0 ) .
Figure 4. (a) Calculated contributions from the GT transition strength to the 208Pb(ve,e-)20sBi reaction cross section induced by v,’s produced by the oscillations of up’s and vr’s from the supernovae. Dashed and solid curves are obtained by the HF RPA model and the method with the spreading effects. The neutrino spectrum given by Eq. (1) with (T, a)= (8.0, 0) is used. (b) Comparisons of the calculated cross sections RPA model and the method with the spreading effects. Neutrino obtained by the HF spectra with (T, a) = (8.0, 0) and (6.26,3) are used.
+
+
Finally, we estimate the count number of events observed by the 208Pb(v,, e-)208Bi reaction. Here, the G T and spin-dipole transitions are taken into account. If y e ’ s are produced in the supernovae at 8 kpc away, the temperature of v, is as low as T = 3.5 MeV and the event number for 1 ton of P b is as small as 0.1. When v,’s produced by oscillations of v p and v, from the supernovae are involved in the reaction, the temperature of u,
450
is T = 6.26-8 MeV and the event number becomes 2 / l t for P b target for the HF+RPA model. When the spreading of the G T strength is taken into account, it is enhanced up to 3.9/lt and 2.8/lt for the spectra ( T , L Y=) (8, 0) and ( T , a )= (6.26, 3), respectively. These numbers are also quite large compared to the number of events observed by the neutral current reaction '08Pb(u, u ' ) "'Pb induced by the supernova up's and u,'s at 8 kpc away. It is estimated t o be about 0.2/lt when we take into account the G T and spin-dipole transitions. These numbers are large enough to be measurable in future experiments palnned16 We would be able to get evidence and rate of the u oscillations by measuring "*Pb(ve, e-)'08Bi reaction. Determination of the rate of the oscillations depend both on the nuclear structure and the neutrino spectrum. It would be quite important to take into account the spreading of the transition strength to obtain reliable information on the properties of v's from the supernovae.
References 1. E. K. Warburton and B. A. Brown, Phys. Rev. C46,923 (1992). 2. D. J. Millener and D. Kurath, Nucl. Phys. A255, 315 (1975). 3. OXBASH, The Oxford, Buenos-Aires, Michigan State, Shell Model Program, B. A. Brown, A. Etchegoyen, and W. D. M. Rae, MSU Cyclotron Laboratory Report No. 524, 1986. 4. T . Suzuki R. Fujimoto and T . Otsuka, Phys. Rev. C67,044302 (2003). 5. R. E. McDonald, J. A. Becker, R. A. Chalmers, and D. H. Wilkinson, Phys. Rev. C10,333 (1974). 6. E. Kolbe, K . Langanke and P. Vogel, Nucl. Phys. A652,91 (1999). 7. C. Volpe, N. Auerbach, G. Colo, T. Suzuki and N. Van Giai, Phys. Rev. C62,015501 (2000). 8. A. C. Hayes and I. S. Towner, Phys. Rev. C61,044603 (2000). 9. LSND Collaboration, L. B. Auerbach et al., Phys. Rev. C64,065501 (2001). 10. A. C. Hayes, P. Navratil and J. P. Vary, Phys. Rev. Lett. 91,012502 (2003). 11. T . Suzuki and H. Sagawa, Eur. Phys. J. AS,49 (2000). 12. T. Suzuki and H. Sagawa, Nucl. Phys. A718,446 (2003). 13. A. Krasznahorkay et al., Phys. Rev. C64,067302 (2001). 14. H. Akimune et al., Phys. Rev. C61,011304 (2000). 15. N. Dinh Dang, A. Arima, T. Suzuki and S. Yamaji, Phys. Rev. Lett. 79, 1638 (1997); Nucl. Phys. A621,719 (1997). 16. R. N . Boyd, A. S. J . Murphy and OMNIS Collaboration, Nucl. Phys. A688, 386 (2001).
451
COMPARATIVE ANALYSIS OF THE 178m2Hf YIELD IN REACTIONS WITH DIFFERENT PROJECTILES S.A.KARAMIAN Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia E-mail: [email protected]
The long-lived high-spin ‘78”’2HfK-isomer can be produced in nuclear reactions with different projectiles. The reaction yields and cross-sections have been measured in the series of experiments and the results are now overviewed. The systematics of isomer-to-ground state ratios are drawn and real production capabilities are estimated for the best reactions. Such a summary is relevant to the significance of the isomer studies both for the nuclear-science knowledge and for possible applications. Potential isomer applications have been earlier stressed in popular publications with probably overestimated expectations. The real possibilities are restricted in part by the production yield, and by other shortcomings, as well.
1
Introduction
Nuclear isomers in the mass range close to A=180 are of special interest because they are characterized by unique combinations of high excitation energy, high spins and K-quantum numbers with long lifetimes. Such features make these isomers extremely attractive for applications to y-ray pulsed sources because they may store the nuclear excitation energy for long time and also provide the high density of energy. In the classical example of the 31-year-lived ‘78m2Hfisomer, the energy density reaches 1.3 GJ/g. An excited nuclear state manifests itself as a metastable isomer when its’ decay is significantly retarded due to some kind mismatch between the wavefunctions of initial and final states. Such hindrances for the decay typically arise because of collective deformation of a nucleus in excited state, either of the angular momentum step at the decay transition or of a structure inhibition. In the region of statically deformed nuclei, the structure hindrance may play an important role, in addition to the selection rules by the spin and parity (I”) for the multipole electromagnetic transitions. For instance, a change in the orientation of the angular momentum vector generates special structure inhibition known in literature as K hindrance. Remind that K is a quantum number of the I vector projection to the deformation axis. The axial symmetry of the nuclear deformation is a major assumption. Many K-hindered isomers are known, and their properties are described in literature, see for instance, in the review articles [l-41 and in the references therein. ‘78m2Hfis one of the clearly identified K isomers because the K-hindrance factor is as high as of about lo9 at this case. Without the latter value, the isomeric state should decay fast, with half-life of 0.8 s, instead of real T1,2=31 years.
452
Thus, the methods available for the unique isomer production are of interest in different aspects, in particular for extension of the exotic-nuclei phenomenology. A field of present report is intentionally narrowed to the nuclear reaction features, only. The nuclear structure properties are touched as far as they influence the isomer production cross-section. The discussion of applications is excluded because we have no intention to support a “sensation” appeared in some newspapers and magazines, for instance, in the “Washington Post”, “Der Spiegel”, etc. One can find more professional and responsible approach in the review articles published in physical journals, Refs. [ 1-51, They contain the explanation that the isomer research is at the very basic stage, far from real use the isomer sample as an energy or weapon unit. At the summary of the present report, the restrictions are discussed for the amount of isomer material that can be really produced. Decay retardation, being useful for energy accumulation, at the same time is accompanied with the suppression of the isomer production cross-section because of the similar factor of the wave-functions mismatch. In theory, the conservation of Kquantum number is not an absolute imperative because it is conserved until the axial symmetry of nuclear shape is perturbed. After the experiments of Refs. [6,7],it was evident that the K-hindrance factor decreases with the excitation energy growth. The long-lived states (isomers) are populated in nuclear reactions through the cascade of yquanta emitted by the excited reaction residue. At typical residual excitations, the K hindrance must be significantly diminished and the isomer yield should be fortunately increased. But yet, the isomer-to-ground state ratio in many cases remains not high, (J,/(J~<<~,as known from experiments. This is because of high spin of the isomeric states. In y-cascades, the most probable are the transitions of low multipolarity, E l , M1, E2, and they can not directly supply the required spin deficit if the isomer spin is much higher the residual angular momentum. Many stretched transition in the cascade are needed and the probability is decreased. There is no possibility to violate the angular momentum conservation because it is an integral of motion and should be conserved absolutely both in classical and quantum mechanics. The correlation of isomer cross-section (J, with the spin deficit was qualitatively clear even before the experiments reviewed here. However, it does not mean that the isomer yields and m/g ratios could be reliably calculated in theory and used for practical estimations. In reality, the spin distribution of the residual nucleus can not be easily predicted for many reactions. Opposite way, the measured m/g ratio sometimes serves as a basis for estimates of the mean angular momentum of the residual nuclei, for instance, in the spallation reaction with the intermediate-energy protons. Another uncertainty is due to the structure peculiarities of the level scheme and the y-cascade branching for some individual nucleus. Simplified statistical model calculations may not be very accurate, especially if they are applied to the excited levels below 3 MeV.
453
So, as normally in nuclear physics, the experimental measurements are needed to get reliable values of the reaction cross-section and yield. Experiments are reviewed below. One introductory remark else concerns the discussion of the isomer application in a mode of the controlled source of energy and radiation. Assumed that the isomer decay can be artificially stimulated (triggered) by the external radiation. Within the "up-conversion'' scheme, a photon is absorbed and provides the transition from the isomer to some higher lying level. The latter one should decay fast to the ground state and the isomeric energy is released this way. But the efficiency of such process should be again restricted by the wave-function mismatch between the isomer and other levels. Triggering cross-section can be too small, even if so lucky intermediate level exists. Extensive experimental studies may clarify the triggering efficiency. In the present report, triggering experiments are out of discussion, and the review of experimental attempts for triggering known up to date is given in Ref. [I]. At the present report, is described a new experiment aimed at the testing of the 178m2 Hf yield in neutron irradiations at Dubna IBR-2 reactor. At the conclusive part, other reaction yields are summarized and their productivity is compared with the neutron results and with other known experimental data. Among them, there are reactions induced by bremsstrahlung at 22 MeV and 4.5 GeV, Refs. [S] and [ 9 ] , respectively, spallation of Ta and W with the intermediate-energy protons [ 10,I 11, and the (4He,2n) reaction at a low-energy a-particle beam [12]. 2
Neutron-induced reactions
Production cross-section in neutron capture reactions with thermal neutrons are typically low for high-spin isomers with 1210. The isomer 177mLu(I"=23/2')is an exception that confirms the general tendency, because the high spin of the target 1766 Lu(I"=7-) nucleus provides a rather modest spin deficit AI=4A in the 176L~(n,y)177mL~ reaction. In contrast with neutron capture, fast neutron reactions supply additional possibilities. The yield of high-spin 178m2Hf isomer in reactions with neutrons was tested in experiments [ 13-15]. Relatively high cross-section was found [14,15] in the 179Hf(n,2n)178m2 reaction induced with 14.5 MeV neutrons. But a productivity is restricted by the neutron flux available when the T(d,r~)~He reaction is used for neutron generation. Much higher fluxes are created in reactors, but the spectrum is soft and needed energies of E&10 MeV have very low probability. Unfortunately, slow neutrons are not effective for the 178m2Hf production because the neutron capture 177Hf(n,y)178"2Hf
454
reaction is characterized by very low isomer-to-ground state ratio -0.5.10-9, accordingly [ 131. At the same time, the isomer population in the 178Hf(n,n'y)178"2Hf reaction was never experimentally tested. In addition, the low yield observed [ 131 in (n,y) reaction can be a result of burnup of the produced isomeric nuclei. Burnup process may be significant at high fluence applied in [13], but its' cross-section was not known. Even today, the data on the 178m2Hf burnup in reactor irradiations are not complete. Only the branch of the 178m2Hf(n,y)179m2 Hf transmutation was experimentally characterized in Ref. [16],but the total 178m2Hf(n,y) cross-section was not yet measured. In such a context, a new experiment has enough motivation to be performed testing both burnup and (n,niy) processes. Metal natHffoils 1 mm in thickness were activated in an external channel of the IBR-2 reactor at FLNP, JINR, Dubna, and were then studied using a 20% efficiency Ge gamma detector. This was accomplished by spectrometric electronics, which allowed a count rate up to 20 kCs/s with a reasonable dead time and conservation of spectral resolution. The neutron spectrum at the location of the target was known from previous experiments. But in addition, NiCr-alloy samples were used as spectators for the calibration of the thermal and fast neutron fluence. The Hf samples were irradiated with and without Cd shields and the method of Cd difference allowed isolation the effect of thermal neutrons and deduction of the thermal cross-section. In measured spectra of activated Hf, the y lines were observed and quantitatively determined for the following radionuclides: 175Hf,179m2Hf, lBomHfand IS1Hf.The bulk of the activity was defined by 175Hfand lslHf formed in (n,y) reactions. They served for the intrinsic calibration of the thermal and resonance neutron fluxes in a presence of the flux attenuation due to the self-absorption in the 1-mm Hf samples. This way, the thermal cross-section 0 t h and resonance integral 1, values were figured out for the IsomHfisomer formation and the results are in accordance with the tabular values [17]. The yield of the high-spin 179m2Hf isomer was newly obtained and attributed to the 179Hf(n,niy)179m2Hf reaction with neutrons of fission spectrum. The isomer-to-ground state ratio: 0m/0gs1.6.1 O", does not contradict the systematics of [181. The activity of 178m2Hfwas too low, and it could not be distinguished and estimated even past long (1.5 years) "cooling" of the sample after irradiation. Only upper limit was established for the number of produced 178m2Hf nuclei. Respectively, a limit for the cross-section of the 178Hf(n,n'y)'78"2Hfcould be evaluated immediately. This reaction may be productive only at neutron energies above 3 MeV because of the isomer excitation energy 2.45 MeV plus 0.5 MeV spent for the ejected neutron and gammas. A number of such neutrons was estimated using the known spectrum of fast neutrons at the location of the irradiated sample and the measured activity of 58C0in
455
the NiCr spectator. As a result, the cross-section of the ‘78m2Hf production at (n,n’y) reaction has been restricted by the upper limit of 0 ~ 1 0 . 0 4mb, and the corresponded omlogvalue has been found being as low as 11.5.10”. More complicated would be an estimation of the definite value of a limit for the neutron capture cross-section leading to the 178m2Hf isomer. Complications are due to the contribution of both thermal and resonance neutrons and due to the unknown burnup cross-section for the produced isomeric nuclei. The approach of Ref. [ 131 was obviously too simplified because rather low burnup cross-section 120 b was assumed, and the effect of resonance neutrons was neglected. At the present experiment with total fluence 110” n/cm2, the burnup can be completely neglected, but not at (3-4) orders of magnitude higher fluences in [ 131. However, the isolation of individual yields induced by thermal and resonance neutrons is still a problem in our case too. The Cd-difference method is not applicable, when the product yield is not really observed and only the upper limits restrict the numbers of produced nuclei. So that, we propose to operate with the effective crosssection aef,As known, after the irradiation during r,rr in reactor, the product yield in linear approximation is proportional to:
where Fib and Fr,,sare the fluxes of thermal and resonance neutrons, respectively, and ozf1 is defined as followings:
c = - FIh In( E2I E,) . F,.S
(3)
c is a constant for definite irradiation position at definite reactor. El and E2 define the “resonance” range of energies, and for heavy nuclei, the typical value is known as:
li
In 2 = (8 - 10) El
(4)
The FidFresratio should be below unity, at least, for Dubna IBR-2 reactor, and the recommended value c=5 can be realistic in our case. Such approach expressed in Eqs. (1-4) has the advantages that: 1. qjf value for the isomer can be deduced simply from the measured number of produced nuclei using the measured thermal neutron flux, and 2. the isomer-to-ground state ratio can be evaluated exploiting the tabular values of qhand I , for the production of the ground state nuclei. Eq. (2) allows to calculate
456
uef for the ground state products and to compare it with that determined in experiment for the isomer. This way, the results were deduced for slow neutrons in the present experiment. The upper limits for (n,y) and (n,n'y) production reactions are compared in Table 1 with cross-sections known in literature. In order to specify the resonance integral Zi, we apply another method for the result processing. The additional sample was irradiated at the straight channel of the reactor (behind the neutron mirror) within the container surrounded with the CB4 layer of 3 mm thickness. The thermal neutrons were completely screened out, and low energy resonances at E,S10 eV were suppressed too. After y-spectra measurements, the number of n8m2Hf in the sample was restricted by the upper limit and the flux calibration was done by such products as 95Zr, 175Hf,lglHf and lg2Ta.The resonance integral values for such products were corrected taking into account the neutron spectrum after the CB4 filter. However, such correction is not enough because at the Hf sample of 1 mm thickness the self-absorption near the resonance energies may be significant. The self-absorption can be neglected for 95Zr and Ig2Ta,it appears for 175Hfand is significant for Ig1Hf. Remind, that 9sZr is produced due to the little admixture (3%) of Zr in the Hf material, and Ig2Tais formed at the two-step capture process, the same as in the astrophysical sprocess: '80Hf(n,y)18'Hfp- -+ "lTa(n,y)Ig2Ta. The IglHf yield is measured directly, means, self-absorption is already in account and the second reaction is not influenced because of very low number of the IglTa nuclei in the target during the irradiation. Finally, the real flux values are estimated using 95Zrand Ig2Taactivities, both are in agreement, while the calibrations by I7'Hf and "'Hf show noticeable reduction of the flux. Using such quantitative measurements, we could estimate the resonance flux attenuation for the 177Hf(n,y)reaction, as well. Finally, the upper limits for Zy and for the isomer-to-ground state ratio were deduced. They correspond to the production of 178m2 Hf with resonance neutron capture and are given also in Table 1. It seems that Ref. [13] provided higher sensitivity of measurements. But, as mentioned above, the burnup effect of the produced 178m2Hf could be strong at fluences above 1O2I n/cm2 and was probably underestimated in publication [ 131. In addition, the resonance flux was neglected, and that could not be correct for the irradiation inside any reactor. Even assuming the best thermalisation, one has to use Eq. ( 2 ) for uef with the numerical coefficient c FZ 20. Because of that, we recalculated the results [13] in more realistic approximations including the resonance neutron contribution and the burnup effect. It would be clear that the radiative neutron capture is most destructive process, because the cross-section ueff can be as high as thousands barn. Respectively, at the fluence range of 0 2 10" n/cm2 the isomer burnup can be manifested. Other nuclear
457
processes in reactor irradiations are characterized by much lower cross-sections, but due to the neutron capture, even the feedstock (target) isotopes are in danger of useless depletion. The transmutation functions for stable Hf isotopes are calculated and are shown in Fig. 1. q h and Zy values are taken from [ 171, and they both are combined to get the unified parameter oe8using Eq. (2) and numerical coefficient c = 20. The latter choice should be adequate to the conditions of the Ref. [13] experiment. One can see that 177Hfis most unstable in neutron flux among other Hf nuclei.
Projectile energy AV)
a) b) c)
thermal
resonance
E,>3 MeV
En=14.5MeV
12
12
3 112
11
recalculated results of [13]; limits established in the present work; Ref: [14].
In presence of burnup of the target and isomer nuclei, the isomer accumulation hnction is expressed as following:
where Nm and No are the numbers of the isomer and target atoms, CD is fluence, omp is the isomer production cross-section, o m b and q b are the burnup cross-sections of the isomer and of the target. In (5) we use again oefquantities. The production and destruction oefvalues are needed for the calculation of the accumulation function for 178112 Hf. One can estimate the destruction cross-section using the results of experiment [16]. The values of q h = 45 b and Zy = 1070 b were obtained for the partial branch of the 3781112Hf(n,y) reaction with the population of the 179m2Hf isomeric state. Combined together with c = 20 they lead to aef=100 b. But total destruction cross-section should also include that for the (n,y) branch leading to the 379gHfground state. In rough approximation, we took oef= 200 b as a total destruction cross-section for '78m2Hfdue to the (n,y) capture.
458
With such choice, the processing of experimental results [I31 should be revisited and finally, the production cross-section is increased significantly. More intense destruction requires respectively the higher production probability to get the same number of produced atoms, see in Eq. (5). Recalculated cross-section is given in Table 1, and the corresponding cim/ogvalue, as well. They characterize the (n,y) reaction observed in [ 131 and evaluated in the present work in more realistic assumptions. The accumulation function is shown in Fig.] for the 178'n2Hfisomer as is calculated within the approach described above with the same numerical parameters. One can see that at fluences above lo2' n/cm2, the accumulation curve deviates strongly From linear function and then decreases. This is due to both the transmutation of the '77Hftarget nuclei and to the burnup of accumulated 178'1'2Hf. The experiment of present work also do not promise much higher yield of the '78n12Hfisomer. Finally, the conclusion follows that the reactor irradiation can not serve as a high efficiency method for the 178m2Hf production.
Nt No
0.8
0.6
0.4
0.2 0
Nm No [ 103
Accumulation and
burnup
0.8
0.6 0.4
0.2
0
Fig.1. Calculated transmutation functions for the Hf target stable isotopes in (n,y) reaction - a); and the 178m2 Hf accumulation curve - b). For the stable isotopes, burnup cross-sections are taken from 1171 and for the isomer, are estimated using the results of [16]. Condition of irradiations corresponds to that in Ref. [13].
459
3
New possibilities at low energy
A method of the 178m2Hf isomer production using the 176Yb(4He,2n)reaction was proposed and studied in Refs. [12,19]. Good cross-section (with respect to other products) allowed the accumulation of a high-purity isomeric material, but the yield was nevertheless restricted because of the 4He-ion energy losses and corresponded limitation on the target thickness. Only 1 pg 178m2Hf could be produced after extensive irradiations with high-current 4He-ion beam. Relatively high cross-section of the '76Yb(a,2n)'78m2Hf reaction leads to the idea of possible use such reactions as IglTa(p,a); I7'Hf(a,a'); "'Hf(a,dn) and 176Lu(7Li,an) in these reactions was not yet studied, but it was at low energies. Production of 178m2Hf known from the nuclear-reaction phenomenology that all of them are more or less probable processes at energy well above the interaction barrier. Means, the total crosssection should be of about hundreds millibam, and reasonably high angular momentum of the product provides not very low isomer-to-ground state ratio. So that, the 1781112Hf production cross-section is expected to be comparable with the known for the 176Yb(4He,2n) reaction, though not much more preferable. Special attraction is the '76Lu(7Li,an)reaction because 176Luis a unique case of the high-spin (7-) target. Respectively, the om/og ratio can reach a level of 50% in this reaction, i.e. to be 10 times higher as compared to '76Yb(a,2n). But at the same time, a maximum current of the 7Li ions is restricted due to the higher density of energy released in the target layer. In total, a factor of (3-5) can be the gain if one uses the high-current 7Li beam and the 90% enriched 176Lutarget of the best design in the sense of heat removal. A few orders of magnitude higher productivity is yet invisible. Nevertheless, indicated above reactions should be experimentally studied in order to operate with the reliable results, instead of some realistic estimations. 4
Comparison of different reactions
It was established that the largest quantity of 178m2Hf was produced at Los Alamos with 800 MeV protons from a high-current accelerator (formerly LAMPF). The yield of 178m2Hf was reported in Ref. [20], but the experimental details were described schematically and the productivity was only estimated. Recently, the reactions of proton-induced spallation were systematically studied for the Ta, W and Re targets of natural isotopic composition and for the enriched 186Wtarget, as well, using the 660 MeV synchrocyclotron at Dubna, Refs. [10,11]. Yields of the long-lived high-spin isomers of 179m2Hf, 178m2Hf and 177mLu are quantitatively determined and the measured values can be used for the productivity optimization in some future irradiations.
460
The highest production cross-sections for the Hf and Lu isomers have been found at the case of enriched lX6Wtarget, but in practice, it must be very expensive if one uses kilograms of the isotopically enriched substance as a target, The reasonable substitution would be the natTatarget, not expensive, and also characterized with rather good cross-sections for the production of mentioned above isomers. The absolute maximum of the I7'"'*Hf yield in p+Ta irradiation is estimated to be as high as 10l2 nuc1.h due to extremely high beam current achievable at Los-Alamos, of about 1 mA, and assuming thick target of 10 cm. Recently, we have obtained [9] the 178m2Hf yield in the Ta target irradiation with a 4.5 GeV Bremsstrahlung. As was expected, even at the best condition, the yield was lower than in the proton-induced spallation, but higher than in the reaction with lowenergy projectiles. In Table 2, the absolute productivities of the reactions induced by different projectiles are compared for the 178m2Hf isomer, following the measurements discussed above. The comparison is somewhat conventional, because the absolute yield depends Table 2. Quantitative parameters characterizing the different methods of the 178'"*Hf isomer production Projectile
Phoi
22
Intensity Target Amount
e100 pA 17'Hf 1og (total)
1%
Y 4500 e-
100 pA
Ta 33 g/cm'
om(mb) omlmg
Productivity, (atomsls) Rank
4.1O7
0.03 [91 3.10'
6
3
3.10"
PI
Neuti thermal 5,101' n/cm2s 177Hf 1g (total) 2.104
IS, In
101' n/cm2s 179~f
1og (total) 1.3 0.5.10-9 3.5.10;' ~ 3 1 ~ 5 1 3.4.10' 2.5.10' 1
Protons, IH'
Alphas, 'He"
14
4
100 pA 1 8 6 ~
Ta 33 g/cm' 0.3 0.02 [lo1
g/cm' 0.5 0.09 [Ill 5.10' 1
2
1°0pA 176yb
0.07 g/cm2 7 0.05 1121 5.10'
5
2.1O1O
1
I
1
5
Remark ont is not given for the Bremsstrahlung induced reactions because of the continuou3 spectrum ofphotons The yield ratio was measured
on the beam intensity and on the appropriate amount of the target material. Despite that, we want to get some ranking of reactions; therefore they should be compared a1 similar conditions in respect to input parameters characterizing a strength oi irradiation. For instance, a beam current is chosen to be 100 pA for all accelerators. and the same target thickness is assumed unless it is physically restricted due to the flux absorption, or the target material price. The chosen parameters are absolutely real. i.e. already reached at the facilities described in literature and remaining in operatior today. No extraordinary powerful systems are involved in the comparison. The
461
quantities of enriched target isotopes are restricted by the value of 10 gram because of high price of such substances. The ranks in Table 2 reflect the absolute yield of the reaction at comparable conditions. The p+Ta spallation is the most productive and its first rank could be expected. A productivity of 2.10'' atoms/s is given in Table 2 as the best, but in the condition comparable with other reactions. The absolute maximum has been estimated above assuming that the beam current can be as high as 1 mA with the target thickness of 10 cm. Even so, the production of 178m2Hfis restricted by mg amounts, while effective applications require kilograms. The latter amount is out of reality, at least at modem status of experimental physics. Despite such orders of magnitude mismatch, the results reviewed at the present report and summarized in Table 2 are of importance. They give a real basis for some speculations and estimations and also stimulate a nuclear-science progress in understanding of the processes with high-spin nuclear states. For nuclear reaction theory, even more significant are the isomer-to-ground state ratios, in addition to the production yields. In o,,,/og ratio, the scale factors in the reaction cross-section are excluded, and the ratio value has eventually strong implications for study of the nuclear reaction mechanism. In particular, mean angular momentum of the reaction residue has strong influence on the o,,,/og ratio. Fortunately, o,,,/og is measurable, and for some reactions, the residual spin can be figured out in theory. Thus, the correlation between o,/og and the reaction-product spin can be verified after the measurements. Such dependence is plotted in Fig.2 for the '78m2Hf isomer production. When the reaction product spin I, is increasing, the spin-deficit parameter A1 is respectively decreasing, and the probability of isomer population is growing up. Such natural behaviour is experimentally confirmed and quantitatively characterized in Fig.2. It would not be easy to calculate in theory the value for the reactions with the intermediate-energy protons or with high-energy Bremsstrahlung. In such cases, the parameter basing on the systematics of Fig.2 can be used for estimation of the measured om/og ratio. This way, the unique information is deduced confirming that the reaction residue receives rather high spin, like -10A, both in proton-induced spallation and in the reaction of photon absorption at GeV energies. In addition, the systematics can be used in application to other processes for estimation of the production possibilities with not yet studied reactions. At the end, let's discuss a somewhat fantastic idea of using the lSomTamaterial as a high-productivity target. Because of high spin (93 of this exotic nucleus, the 178m2Hf high-spin isomer can be produced in the spallation reaction with much higher isomerto-ground state ratio. The productivity can be enhanced by a factor of 10 using such a target, as compared to the regular natTatarget. This follows from the systematics of
5
462
Fig.2. However, a kilogram amount of the 90% enriched I8'"'Ta material is out of reality today. Creation of a special facility for the I8'"'Ta separation and the accumulation it in large amount should be extremely expensive, and even technical restrictions for that are not yet clear. Ignoring the cost arguments, one can deduce the absolute maximorum of the productivity, as following: Y,,, = I oi3 atoms/s, (6) if a 1 kg target made of 90% enriched IgornTa is exposed to the 800 MeV protons at a beam current of 1 mA. This way, of about 100 mg 178m2Hf can be accumulated in oneyear effective irradiation run.
6,,
I
I
I
(LHe 2nl
%l
lo-'
.oh(ung,
4.5 GeV
1
10.'
t 0
5
10
15
Mean ang. m o m . ( h )
Fig.2. Systematics of the isomer-to-ground state ratio versus the reaction product spin for the '7X"'2Hfisomer as is measured in reactions with different projectilcs.
463
5
Summary
Known experimental results are reviewed for the production cross-sections of the Hf exotic isomer. The productivity of different reactions is compared and they are ranked in an order of decreasing yield. Respectively, the values are estimated for the 1781112 Hf material amount that can be accumulated in irradiations with different projectiles. Realistic parameters of existing experimental facilities restrict the production of large amount, while the discussed in literature applications require by orders of magnitude higher quantities. A thinkable maximorum of productivity is estimated in assumption that the parameters of irradiations can be significantly enlarged using new facilities specially constructed for such irradiations and new isotope separator for the preparing of a kg amount of the '""'Ta, 179Hfand 176Lu isotopes. Measured isomer-to-ground state ratios are systematized, because they define the quality characteristics of the accumulated 178m2Hfmaterial. In addition, such systematics is significant in the nuclear-reaction phenomenology and can be used for the prediction of productivity at the case of unstudied reactions. 178m2
6
References
1. J.J. Carroll, Las. Phys. Lett. 1, No.6 (2004) 275. 2. R. Coussement, R. Shakhmouratov and G. Neyens, Euro Phys. News, 34 (2003) 190. 3. P.M. Walker and G. Dracoulis, Nature, 399 (1999) 35. 4. J.J. Carroll, S.A. Karamian, L.A. Rivlin and A.A. Zadernovsky, Hyperfine Znt., 135 (2001) 3. 5 . H. Roberts, Hyper-ne Int., 107 (1999) 91. 6. S.A. Karamian, C.B. Collins, J.J. Carroll and J. Adam, Phys. Rev. C57 (1998) 1812. 7. S.A. Karamian, C.B. Collins, J.J. Carroll, et al., Phys. Rev. C59 (1999) 755. 8. S.A. Karamian and J.J. Carroll, Las. Phys., 12 (2002) 3 10. 9. S.A. Karamian, J.J. Carroll, J. Adam and N.A. Demekhina, Preprint JINR, El2004-36, Dubna; submitted to Nucl. Instr. Meth., A. 10. S.A. Karamian, J. Adam, D.V. Filossofov, et al., Nucl. Instr. Meth., A489 (2002) 448. 11. S.A. Karamian, J. Adam, P. Chaloun, et al., Nucl. Instr. Meth. A527 (2004) 609. Preprint JINR, E6-2004-7, Dubna. 12. Yu.Ts. Oganessian, S.A. Karamian, Yu.P. Gangrski, et al., J. Phys. (London) G18 (1992) 393.
464
13. 14. 15. 16. 17. 18. 19. 20.
R.G. Helmer and C.W. Reich, Nucl. Phys., A21 1 (1973) 1. Yu. Weixiang, et al., Chin. J. Nucl. Phys., 14, No.4 (1992) 326. M.B. Chadwick and P.G. Young, Nucl. Sci. Eng., 108 (1991) 117. S.A. Karamian, Yu.Ts. Oganessian, J. Adam, et al., in Proc. Int. School-Seminar on Heavy-Ion Physics, Singapore, World Scientific, 1998, p.565. S.F. Mughabghab, Neutron Cross Sections, Academic, N.Y., 1984, v.1, Part B. S.A. Karamian, J.J. Carroll, et al., Las Phys., 14 (2004) 438. Yu. Ts. Oganessian, S.A. Karamian, Yu.P. Gangrski, et al., In Proc. Int. Symp. on “Nuclear Physics of Our Times”, Singapore, World Scientific, 1993, p.521. H.A. O’Brien, Nucl. Instr. Meth., B40/41 (1989) 1126.
467
SHELL STRUCTURE OF EXOTIC NUCLEI AND NUCLEAR FORCE
TAKAHARU OTSUKA Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 11 3- 0033, Japan Center for Nuclear Study, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan R I K E N , Hirosawa, Wako-shi, Saitama 351-0198, Japan E-mail: [email protected] TOSHIO SUZUKI Department of Physics, Nahon Cniversity, Setagaya-ku, Tokyo, Japan RINTARO FUJIMOTO, TOSHIAKI MATSUO, AND DAISUKE ABE Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan HUBERT GRAWE G S I , 0-64291, Darmstadt, Germany YOSHINORI AKAISHI KEK, Oho, Tsukuba-shi, Ibaraki 305-0801, Japan The effect of the tensor force on the single-particle energies is presented, exhibiting how spherical single-particle energies are shifted as protons or neutrons occupy certain orbits. An analytic relation for such monopole shifts is shown, and their general features are explained intuitively. Single-particle levels are shown to change in a systematic and robust way, by using the ~ + meson p exchange tensor potential. Several examples are shown with the corresponding experimental data.
1. Introduction
The shell structure characterizes finite quantum many-body systems. Atomic electrons confined by the Coulomb potential are subject to a wellknown shell structure. In the case of nuclei, the shell structure was con-
468
ceived by Mayer and Jensen in terms of the harmonic oscillator potential and spin-orbit splitting. This shell structure has enjoyed its tremendous success for a half century, and, indeed, it holds valid in stable nuclei around the B stability line on the nuclear chart. Recently, much progress has been made in clarifying the structure of exotic nuclei, which have rather extreme (i.e., asymmetric) ratios between the proton number ( 2 )and the neutron number ( N ) , being located far from the stability line. Naturally, it is of great and general interest what new features can be found in their shell structure. In this talk, along this line, we present the variation of the nuclear shell structure due to the tensor force. The nucleon-nucleon ( N N ) interaction is originally due to meson exchange processes as predicted by Yukawa 2 , and the tensor-force part of the N N interaction is one of the most distinct manifestations of this meson exchange origin. As we shall show, the tensor force does indeed change the shell structure in a unique and robust way throughout the nuclear chart, diminishing the conventional shell or magic structure in some cases, and creating new (sub-)shell gaps in other cases. The tensor force has been discussed over many decades. For instance, its contribution to the spin-orbit splitting has been discussed first by Terasawa and Arima in terms of the second-order perturbation '. The importance of the tensor force for the nuclear binding energy has been demonstrated for instance by Pudliner et al. 4 , and its crucial role in the structure of deuteron is a textbook example We shall present, in this talk, how single-particle energies are changed by the tensor force in the first order, following a simple, basic and novel rule. Several systematic features based on this rule will be presented, while the tensor force itself has been included in various numerical calculations as one of possible channels.
'.
2. Tensor Force
The change of the shell structure, or the shell evolution, may have different origins. We focus upon the shell evolution by the tensor force in this talk. It is well-known that the one-pion exchange process is the major origin of the tensor force, which is written as
where 71,~($1,~) denotes the isospin (spin) of nucleons 1 and 2, [ means the coupling of two operators in the brackets to an angular momentum (or rank) K , Y implies the spherical harmonics for the Euler angles of the
469
relative coordinate, and the symbol ( . ) means a scalar product. Here, f ( ~ is) a function of the relative distance, T . For the one-pion exchange process, f ( r ) can be derived from the well-known S12 function. Because the spin operators s< and ;s are dipole operators and are coupled to rank 2, the total spin S of two interacting nucleons must be S=l. The matrix element of VT vanishes for S=O. We make a remark on the states of the relative motion, using the usual notation of s, p and d for the L=O, 1 and 2, respectively, with L being the relative orbital angular momentum. If bra and ket states of VT are both s, the matrix element is vanished because of the Y(') coupling. Other combinations, for instance, s and d are allowed. These properties are used in later discussions. 3. Monopole Interaction and Effective Single Particle Energy
We then discuss how the single-particle energy of an orbit j1 is shifted as another orbit j 2 is occupied by some nucleons. Here, jl and j 2 are total angular momenta of orbits 1and 2. These nucleons in orbit j 2 can form various many-body states, but we are interestTedin monopole effects independent of details of such many-body states. For this purpose, the monopole component of an interaction, V , is extracted, by using the following two-body matrix elements 6 :
where < j1j2lVlj1j2 > J T stands for the (diagonal) matrix element of a state where two interacting nucleons are coupled to an angular momentum J and an isospin T . In the summation in this equation, J takes values satisfying antisymmetrization. We then construct a two-body interaction, called VM, consisting of two-body matrix elements I,$ j z: , in eq. (2). Because the J-dependence is averaged out in eq. ( 2 ) , the monopole interaction, V M ,represents the angular-free, i.e., monopole property of the original interaction, V. On the other hand, the isospin dependence remains in VM. As the orbit j 2 is occupied by more particles, the orbit j l moves in energy. If neutrons occupy j 2 and one looks into the orbit jl (f j 2 ) as a proton orbit, the shift of the single-particle energy of j1 is given by
where nn(j2) is the number of neutrons in the orbit j2. The same is true for Aen(j2) as a function of n p ( j l ) .
470
proton
neutron
Figure 1. Schematic picture of the monopole interaction produced by the tensor force between a proton in j>,<= 1 f 112 and a neutron in j > , < = 1’ f 112.
i’ Figure 2. force.
Exchange pro Zsses contributing to the monopole ir eraction of the tensor
If the orbit j z is fully occupied by neutrons, only the monopole effect remains and eq. (3) gives the energy shift for this (sub-)shell closure. The single-particle energy including monopole effects from valence nucleons is called effective single-particle energy (ESPE), and the filling configurations are taken usually t o evaluate many-body monopole effects. If protons and neutrons are filling the same orbit, the change of ESPE becomes slightly more complicated in general due to isospin symmetry
‘.
471
4. Identity for the Tensor Monopole Interaction
We begin with cases like Fig. 1: with orbital angular momenta being denoted by 1 or I' (1 # l l ) , protons are in either j , = I + 112 or j , = I - 112, while neutrons are in either j > =I' 112 or j : = 1' - 112. From now on, V is the tensor force. For the orbits j and j', the following identity can be derived,
+
where T = 0 and 1, and j ' is either j: or j : . Note that this identity is in the isospin formalism, and can be applied not only cases in Fig. 1 but also for cases between neutrons or between protons. The identity in eq. (4) can be proved by angular momentum algebra by summing all spin and orbital magnetic substates for the given 1. It is assumed that the radial wave function is the same between j , and j , orbits, which is exactly fulfilled in the harmonic oscillator and practically so in other models if the orbits are well bound. This identity is a general one for the tensor force. It does not hold if the single-particle states j and j ' are identical (as excluded in Fig. l ) , because the substate summation is affected by the isospin symmetry. However, the actual monopole matrix elements follow the relation in eq. (4) semi-quantitatively. Only exchange processes in Fig. 2 contribute t o VM for the tensor force. The direct processes yield no contribution. Because of this, the spin-coordinate part of the T=O and 1 matrix elements are just opposite. Combining with (?I . ?.) in eq. ( l ) , one obtains
Thus, the proton-neutron tensor monopole interaction is twice stronger than the T=l interaction. We now discuss the implications of the identity in eq. (4): the tensor monopole interaction between proton j , and neutron j > has the opposite effect t o that between proton j , and neutron j : (See Fig. 1). The same property holds for other but similar combinations of the orbits. For instance, as neutrons occupy f712, the proton d5I2 and d312 should move in opposite directions. The questions then arise as to which directions they move, and as to whether there is a general rule for this movement. We reply t o such questions now.
472
5. Intuitive Picture of the Tensor Monopole Interaction Here, one needs another argument to determine the sign of the effect. This can be given in an intuitive way as in Fig. 3. In Fig. 3 (a), a nucleon on j , is colliding with another on j > . Due t o high relative momentum between them, the spatial wave function of their relative motion is narrowly distributed in the direction of the collision which is basically the direction of the orbital motion. The spins of two nucleons are parallel in this case, giving rise to S=l basically. Thus, the spatial distribution is narrower in the direction perpendicular to the composite spin S=l. From the analogy to the deuteron, the tensor force works attractively. The same mechanism holds for two nucleons in j , and j : . On the other hand, as in Fig. 3 (b), the tensor produces a repulsive effect for two nucleons in j , and j > (or vice versa), because the wave function of the relative motion is stretched in the direction of the collision. Thus, we can obtain a robust picture that j , and j > (or vice versa) orbits attract each other, whereas j , and j > (or j , and j : ) repel. This effect should be significant if the orbital angular momenta of the two colliding nucleons are both large or are equal. The radial wave functions of the two orbits must be similar to have a large overlap in the radial direction. A narrow spacial distribution is favored in the radial direction, in order to have a “deuteron-like” shape. This is fulfilled if the two orbits are both near Fermi energy, because their
[izziiz]
[repulslonl
0wave function of relative motion 4 Figure 3.
spin
Intuitive picture of the tensor force acting two nucleons on orbits j and j’.
473
t#
> >
0
Figure 4.
I
I
f I
I
I
I
I
'
I
I
I
I
I
b
r (fm)
8
3
I
2
I
3
I
I
3
I
li
3
Triplet-even potential due to the tensor force for various interaction models.
radial wave functions have rather sharp peak at the surface. If the radial distributions differ between the two orbits, not only the overlap becomes smaller but also the relative spacial wave function is stretched in the radial direction, which is against the deuteron-like shape, resulting in quick damping of the effect. 6. Examples Having this mechanism clarified, we assess the effect quantitatively by using a reasonable tensor force. Figure 4 exhibits the triplet-even potential due to the tensor force in various potential models, including 71 exchange, 7r p exchange, M3Y 7 1 AV8 ', and G-matrix (G-M) potential for the normal nuclear density. The first two are fixed from standard meson-nucleon coupling constants 9,10. Although there are large differences in the short distance part, these potentials do not differ much for T >0.8 fm except for the IT exchange. Since two nucleons interacting through the tensor force are not in the relative s state (as discussed earlier), the differences at the short distance are irrelevant in the following discussions. We use the 7r + p exchange potential with a radial (inner) cut-off at 0.7 fm, for simplicity. In fact, all these interactions but 71 exchange produce quite similar results. Figure 5 shows effective single particle energies (ESPE) of 1d5/2,3/2and 2 . ~ 1 1 2 orbits of protons as a function of N. As more neutrons occupy 1f7p, these proton orbits are shifted. In Fig. 5, their changes due to the tensor force are indicated starting from experimental energies of 40Ca. Following the general rule discussed with Fig 3, the monopole interaction between
+
474 Proton Single Particle Levels
I
-8'
20
24
28
Neutron Number Figure 5. Proton ESPE in Ca isotopes as a function of the neutron number. Lines are calculations obtained with the H p tensor force. Points represent analyzed experimental data". The values are relative to that of ld3/2, and their changes due to the tensor force are shown by lines, starting from experimental ones for 40Ca.
+
proton d 3 p and neutron f 7 p is attractive, whereas that between proton d5/2 and neutron f712 is repulsive. Hence, as more neutrons occupy 1f 7 / 2 , proton ld3/2 goes down while ld512 comes up. Since the energies are shown relative to ld312 in Fig. 5, as N increases, 2s1/2 approaches to ld312 and the splitting between ld512 and ld312 becomes narrower. A compilation of experimental data is included in Fig. 5 11, showing the narrowing between ld312 and 2.3112 in agreement with the calculation '. The situation is more open for ld512, because of higher ambiguity due t o deep hole states. Figure 6 exhibits proton pf-shell orbits from 68Ni to 78Ni as neutrons occupy 1 9 9 1 2 . The single-particle energies for 68Ni predicted by the shell model with the G X P F l interaction l2 are used as the starting point, and changes due to the tensor force are shown. Again as in Fig. 1, proton 1f 5 1 2 is pulled down while lf712 is lifted up, as N becomes larger. The Z=28 gap becomes rather small at 78Ni, and the sequence of the orbits are quite different between 68Ni and 78Ni 17. There should be other monopole effects, but those are likely more equal among the pf-shell orbits as expected from the Woods-Saxon potential, for instance. Thus, the predictions shown in Fig. 2 may produce exciting features in the structure of exotic Ni isotopes. In fact, unusually low-lying 2+ states are known 13. The N=51 isotones provide us with another example. As the proton number increases from 2=40 t o 50, the 19912 orbit is filled by protons. These protons pull, through the tensor force, neutron 19712, whereas neu-
475 Proton Single Particle Levels
$-12I/
-
Figure 6. Proton ESPE in Ni isotopes, starting from the values for @‘Ni (see the text) as a function of the neutron number. Their changes due to the tensor force, as evaluated by the A p tensor force, are shown by lines.
+
4
Neutron Single Particle Levels
2djiZ O
40
50
L
Proton Number Figure 7. Neutron ESPE in N=51 isotones starting from experimental values at 2=40 as a function of the proton number. The energies are shown relative to 2d512. Lines indicate the changes due to the tensor force as evaluated with the A p tensor force. Points represent e ~ p e r i m e n t s l ~ .
+
tron 1hlll2 is pushed up, as shown in Fig. 7. The growing spacing between neutron lg712 and lhll12 is well explained. In the sense that the tensor effect is calculated from the underlying N N interaction, the calculations in Figs. 5 7 are all predictions. This lowering of 19712 is nothing but the phenomenon pointed out by Federman and Pittel 1 4 . Figure 8 shows proton lhll12 and 1 9 7 1 2 single particle energies as a function of N 16. In Fig. 8, their changes due to the tensor force are
-
476 0
-
Proton Single Particle Energies
-5 , :
za -5-F
-
2 :
Y
-10I
64
72
80
Neutron Number Figure 8. Proton single-particle energies in S b isotopes as a function of the neutron number. Points are experiment16. Lines are calculations obtained with the T p tensor force (see the text).
+
indicated starting from experimental energies for N=64. Here, in order to shed a light on the increasing spacing between 1h11/2 and lg7/2 in the full energy scale of single-particle states, a common monopole shift for both lhll/a and 197/2, -0.3 A N (MeV) is included so as to accommodate other mean field effects approximately. This common linear shift is consistent with the usual Woods-Saxon potential. As more neutrons occupy 1hll12 , proton 1hll/2 and 19712 become more apart due to the tensor force. The agreement to experiment on this feature is remarkable. We note that this case does not belong to the one in eq. (3). In the present case, all magnetic substates of the neutron lh11/2 orbit are occupied, in a good approximation, with equal probability due to strong pairing correlation. It can then be proved that eq. (3) is still valid. In the present case, not only l h l l l 2 but also other orbits are occupied between N=64 and 82. We then assume that the occupation of 1hll12 increases linearly. There are many other observed cases l7 to which the tensor shell evolution is relevant, for instance, the inversion between the neutron ld5/2 and 2s112 between 15C and 170. The present tensor monopole effect has been compared to the neutron skin effect by extending mean field calculations. It has been seen that the tensor and neutron-skin effects are of the same order of magnitude.
477
7. Summary
In summary, the shell evolution due to the tensor force was presented for the first time with its mechanism. A relevant identity was shown, and an intuitive explanation is given for this mechanism, which produces general and robust effects. This shell evolution can change largely the shell or (sub)magic structures of exotic nuclei, from pshell t o superheavy regions. The significant role of the tensor force as direct effects of T and p mesons seems t o be related t o the Chiral Perturbation idea of Weinberg 18. Indeed, the long-range part of the N N interaction seems to manifest itself quantitatively in the single-particle spectra in a unique and systematic way. As the tensor force somewhat resembles the magnetic dipole-dipole interaction 21 , its effects may be of interest. The importance of the proton-neutron j > - j < coupling has been pointed out in relations to the shell evolution within one major shell in 19,20. The tensor force should be one of the origins of this coupling, and details will be presented in a forthcoming paper. This work was supported in part by a Grant-in-Aid for Specially Promoted Research (13002001) from the MEXT. This work has been a part of the RIKEN-CNS joint research project on large-scale nuclear-structure calculations. References 1. M.G. Mayer, Phys. Rev. 7 5 1969 (1949); 0 . Haxel, J.H.D. Jensen and H.E. Suess, Phys. Rev. 75 1766 (1949). 2. H. Yukawa, Proc. Phys. Math. SOC.Japan 17 48 (1935). 3. T. Terasawa, Prog. Theor. Phys. 23, 87 (1960); A. Arima and T. Terasawa, Prog. Theor. Phys. 23, 87 (1960). 4. B.S. Pudliner, V.R. Pandharipande, J . Carlson, S.C. Pieper and R.B. Wiringa, Phys. Rev. C 5 6 , 1720 (1997). 5. M.A. Preston and R.K. Bhaduri, Structure of the Nucleus (Addison-Wesley, Reading, 1975). 6. R.K. Bansal and J.B. French, Phys. Lett. 11, 145 (1964). 7. G. Bertsch, J. Borysowicz, H. McManus and W.G. Love, Nucl. Phys. A 284, 399 (1977). 8. R.B. Wiringa, V.G.J. Stoks and R. Schiavilla, Phys. Rev. C 51,38 (1995). 9. F. Osterfeld, Rev. Mod. Phys. 64, 491(1992). 10. S.-0. Backman, G.E. Brown and J.A. Niskanen, Phys. Rep. 124, 1 (1968). 11. P.D. Cottle and K.W. Kemper, Phys. Rev. C 5 8 , 3761 (1998). 12. M. Honma, T. Otsuka, B.A. Brown, and T. Mizusaki, Phys. Rev. C 6 5 , 061301(R) (2002); Phys. Rev. C 69,034335 (2004). 13. For instance, M. Sawicka et al., Phys. Rev. C 6 8 , 044304 (2003).
478
P. Federman and S. Pittel, Phys. Lett. B 69,385 (1977). H. Grawe et al.,, Nucl. Phys. A 704,211 (2002). J.P. Schiffer et al., Phys. Rev. Lett. 92,162501 (2004). H. Grawe, Springer Lect. Notes in Phys. 651, 33 (2004). S. Weinberg, Phys. Lett. B 251,288 (1990). T. Otsuka, R. Fujimoto, Y. Utsuno, B.A. Brown, M. Honma and T. Mizusaki, Phys. Rev. Lett. 87,082502 (2001). 20. T. Otsuka, Prog. Theor. Phys. Suppl. 146,6 (2002). 21. A . Bohr and B.R. Mottelson, Nuclear Structure, vol. 1, p. 249 (Benjamin, 14. 15. 16. 17. 18. 19.
1969).
479
STUDIES OF FINE STRUCTURE DECAY IN PROTON EMITTERS AT THE HOLIFIELD RADIOACTIVE ION BEAM FACILITY JON BATCHELDER UNIRIB/Oak Ridge Associated Universities, Oak Ridge, TN 37831 USA
M. TANTAWY, C. R. BINGHAM, R. K. GRZYWACZ, C. MAZZOCCHI University of Tennessee, Knoxville, TN 3 7996 USA C. J. GROSS, K. P. RYKACZEWSKI, C.-H. W Oak Ridge national Laboratory, Oak Ridge l" 3781 USA
D. J. FONG, J. H. HAMILTON, W. KROLAS, A. V. RAMAYYA Vanderbilt University, Nashville, TN 37235 USA T. N. GINTER, A. STOLZ NSCL, Michigan State UniversiQ, E. Lansing, MI 48824 USA A. PIECHACZEK, E. F. ZGANJAR Louisiana State University, Baton Rouge, LA 70803 USA J. A. WINGER Mississippi State University, Mississippi State, MS 39762 USA
M. KARNY Warsaw University, PI-00681 Warsaw, Poland K. HAGINO Dept. of Physics, Tohoku Universiq, Sendai 980-8578 Japan Measurement of fine structure in proton emission allows one to deduce the composition of the parent state's wavefunction and the deformation of the daughter state. In this paper the results from a systematic study of proton-unbound nuclei in the rare earth region performed at HRIBF are presented. Newly discovered fine structures in the decays of I4'Hoand '46Tmare discussed, as well as advances in our experimental equipment.
480
1.
Introduction
The study of the decay of proton-emitting isotopes allows one to study nuclear structure effects in nuclei that are inaccessible during in-beam experiments. The emitted proton tunnels through the Coulomb and centrifugal barriers, and the decay probability depends strongly on the energy of the proton and on its angular momentum t. In addition to the energy of the emitted proton, the partial half-life for proton emission must be accurately determined in order to understand the tunneling process. Proton emission from a spherical (odd-Z, even N) nucleus typically occurs to the Of ground state of the even-even daughter. The orbital angular momentum (t) of the emitted proton can often be determined through the use of a simple spherical WKB calculation of the expected rate of the tunneling process, revealing the shell model orbital of the least bound proton of the parent emitting state [ 11. Fine structure in the proton emission spectrum allows one not only to define low energy states in the daughter, but from a comparison of the proton energies and partial half-lives, enables determination of the composition of the singleparticle proton states in the proton unbound state 12-41. In the case of an odd-Z, even-N decay to an even-even nucleus, the proton emission proceeds primarily to the 0' ground state with some fraction proceeding to the excited 2+ state. The energy of this 2' state allows one to estimate the quadrupole deformation in the daughter [5,6]. The situation with the decay of an odd-odd nucleus to an (evenZ, odd-N) isotope is quite a bit more complicated. The proton emitting state consists of coupled proton and neutron states, with the final state being a lowenergy (not necessarily the ground state) neutron configuration in the daughter nucleus. The study of fine structure in the decay of these odd-odd nuclei can be used to identify and determine relative energies of these low-energy neutron levels in the (even Z, odd N) daughter nucleus. In this conference proceeding we report on the results of fine structure studies in the proton emission of I4'H0 and '46Tm. 1.1. Experimental Method
The experimental results presented here were obtained at the Oak Ridge Holifield Radioactive Ion Beam Facility (HRIBF), using the 25 MV Tandem Accelerator. After bombardment on the appropriate target, recoil nuclei of interest were separated spatially according to their masslcharge (NQ)values by the HRIBF Recoil Mass Spectrometer (RMS) [7]. A microchannel plate detector (MCP) [8] at the focal plane was used to identify the A/Q of the recoils. In reactions similar to those presented herein, the R M S transmission efficiency has
481
been determined to be -5% for a typical production target of 0.5 - 1.0 mg/cm2 [7]. Following the MCP, the ions were implanted into a 60-pm thick doublesided silicon strip detector (DSSD) [9] with 40 horizontal and 40 vertical strips. This strip arrangement results in a total of 1600 pixels, each acting as an individual detector. Events in the DSSD are defined as a recoil if they are in coincidence with the MCP and a decay if not in coincidence. Signals from the DSSD are read by the preamps and then fed directly into a digital spectroscopy system using 25 DGF-4C modules (produced by X-ray Instrumentation Associates) [lo]. This system uses 40 MHz flash ADC's and on-board digital signal processors. This system serves as a replacement for amplifiers, discriminators and conventional ADC's.
-
1.2. Experimental Results - '"Ho
Holmium-14 1 was studied via the 92Mo(s4Fe,p4n)reaction with a beam energy of 300 MeV and intensity of 20 pnA over a period of -6 days. The half-life of I4'H0 was previously measured to be (3.9(5) ms) [ l 1, 121. The position of the 2+ state in I4'Dy was previously measured at HFUBF [13] to be 202.2(2) keV through the decay of 14"'D ' y (7 ps, I" =83. This was confirmed by an independent experiment at Argonne National Laboratory [ 141. Figure 1 shows the spectra accumulated in the front and back strips of the DSSD by requiring the time between recoil and decay to be < 30 ms. Two peaks are present in both front and back strips. The large peak (-7000 counts) at 1.17 MeV is the previously observed [ l l , 123 ground-state to ground-state ('41Ho-+'40Dy)transition. The smaller peak at 0.97 MeV corresponds to the energy one would expect for the fine structure decay of I4'H0 to the 202 keV 2' state in I4'Dy [15]. The resulting branching ratio for the fine structure is 0.7(2)%, and the value of D2 is estimated to be 0.23-0.24 for I4%y.
Figure I . Decay energy spectra from front and back strips of the DSSD obtained during the I4'Ho study.
482
Calculations on I4'H0 using a non-adiabatic coupled-channel method [4] indicate that the wave function is composed of -80% nhlIl2and -13% nfl12, both of which are coupled to rotational states of the ground state band of the I4'Dy core. The observed fine structure decay is governed by the nfl12 component. The resulting half-life, however, is overestimated by a factor of 10, and the branching ratio for fine structure decay by a factor of 3. In contrast, the only other known example of fine structure in the proton decay of a highly deformed nucleus is I3'Eu (p2 = 0.32) [2,11]. Interpretation of the I3'Eu data via the same method gives results that are in good agreement with the measured partial halflives and branching ratios. Another set of theoretical calculations, based on the adiabatic particlerotorapproach [16], investigated the effect of a static triaxial deformation on the fine structure decay rate of I4'H0. The results from this calculation agree with experimental values only if the triaxial angle y is < 5 degrees, which indicates that triaxiality does not play a significant role in the decay of 14'H0. For the case of y = 0 degrees, values of tl/2= 3.0(7) and a branching ratio for fine structure decay of 0.71(5)% are obtained, and are in good agreement with the experimental values from this work. 1.3. Experimental Results - '"Tm
In a previous experiment by this research group [ 171, fine structure in the proton radioactivity of '46Tmwas observed, which populated excited neutron states in I4'Er. Three fine structure transitions of energies 0.89( l), 0.94( 1) and l.Ol(2) MeV were observed. Due to the low statistics of the data, only the 0.89 and 0.94 MeV transitions could be unambiguously assigned to the isomer and ground state respectively, based on the similarity of the half-life values. Because of this, we re-investigated the decay of '46Tmwith an improved setup at the focal plane of the RMS.
-
Hack Strips. Si Wax Vato, 'I'c 200 ins
dln,,,+i I
_
__xxI
Figure 2. Spectrum of proton events obtained in the '46Tmstudy.
~
-
"
~
483 Table I . Preliminary values for the proton energies, half-lives, and counts from '46Tm.
E (keV) 888(10) 1120(5) 938(10)
TIR(ms)
# counts
190(80)
170(30)
198(5)
9450(250)
Relative Intensity 1.8(3) 100
60(20)
290(30)
22(2)
1016(10)
70(7)
743)
1 190(5)
75(3)
370(40) 1350(80)
100
We produced '46Tmvia the 92Mo(5ENi,p3n) reaction with a beam energy of 297 MeV (292 MeV at the target mid-point). In this experiment, a "Si-box" consisting of four 700 pm thick Si detectors was added to the system to veto escaping alphas and protons. With these detectors, we were able to significantly clean up the proton spectra. The existence of the three new fine structure peaks was confirmed, as is shown in Fig. 2 . The measured energies and half-lives are detailed in Table 1. Based on the half-lives, we assign the 0.89 MeV transition to the 198-ms high spin state (along with the 1.12 MeV line). The 0.94(1) and 1.02(1) MeV transitions are assigned to the 75-ms low spin state (along with 1.19 MeV). From a simple shell model picture, one expects that 146Tmwould have 5 proton particles above the Z = 64 proton subshell, and 5 neutron holes below the N=82 closed shell. The available single particle orbitals for both protons and neutrons are therefore hI1/2,d3/2and s112. From the experimental level systematics for heavier odd-odd Tm isotopes, one would expect an isomer with a spin of 8' to 11' (xh11/2@vh1112), and a ground state of 5 - or 6- (nh11~2@v~112). The lower spin states can have a complex structure with admixtures of n ~ I , ~ @ v h xd31z@vhI112,and xhl 112@vd312 contributing to their wave functions. The possible wave function compositions of both the isomer and ground state of 146Tmwere analyzed in the particle-core vibration coupling model [18] and compared with the experimental data. In the case of the high spin isomer, the calculations show that if the level was 8' as previously assigned [ 171, both the branching ratio for the fine structure decay and the half-life are much smaller than the experimental data. Of the three possibilities that give values consistent with the experimental values, fine structure decay to the 13/2' state in the daughter is more likely than to the 1512' based on the relatively low excitation energy (0.48 MeV) of the daughter state. The relatively long proton half-life (compared to the measured half-life) indicates that the decay of this state proceeds mostly (-75%) via beta decay. For the low spin ground state, the two scenarios that agree with experiment are 5'+3/2+ and 6-+5/2+. From the
484
systematics of the N = 77 isotopes, one would expect the 5/2'(vs1/2@2') state to be > 200 keV, and the 312' (vs1/2 @ 2+ ) state to lie somewhere between 160 and 180 keV. We therefore assign the 1.02(1) MeV transition to the 312' state in 145Er. The resulting decay scheme is shown in Figure 3. Figure 3. Partial decay scheme of'46Tm. All energies are listed in Mev
Acknowledgments This work is supported by the U.S. D.O.E. under contract No. DE-ACO5760R00033 (UNIRIB), DE-AC05-000R22725 (ORNL), DE-FG02-96ER40983 (U. TN.), DE-FG02-96ER40978 (LSU), DE-FG05-88ER40407 (Vanderbilt), DE-FG02-96ER4 1006 (MSU), and DE-FG05-87ER40361 (JIHIR). References
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
S . Aberg et al., Phys. Rev. C58,3011 (1998). A. A. Sonzogni, et al., Phys. Rev. Lett. 83, 1116 (1999). A. T. Kruppa, et al., Phys. Rev. Lett. 84,4549 (2000).
B. Barmore, et al.,Phys. Rev. C62,0543 15 (2000). L. Grodzins, Phys. Lett. 2, 88 (1962). S . Raman et al., At. Data Nucl. Data Tables 78, 1 (2001). C. J. Gross et al., Nucl. Instr. Meth. A450, 12 (2000). D. Shapira, et al., Nucl. Instr. Meth. in Phy. Res. A454 409 (2000). P. J. Sellin, etal., Nucl. Instrum. Methods Phys. Res. A311,217 (1992). B. Hubbard-Nelson et al., Nucl. Instr. Meth. A422,411 (1999). C. N. Davids et al., Phys. Rev. Lett. 80, 1849 (1998). K. Rykaczewski, et al., Phys. Rev. C 60, R2970 (1999). W. Krolas, et. al., Phys. Rev. C. 65,031303(R) 2002. D. M. Cullen, et al., Phys. Lett. B529,42-49 (2002). K. P. Rykaczewski et al., in Proc. of Int. Conf. on Nucl. Structure "Mapping the Triangle", Wyoming 2002, AIP Proc. 638,2002, p. 149. 16. C. N. Davids and H. Esbensen, Phys. Rev. C 69,034314 (2004). 17. T. N. Ginter, et. al., Phys. Rev. C 68,034330 (2003). 18. K. Hagino, Phys. Rev. C64,041304R(2001).
485
NUCLEAR MODEL OF BINDING ALPHA-PARTICLES *
K.A. GRIDNEV, S.YU. TORILOV Institute of Physics, St.Petersburg State University, 198904, Russia V.G. KARTAVENKO Joint Institute for Nuclear Research, Dubna, Moscow District, 141980, Russia W. GREINER, D.K. GRIDNEV J . W. Goethe University, Frankfurt/Main, 0-60054, Germany J. HAMILTON Vanderbilt University, Nashville, T N 37235, USA
The model of binding alpha-particles in nuclei is suggested. It is shown good (with the accuracy of 1-2%) description of the experimental binding energies in light and medium nuclear systems. Our preliminary calculations show enhancement of the binding energy for super heavy nuclei with Z ~ 1 2 0 .
We assume that atomic nuclei might be considered as a tight packing of alpha-particles. The binding energy could be expressed as a sum of energies coming from interactions between N , alpha-particles and their self-energies.
EB(Na)= EB(Q)Nol -k EC
-k Va,(Na)
(1) where V,,(N,) = Cr;i VaOl(rij), E~(cr)=28.296MeV is the binding energy of alpha-particle, and Ec is the Coulomb part of the potential energy. For the alpha-alpha interaction we use the Lennard-Jones (LJ) potential, which can be written as
where r is the distance between centers of clusters, VOis the depth of the *This work is partially supported by Deutsche Forschungsgemeinschaft (grant 436 RUS 113/24/0-4), Russian Foundation for Basic Research (grant 03-02-04021) and the Heisenberg-Landau Program (JINR)
486 ES[MeV] -0-
theory
-m-
experiment
I
0 '
6
1-
m
\ , 12
,
18
f
Figure 1. Separation energy of a single alpha-particle versus the number of alphaparticles N, . The alpha-particle sets and the differences of inter-alpha-particle b o r h ( A N = B,(N, 1) - B,(N,),where B,(N,) is the number of the bonds for nucleus with N, alpha-particles.)
+
potential (Vola(a) = -Vo, d V & / d ~ l ~ = = ~0), and u is the pair bonding length. The positions of alpha-particles are found from the condition of
487 Distance [fm] 14
..
-
20
1000
500
1500
2500
2000
Number of inter -alpha particle bonds
The area of expected stability island
ES[MeV1 12-
0-
-4,
0
10
'
,
20
*
,
30
.
l
40
,
,
50
.
l
60
.
l
70
Na Figure 2. The set of distances between alpha-particles in the nucleus versus the number of bonds between alpha-particles B,. Separation energy of an alpha-particle as the function of the number of alpha-particles N , .
the minimal total potential energy. The volume of the nucleus is set on a grid and the first alpha-particle is positioned in the center. The potential is calculated in all nodes of the grid and the next particle is positioned into the node with the minimum potential energy (T = n), Then we take two
488
particles and calculate their potential and put to its minimum the third cluster. And so on and so forth. This simple algorithm gives possibility to cast the binding energy (1) in the following form1
EB(No)= EB(Q)N,+ VoB, 4-E&
(3) where the effective geometrical factor B, is the number of bonds between alpha particles, and all Coulomb terms are collected in E’c. Assuming six equal nucleon bonds in each close-packed tetrahedral alpha particle, the unit bond energy will approximately equal to VOx E B ( Q ) /M~ 5 MeV. The cluster structure of nuclear system, and the selected type of interuction (LJ Eq. (2) results to enhanced stability of tight cluster configurations with icosahedrul symmetry (see Figures). It is seen, in the upper part of Fig. 2, that the first set of distances between alpha-particles are nearly CT x 3 fm, and then there are stripes corresponding to non-bordering alphaparticles and etc. Gradually the stripes wash out because the tetrahedrons have to deform in order to constitute closed figures. The cluster structure of nuclear system leads to (many-body) shell effects (see Figures), which are not reduced to the traditional nucleon singleparticle ones. Such shell effects could be very important for heavy and superheavy nuclei, which stability is defined mainly by the shell effects. Assuming that the main trend remains when adding neutrons, our calculations show enhancement of the binding energy for super heavy nuclei with Z -120, which corresponded to completed icosahedral shell of 55 alpha-particles. One of us has predicted recently fullerene-like nuclei in this region. The details (exact binding energy and the 2 value) depend on the details of the potential, but the main trends conserve, as seen from the lower part of Fig. 2, where results are presented for two different values of the potential width (Rw = 21/12a((& 1)1/6 - (& - 1)1/6). It turns out also, the structure of heavy even-even nuclei looks like the Fe nucleus coated with the Bose condensate of alpha-particles with additional neutrons playing the role of electrons in the covalent bond. A similar mechanism of alpha-particle formation analogous with formation of Cooper pairs has been considered in ’.
+
References 1. P.D. Norman, Eur. J . Phys. 14,36 (1993). 2. W.Greiner, Prog. Theor. Phys. Suppl. 146,84 (2002). 3. S. Koh, Prog. Theor. Phys. Suppl. 132,197 (1998).
489
STRUCTURE OF THE HEAVY CA ISOTOPES AND EFFECTIVE INTERACTION IN THE SD-FP SHELL
F. MARECHAL,~ F. PER ROT,^ PH. DESSAGNE,~J.C. ANGELIQUE,’ G. BAN,’ P. BAUMANN,l F. BENRACHI,3 C. BORCEA,4 A. BUTA,4 E. CAURIER,~s. COURT IN,^ s. GREVY,’ c. JOLLET,~F.R. LECOLLEY,’ E. LIENARD,’ G. LE SCORNET,’ CH. MIEHE,l F. NEGOITA,4 F. NOWACK1,l N. ORR,’ E. POIRIER,l M. RAMDHANE3 AND I. STEFAN4 IReS, I N z P ~ - C N R S ,F-67037 Strasbourg Cedex 2, France 2LPC, IN2P3-CNRS, F-14050 Caen Cedex, France University of Constantine, Constantine, Algeria Institute of Atomic Physics, IFIN-HH, Bucharest, Romania ISOLDE, Division EP, CERN, CH-1211 Geneva, Switzerland
’
p decays of the neutron-rich nuclei 51,52,53K have been used to populate
bound and unbound levels in 50,51,52,53Ca.y r a y spectroscopy as well as P-delayed neutron spectroscopy by time of flight were carried out to obtain a detailed decay scheme to levels in 53Ca, 52Ca,51Ca and 50Ca. A half-life of 1 1 6 f 6 ms was deduced and new PI, and Pzn delayed neutron emission probabilities have also been determined for 5 2 K . A total of 11 and 3 new y transitions have been observed in the decay of 52K and 53K, respectively. The low-lying level structure of 52Ca is compared to shell-model calculations. Implications of these results on the determination of the Y-Y interaction in the f p shell are discussed.
1. Introduction
It has been experimentally observed that the first excited 2+ state in the N = 32 calcium isotope is well above that observed in the even-even neighbors, suggesting a subshell closure at N = 32 corresponding to the filling of the ~ p 3 / 2neutron orbital. This gap can be explained as the result of a reduced u f5p-nf7/2 monopole interaction as protons are removed from the f 7 p orbital. Indeed, one expects the very attractive rj>-uj< monopole term in the nucleon-nucleon interaction t o play an important role in defining the magic numbers in exotic nuclei1. The excitation of the lowest Gamow-Teller states in heavy Ca isotopes depends strongly on this proton-neutron interaction across the sd- f p shell. Because an energy around 4 MeV is necessary to excite these unnatural
490
parity states, one expects the Gamow-Teller states within reach of p decay to be located close t o or above the neutron separation energy, giving rise to a large delayed neutron emission. Nevertheless, because of the simplicity of their wave functions, the Ca isotopes correspond to the optimal choice to fix these two-body matrix elements in the A = 50 mass region. Calcium isotopes are also interesting for the information they can bring to our understanding of the neutron-neutron interaction in the f p shell. Because of the Z = 2 0 shell closure, the lowest energy states in even Ca isotopes are of positive parity. These natural parity states are populated either by forbidden transitions or indirectly. Their properties (E,, decay modes) are governed by the neutron-neutron monopole interaction in the f p shell. They are therefore indicative of the neutron sub-shell sequence, and of the evolution of the effective interaction between valence neutrons. Little has changed in our knowledge of the heavy potassium decay properties since the first experiments performed at CERN/PS2 and ISOLDE/SC334, yielding information on half-lives, delayed neutron emission and excitation energies of some low-lying states in heavy calcium isotopes. To bring new insights in the description of the A - 50 nuclei, we have undertaken a study of the neutron-rich calcium isotopes through the p decay of the corresponding neutron-rich potassium precursors. The experiments were performed at CERN using the online mass separator ISOLDE. Here, we report new data on the low-lying structure of 51Ca, 52Ca and 53 Ca. The results are compared with shell-model calculations performed in the full f p valence space.
2. Experiment
The experiment was carried out at the ISOLDE online isotope separator at CERN. The K ion beams were produced by spallation of a thick (53 g/cm2) UC, target induced by an intense 1.4 GeV proton beam delivered by the CERN-PSB accelerator. A hot transfer line connects the target and the tungsten surface ion source from which the ions are extracted in their 1+ charge state and electrostatically accelerated to 60 keV. The spallation products were analyzed using the HRS mass separator and collected on a 55 pm thick aluminized mylar tape that can be moved in order to limit the build-up of daughter activity. The average production yields for “K, 52K and 53K were 2.4x103, 50 and 2 atoms/&, respectively. The beamline transmission was as high as 90 %. The target received six to ten of the sixteen proton pulses produced per
491
supercycle. The ions were collected on the tape for 500 or 600 ms at the beginning of each measurement cycle that lasted 1 or 2 s in total. At the beginning of each cycle, a real time clock was started and the time of each event was recorded. During the experiment, 0-7 as well as p-7-7 and P-n-7 coincidences were recorded. The p particles were measured in a 2 mm thin cylindrical plastic scintillator surrounding the collection point in a near-47r geometry with a total detection efficiency of about 70%. The p-counter signal was used as the acquisition master trigger t o time-stamp the events and t o start the neutron time-of-flight measurements. The y rays were measured using two large Ge clusters from the MINIBALL array5. The energy resolution and the total photopeak efficiency for 7-ray detection were measured t o be around 3 keV and 5 % at 1.33 MeV. The ,B-delayed neutrons were detected using two types of detectors and their energies were measured using the time-of-flight technique. For the low-energy neutrons, a group of 6 detectors, each consisting of a 1 cm thick plastic scintillor (10 cm diameter) readout by two photomultiplier tubes operated in coincidence, has been used with a 66 cm flight path6. With a threshold adjusted below the one photoelectron level, the resulting energy threshold was about 90 keV. For the upper part of the neutron spectrum, complementary information was obtained from 11 curved scintillating plastic bars from the TONNERRE array7. Each individual module, 1 6 0 x 2 0 ~ cm3 4 in volume, bends to a radius of 120 cm (ie. the flight path) and is connected t o a photomulitplier tube on each end. The absolute neutron detection efficiency for both group of detectors was calculated from the observed intensities of the well-known @-delayedneutron decay of 49K. Total efficiencies (including the solid angle) for the low energy neutron detectors and the TONNERRE modules are 0.2% and 8 % at 1 MeV, respectively. Energy resolutions of 6 % and 11% were measured at 440 keV for the two neutron detection systems, respectively.
3. Results Decay curves for the three 51,52,53Kisotopes were obtained from P-7 coincidence data. A new value of the half-life was deduced only for 52K. Independent values of Tl/2 in that case were obtained from the two most intense 7 transitions. The weigthed average from the two values yields =116.6f6.0 ms, in agreement with the previous measurements of 1 0 5 f 5 ms2 and 110f30 ms4.
492
Decay schemes for 51K, 52K and 53Kwere obtained from the ,B-y-y and P-n-y coincidence analysis. No new transition was found in 51Ca following From the decay of 52K, we have identified four new y the decay of 51K. transitions in 52Cain addition to the previously known 2.56 MeV transition deexciting the 2; excited state4. Seven new transitions have been observed in 51Ca following the &delayed neutron emission in 52Ca, and five new levels at 1718, 2378, 2934, 3500 and 4493 keV have been identified in this nucleus. We have also observed for the first time a feeding to the 2; excited state in 50Cafollowing a P-delayed two-neutron emission. Finally, we have observed for the first time in the decay scheme of 53K the transitions at 2220 keV in 53Caand 2563 and 3150 keV in 52Ca. Complete decay schemes for 51K, 52K and 53K as well as the energies, relative intensities and level assignements of all observed y transitions will be found in a forthcoming publication'. The emission of one and two neutrons has been observed through the related y activities in the A-1 and A-2 daughter nuclei, respectively. New neutron emitter states in 52Cadecaying to the ground state and the excited states located at 1718, 2378 and 2934 keV in 51Ca have been observed. A detailed level scheme of these unnatural parity states located above the neutron emission threshold is currently being constructed from the P-ny coincidence data. Tables grouping the energies and the intensities of the neutron transitions assigned to the decay of 51K, 52K and 53Kare in preparation and will be available soon'. In order to determine the total P, neutron emission probability, one has unfolded the decay curves to disentangle all direct and daughter P activities. In the case of 52K, the intensity of the 2563 and 3150 keV transitions in 52Ca yields a P, of 75&8%. The one- and two-neutron emission probabilities were then calculated based on the number of 0-n coincidences corrected from the neutron detection efficiency. Values of 7 3 f 1 0 % and 3 f l % were obtained for P 1 n and Pzn, respectively. Because the intensity of the B transition to the ground state of 52Cawas not measured, the PI, value should be considered as an upper limit. However, this Po transition is expected to be very weak based on the relative y intensities observed in 51Sc and 50Sc. For 51K, a Pi, value of 63 % has been determined, and no delayed emission of two neutrons has been observed for this nucleus. Results for the decay of 53K are not available yet. The very low statistics in that case make it difficult to extract any delayed neutron emission probability. Morover, neutrons coming from the decay of 53Ca also contribute to the time-of-flight spectra and are difficult to distinguish from those coming from the decay of 53K.
493 4. Discussion
The experimental low-level structures of the 51752753Caisotopes have been compared to full fpshell-model calculations using the GXFPl’ and KB3G1° effective interactions. Results for 51,52Cashow that the effective interaction in the lower part of the fp shell is relatively well understood, the N = 3 2 sub-shell closure being well described by both interactions (see Fig. l ) , eventhough the first (1+,2+) doublet splitting is better reproduced by the KBSG interaction.
1+3+5.77
5.95
-243 4+ 2+ 4+
4; 3
5.93 5.64
0+1+ 4+2+
4.28 4.23 3.94 3.90
5.22
2+ 4.11
O+ -
3.42
3.99
?”?-_.._2.56
--“.-___ 2+
3.08 7.35
Figure 1. Low-energy levels for 52Ca. Experimental results are compared to full fp shell-model calculations using either the G X P F l or KB3G interactions.
New information on the behaviour of the upper f5/2 shell can now be brought from the location of the first 4+ state in 52Ca. A possible candidate for such a state is the one observed at 3.99 MeV of excitation energy. If this state is of natural parity, it corresponds most probably to a (~p3/2)’@(vp1/2)’ or ( ~ ~ 3 1 @( 2 ~ ) f~5 / 2 ) ’ configuration. However, one does not observe any y-y coincidence between the 1961 keV transition deexciting the 5.95 MeV state (see Fig. 1)and 511 keV annihilation photons that would come from internal pair production in the 0;’ to O L s , decay. This rules out the ( ~ p 3 / 2 ) ~ @ ( ~ ~ possibility 1/2)~ in first approximation. The location of the 3.99 MeV state would thus support the idea of a smaller vp,/, - ~ f 5 / 2 gap corresponding t o a more attractive Vf7,2-f5,2 monopole term as it is in the KB3G interaction (see Fig. 1). However, ambiguities remain as for the parity of this state and one cannot draw any firm conclusion on the different monopole terms of the interaction yet. Gamow-Teller strength calculations
494
are especially needed to try to locate the non-natural parity states in 52Ca and eventually preclude this possibility for the 3.99 MeV state. 5 . Summary
,f3 decay of the neutron-rich 51152753Kisotopes was used to populate excited
states in the daughter 51952953Canuclides, respectively. A-1 and A-2 calcium isotopes were also populated in some cases through delayed neutron emission. Several new y and neutron transitions have been observed in the The complete and detailed evaluation of the ,B-y--y decay of 52K and 53K. and B-n-y coincidence data is under way. The low-energy levels of the even-even 5 2 Ca isotope have been compared to full fpshell-model calculations using two different Hamiltonians. Both GXPFl and KB3G interactions yield good predictions for the lowest 2+ state in 52Ca. Ambiguities remain as for the spin and parity of the state observed at 3.99 MeV. Nevertheless, preliminary results on 52 Ca support the calculations based on the KB3G interaction. However, additional calculations are needed in order to further pinpoint the monopole terms of the n-n effective nuclear interaction in the fp shell. Gamow-Teller strength calculations are currently being carried out to study the negative parity states in 52Ca and tune the n-p effective interaction across the sd-fp shells. Acknowledgments
The authors would like to thank their colleagues, engineers and technicians, from the IReS-Strasbourg and LPC-Caen, whose collaboration has been so valuable during the experiment. References T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001). M. Langevin et al., Phys. Lett. 130B,251 (1983). L.C. Carraz et al., Phys. Lett. 109B,419 (1982). A. Huck et al., Phys. Rev. C 31, 2226 (1985). J. Eberth et al., Progress Part. Nucl. Phys. 46, 389 (2001). M. Bounajma, Ph.D. thesis, UniversitC de Strasbourg, 1996, Internal Report NO. CRN-96-43. 7. A. Buta et al., Nucl. Instrum. Methods Phys. Res. A 455, 412 (2000). 8. F. Perrot, Ph.D. thesis, UniversitC de Strasbourg, 2004, to be published. F. Perrot et al., in preparation. 9. M. Honma et al., Phys. Rev. C 69, 034335 (2004). 10. A. Poves et al., Nucl. Phys. A 694, 157 (2001).
1. 2. 3. 4. 5. 6.
495
STATICAL AND STATISTICAL PROPERTIES OF HEATED ROTATING NUCLEI IN THE TEMPERATURE-DEPENDENT FINITE-RANGE MODEL
E. G. RYABOV AND G. D. ADEEV Department of Theoretical Physics, Omsk State University Prospect Mira 55a, Omsk, Russia E-mail: [email protected] Macroscopic temperature-dependent finite-range model is applied to calculate statical and statistical properties of heated rotating compound nuclei. The level density parameter was approximated by a leptodermous type expression. The coefficients of this expansion are in surprisingly good agreement with those obtained earlier by Ignatyuk and co-workers. The importance of taking into account simultaneously temperature and angular momentum of the nucleus on such properties as heights and positions of barriers, effective moments of inertia is considered. Temperature dependence of ( Z 2 / A ) , , i t parameter and BusinaroGallone point are also investigated. It was revealed that, both parameters moved to the range of lighter nuclei as temperature increased. It was shown that, temperature effects are qualitatively similar to angular momentum ones, but they are significantly smaller in most investigated cases.
1. Introduction
The availability of heavy ion beams in recent decades has stimulated experimental studies of fission-fusion process of atomic nuclei with large amount of angular momentum formed in heavy-ion induced reactions. Theoretical description of such systems includes a quantitative estimation of the behavior of a compound nucleus with respect t o its shape and rotational degrees of freedom as functions of the excitation energy and the angular momentum. Most detailed and exact description of fission process can be obtained in some ways: Microscopic methods (HartreeFock, Thomas-Fermi, e t c . ) , Strutinsky macro-microscopic method. But both are complicated and very time-consuming in numerical calculations. Macroscopic liquid drop approach is much easier, but temperature independence has been its weakness until recent Krappe publication In
'.
496
this work finite-range LDM based on the Yukawa-plus-exponential potential was generalized by Krappe t o describe the temperature dependence of the nuclear free energy. This dependence is obtained by fitting the results of the former temperature-dependent Thomas-Fermi calculation with a finite-range mass formula. This generalization lets us calculate various statical and statistical properties of hot rotating nuclei consistently in the framework of Krappe’s approach. In the current work we will try to pay attention t o angular and temperature dependence of level-density parameters, fission barriers B f , effective moment of inertia Jeff and stiffness with respect to asymmetric deformation and BusinaroGallone point ( Z 2 / A ) ~ c 2. Macroscopic temperature-dependent finite-range model
Free Helmholtz energy in the finite-range LDM based on Yukawa-plusexponential mass formula as a function of the mass number A = N 2, relative neutron excess I = ( N - Z)/A, and a set of collective coordinates q has been suggested ’) in the following form:
+
F ( A , 2,q, T ,L ) = -a,(l
- lc,12)A
+ as(l - k,12)Bn(q)A2/3+ COAO(1)
where a,, a,, and a, are the usual volume, surface, and Coulomb energy parameters of the finite-range LDM at zero temperature and lc, and lc, are the corresponding volume and surface asymmetry parameters. The deformation dependence is taken into account through the shape functions B,(q), Bc(q), and J(q), which model effects of finite range of nuclear forces and the realistic distributions of charge and nuclear densities. The temperature dependence of the 7 coefficients entering equation (1) a,, a,, k,, k,, T O , a , and ad is parameterized in the form Ui(T)
= U i ( T = 0)(1 - XiT2),
(2)
3. Main results Our present work continues investigations of Krappe’s finite-range temperature-dependent liquid drop model. First results of our research was published in our previous work, done in cooperation with Karpov A.V.
497 Table 1. The finite-range LDM coefficients. The first row gives their values at zero temperature and the second one - the coefficients xi. TO
ai(0) 1032i (MeV-')
1.16 -0.736
a
ad
av
kv
as
0.68 -7.37
0.7 -7.37
16.0 -3.22
1.911 5.61
21.13 4.81
ks 2.3 -14.79
and Nadtochy P.N 3 . As mentioned in introduction we paid attention t o fission barriers of hot rotating nuclei. It's clearly seen from figure 1, how v
10
-
50
100
150
200
250
300
A Figure 1. Fission barriers of hot beta-stable nonrotating nuclei (solid lines, temperatures in MeV). Dashed line - barriers for beta-stable nuclei with T = 2 MeV and angular momentum L = 40A. Inverted triangles - experimental data with the shell correction.
both temperature and angular momentum are important when evaluating fission barriers. Also special attention was paid to stiffness to asymmetric deformation and Businaro-Gallone point. Stiffness of the fissioning nuclei in the saddlepoint is one of the main characteristics when considering fission of nucleus with Z2/A< 32 and moderate values of angular momentum. In the framework of temperature-dependent finite-range LDM instead of potential energy one deals with the free energy of the hot rotating nucleus. At the Businaro-Gallone point stiffness turns into 0. As can be seen from figure 2 ( Z 2 / Apoint ) ~ ~moves to lighter nuclei when temperatures increased from 0 to 4 MeV.
498
Z21A Figure 2. The stiffness of fissioning nucleus in the saddle-point - a2F/6’q2 in MeV for the hot beta-stable nuclei (temperatures in MeV). Filled squares - are experimental estimations ‘.
4. Conclusions
From the results it’s obvious that in theoretical description of fission-fusion reactions both temperature and angular momentum of compound nucleus should be considered. Furthermore, in most cases angular momentum plays more crucial role than the temperature of compound system. The model under consideration lets us calculate all important parameters for fissionfusion dynamics modelling consistently in the framework of one approach. This kind of experiments are the next step in our investigations of Krappe’s finite-range temperature-dependent LDM. First results of our dynamical modelling was published in our previous work, done in the cooperation with Karpov A.V., and Nadtochy P.N. ’. References 1. K r a p p e H.J., Phys. Rev. C59, 2640 (1999). 2. Guet C . , Strumberger E. and Brack M. Phys. Lett. B205,427 (1985). 3. Karpov A.V., Nadtochy P.N., Ryabov E.G. a n d Adeev G.D., J. Phys. G. 29, 2365 (2003). 4. Itkis M. G., a n d Rusanov A. Ya., Faz. Elem. Chast. 29(2), 389 (1998).
499
NUCLEUS-NUCLEUS POTENTIALS FROM DEEP SUB-BARRIER FUSION AND THEIR RELATION TO CLUSTER RADIOACTIVITY*
R.N. SAGAIDAK AND S.P. TRETYAKOVA Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia A.A. OGLOBLIN Kurchatov Atomic Energy Institute, Moscow, Russia
S.V. KHLEBNIKOV Khlopin Radium Institute, St-Petersburg, Russia W. TRZASKA JYFL, University of Jyvaskyla, Jyvaskyla, Finland
A combined study of the cluster decay probability of nuclei and fusion of their decay products provides new information on the mechanism of cluster radioactivity and fusion. The analysis of the I6O, 22Ne 208Pb fusion cross sections using different forms of nuclear potentials shows that the “alpha-decay-like” scenario is more preferential than the “fission-like” one for these reactions, which are the reverse to the cluster decay from compound nuclei 224Th and 230U. The consideration of fusion for heavier projectiles allows one to trace a possible transition of the potential from the “alpha-decay-like” to the “fission-like”one.
+
Cluster radioactivity (CR) occupies an intermediate position between alpha-decay and spontaneous fission of nuclei. Twenty nuclides from 221F’r to 242Cmemitting light nuclei from 14C to 34Si,correspondingly, are known. More than a dozen of theoretical models were proposed for the explanation of this phenomenon. Despite the fact that different models are bases for these theories, they reproduce measured decay probabilities quite well’. *This work is supported by the Russian Foundation for Basic Researches (Grant 0202-17297) and Academy of Finland under the Finnish Center of Excellence Programme 2000-2005 (Project 44875, Nuclear and Condensed Matter Physics Programme at JYFL)
500
Differing in details they describe CR either as an adiabatic “fission-like”2or as a sudden “alpha-de~ay-like”~ (cluster) processes corresponding to quite different shapes of the potential barrier. A reason for this lies in some compensation of different factors determining the decay probability. So, a real mechanism of CR still remains an open problem. At the same time, quite unexpected behavior of the fusion excitation function, which was observed recently in nearly symmetric combinations of massive nuclei at deep sub-barrier energies4, implies seemingly that it is the “fission-like” or adiabatic potential that governs the fusion process at low energies. In CR both decay products are formed in their ground states, consequently, study of their deep sub-barrier fusion can contribute in solving this problem as well. The aim of this work is to check the applicability of different nucleus-nucleus potentials to the description of fusion cross sections in the region from well above to deep sub-barrier energies. In our measurements of heavy-ion fusion (fission in the case of strongly fissile compound nuclei) excitation functions we used solid-state track detectors (SSTD)5, as a very effective method for the study of extremely deep sub-barrier fusion. The use of SSTD allows one to go down to the crosssection level of -lop5 mb and lower and to probe the potential far beyond the top of the barrier. An array of SSTD (mica) installed into a targetreaction chamber allows us to detect fission fragments approximately in 4~-geometry.It is also possible to measure the angular distribution of fission fragments and to make a conclusion on their anisotropy6. This could help to estimate possible presence of quasi-fission events (resulted in the non-equilibrated fission process without compound nucleus formation). Analysis of the data was performed with the HIVAP code7, in which the fusion cross section is calculated in the framework of the potential barrier-passing model. In this approach the coupled-channel (CC) effects are reproduced with the fluctuating barrier (expressed via c ( r o ) / r oas the percentage of the radius parameter r , ) . As shown in a number of works, excitations of different degrees of freedom through the coupling effect, cause a splitting in energy of the single fusion barrier resulting in a distribution of barriers. This barrier distribution drastically alters the fusion probability from its value calculated assuming penetration through a single barrier. Our analysis of the l60 + 208Pb reaction data6Is-lo shows (Fig. 1, left panel) that the exponential7 and Woods-Saxon (WS) potentials (with the corresponding parameter values designated in the figure) provide the best fit to the data (for the WS potential we used the systematics of the diffuseness parameter a with the fixed values of V, and ro, as was proposed
501 recently"). Forms of these potentials are close to the "alpha-decay-like" one, whereas the Bass potential12 being weaker than previous ones, does not allow us to reproduce the data in the whole region of energies.
0
oN,, this data (I1run)
A
%*,",
Andreyev el al ro=l 12,o(ro)/ra=35%
-v;""=75,
-
k,=O 68
---- Vyp=70 o(r,)lr0=4 0% ,V
I"
65
70
75
80
80
85
90
100
c(ro)lro=2 8% k,=O 68
110
120
130
Figure 1. Left panel: ER, fission and fusion cross sections6,8-10 obtained earlier and in this work at deep sub-barrier energies (fusion cross sections based on fission and ER data6,l0)for the l60 208Pb reaction in comparison with the corresponding excitation functions calculated with HIVAP7 using different nuclear potentials (adjusted parameter values are designated in the figure). Right panel: the same as in the left panel, but for the 22Ne zosPb reaction with the fission data measured at JYFL recently14 and ER data obtained earlier15 (two values of scaling parameter at the liquid-drop fission barriers ICf have been used in attempts to reproduce these data in the whole energy region).
+
+
The first study of the extremely deep sub-barrier fusion with the products of the cluster decay 230U --+ 22Ne 208Pb,for witch the CR probability was measured earlier13, has been performed recently at JYFL14. Preliminary results and data analysis using different nuclear potentials are shown in Fig. 1 (right panel). The best fit to the data in the whole region was obtained with the WS (using the same a, V, and T , as for l60 'O'Pb) and Bass potentials (see corresponding parameter values in the figure). The best fit to the data with the exponential potential corresponds to some
+
+
502 overestimate of the fusion cross section at the lowest energies. The results of our analysis show that CC effects are very important for both the reactions, since the single barrier calculations ( o ( r o ) / r o = 0) with any potential strongly underestimate the sub-barrier fusion cross sections. At the same time the analysis shows that CC effects are more pronounced in the "Ne fusion than those for the l60one (it is expressed in greater values of o ( r o ) / r o )probably, , due to a significant deformation of the former. A further selection of the potential can be performed assuming that the fusion potential satisfies the cluster decay probabilities for z24Th-+ l60 '08Pb and 230U -+ 22Ne 208Pb. It was made r e ~ e n t l y 'for ~ some fixed potentials. The problem arising in such approach is taking into account the barrier distribution (ro fluctuations in the present consideration). One should mention that main contributions into the calculated fusion cross section give us potentials with greater values of ro providing lower heights and widths of the barrier and, consequently, higher penetration probabilities. One can expect a feasible transition to the "fission-like" (adiabatic) nuclear potential in the deep sub-barrier fusion with the increasing mass of a projectile. Available data on fission cross sections around the fusion barrier for reactions of 28Si, 32S and 48Ca with '08Pb motive us for the future measurements at deep sub-barrier energies using the SSTD technique.
+
+
References 1. A.A. Ogloblin, G.A. Pik-Pichak and S.P. Tretyakova, in Proc. of CRIS 2002, eds. S. Costa et al. (Melville, NY, 2002), p.122. 2. G. Royer and B. Remaud, Nucl. Phys. A444,477 (1985). 3. R. Blendowski and H. Walliser, Phys. Lett. 61,1930 (1986). 4. C.L. Jiang et al., Phys. Rev. Lett. 89,052701 (2002). 5. S.P. Tretyakova, Sow. J. Part. Nucl. Phys. 23, 156 (1992). 6. B.I. Pustylnik et al., JINR Rapid Communication No.3[89]-98, 57 (1998). 7. W. Reisdorf, 2. Phys. A300, 227 (1981); W. Reisdorf et al., Nucl. Phys. A438,212 (1985); W. Reisdorf and M. Schadel, 2. Phys. A343,47 (1992). 8. K.-T. Brinkmann et al., Phys. Rev. C50,309 (1994). 9. C.R. Morton et al., Phys. Rev. C52,243 (1995). 10. R.N. Sagaidak et al., in Proc. of VI International School-Seminar on Heavy Ion Physics, eds. Yu.Ts. Oganessian and R. Kalpakchieva (WS, 1998) p.323. 11. J.O. Newton et al., Phys. Lett. B586,219 (2004). 12. R. Bass, Lect. Notes in Phys. 117,281 (1980). 13. R. Bonetti et al., Nucl. Phys. A686,64 (2001). 14. S.P. Tretyakova et al., Nucl. Phys. A738,487 (2004). 15. A.N.Andreyev et al., Yad. Fiz. 50,619 (1989); JINR Rapid Communication, No.3[77]-96,65 (1996).
503
DECAY SCHEMES OF NUCLEI FAR FROM STABILITY I.N.IZOSIMOVt Khlopin Radium Institute, 2"" Murinski avn.28 St.Petersburg, 194021, Russia It is shown that in the medium and heavy nuclei with Tli22-3MeV may be not identitied in decay schemes. The principles of the more complete decay schemes construction by using the combination of the TAGS spectroscopy with high resolution yspectroscopy are presented. The possibilities of TAGS applications for P-strength measurements, decay schemes completeness testing and more complete data using for decay heat calculations are discussed.
1.
Introduction
The total absorption y-ray spectroscopy (TAGS) is based on summation of cascade gamma quantum energies in the 4n geometry""). The TAGS may be applied for P-decay strength function Sp(E) measurement, for total P-decay energy Qp determination and for decay scheme completeness testing. The combination of the TAGS with high resolution y-spectroscopy may be applied for Sp(E) fine structure study and for detailed decay schemes construction3JoJ'). For the Gamow-Teller beta transition the level occupancy with the excitation energy E of the daughter nucleus after the /?+/EC-decay or p- -decay, I(E), and half-life T, rcan be written as1): I(E) = Sp(E)Tmf(Qp - E),
(1)
(Tin).' = I Sp(E)f(Qp - E) dE,
(2)
Sp(E)dE = C l/(ft) (3) where SP(E)- the beta decay strength function which describe the nuclear part of transition, f(QP- E) - the Fermi function which describe the lepton part of transition and QfI- is the total energy of the beta decay. Fermi functionf(Qp - E) decrease with excitation energy E increasing and as a rule the more intensive beta decays populate the levels with low (less than 2-3MeV) excitation energies. But from the nuclear structure point of view the most interesting beta transition populate the levels with high (more than 3-4MeV) excitation energies where in S,(E) the strong resonances") or it's tails may be observed. Also a lot of nuclear levels and y-transitions may not be identified in decays schemes because of not so strong beta transitions to the levels with high excitation energy. For beta transitions to the high excitation levels study and the decay schemes completeness testing the total absorption y-spectroscopy may be used'-") both for neutron-deficit (P'/EC-decay) and neutron-rich nuclei @-decay). For TAGS applications and for detailed decay schemes construction by using TAGS in +
e-mail: [email protected]
504
combination with high resolution nuclear spectroscopy methods it is necessary to have Z-separated (element separation) and M-separated (mass separation) s o ~ r c e s ~ For ~ ~ ~nuclei ' ~ ) . with TI,*) 30min we used radiochemistry methods for element separation and after it mass separator for isobaric pure sources In our experiments we use the total absorption y-rays producti~n~~"). spectrometer (Fig.1) which consists of the two NaI(T1) crystals 0200mm by 1 l0mm and 0200mm by 140mm. The larger crystal has a 070mm by 80mm well into which the nuclei under investigation are supplied and where a Si(Au) detector is install for P-particles detection5). Isolating total absorption peaks in the total absorption (TAS) spectrum, one can find the occupancy of the levels I (E),and using (1)-(4), find Sp(E)1,5). The end-point energy of TAS spectrum is connected with the total beta decay energy Qp The TAS spectrum and $(E) may be calculated from decay scheme data. For decay scheme construction the high-resolution nuclear spectroscopy methods are using"). Compare the TAGS data (TAS spectrum and Sp(E)) with the data obtained from decay schemes one may estimate the degree of the decay scheme completeness and determine the energies regions where decay scheme is not enough ~ o m p l e t e " ~ ~ ~ ~ ' ~ ) .
D - fi-particlc detccior X x-ray detector
Nal(T1)
y -detector PMT-photornultiplier
Figure 1 .Total absorption y-rays spectrometer.
2.
P+/EC-decay
Using our TAGS spectrometer we dete~ted"~.'~)Gamow-Teller = +1 resonance (Fig.2) for 1478Tb(T112=I.6h) P'/EC-decay as a strong peak in Sp(E) at E-4MeV. Theoretical analysis of observed Gamow-Teller = +I resonance and its fine structure was done in Ref.3). The P/EC-transitions to the levels with
505
excitation energy more than 2MeV were not identified in the decay scheme of I4’gTb (Fig.3) from Ref.I3). This means that the decay ~ c h e m e ’ ~(Fig.3) ) (T112=1 .6h) is strongly incomplete. The more complete decay scheme (Figs.4 & 5 ) of ‘47gTb ( T u F .6h) ~ was constructed in Ref.”). The most interesting region for the beta strength functions study lies at the excitation energies more than 34MeV. The B’/EC-decay strength function (Fig.6) deduced from the more complete decay scheme was c o n s t r ~ c t e d ~ The ~ ’ ~ )strength . functions (Figs.2 & 6 ) 1.7 h
20 -
io -
0
1
-J
36.06h 3
4
Figure 2. ““*TTb (Tliz=l.6h) VlEC-decay strength function from TAGS” I”). GamowTeller p,=+l resonance was observed as a strong peak at Ez4MeV
1 1
0
’;$d Figure 3 . ‘47gTbdecay scheme from’3).There are no indicated !Y/EC-transitions to the excitation energy more than 2MeV. This decay scheme is not complete and not agree with TAGS
are in a good agreement (Figs.4 & 5) of I4’gTTb (T1,,~l.6h)PlEC-decay fiom Ref.”) is enough complete. Comparison of the decay schemes (Figs.3 to 5 ) demonstrate that the decay schemes for transitions especially to the levels with excitation energies more than 2-3MeV in the medium and heavy nuclei may be strongly incomplete. For estimation of the decay scheme incompleteness degree by using TAGS spectroscopy it is necessary to have both Z and M separated sources because of low energy resolution of TAS spectrometer and about one day for measurements and data analysis. For detailed decay scheme and one may conclude that the decay scheme construction it is necessary to have much more time of measurements and data analysis.
506
Figure 4. 14’&Tbdecay scheme”’, a - low energy levels of I4’Gd
Figure 5 . ‘47gTb decay scheme’ ‘1, b high energy levels of 147Gd. There are manyB+/EC -transitions to the region with excitation energy more than 2MeV. This decay scheme is enough complete and is in a good agreement with TAGS data.
507
= 4.6 MeV T O T ) =0.12 in energy &w
3G
g
0,014
-
0,012
-
0.010
-
0.-
-
rq
4.43 MeV
0.m -
o m0.002
1
Figure 6 . The ,f'/EC-decay strength function deduced',"') from the more complete decay scheme").
3. p - -decay, fission products decay schemes completeness and decay heat calculations
Population of the levels at the excitation energies more than 2-3MeV after -decay connected with resonance structurel-12)in S,(E). The information about p-decay strength function and possibility of decay schemes completeness testing is very important for correct decay heat calculations especially for fission products6). The p and y decay energies realized through the natural decay of fission products may be up to 13% of the total energy generated during the fission process and becomes dominate component following reactor hutd down^,'^). This energy source is commonly called decay heat. There are some discrepancies between calculations of decay heat with using such libraries as JNDC, JEF2.2, ENDFIB-VI and the experiments connected with y and p components of the fission products. The p and y-ray discrepancy exist in equivalent studies for 239Pu(Fig.7), 233,23s,238U fission product^^"^). The correct calculations of decay heat is very important factor in the operation with radioactivity. By using TAGS spectroscopy it was demonstrated') that more than 50% of the p- decay intensity to the higher lying states have not been identified for some fission products in the nuclear spectroscopic decays studies. To improve agreement between calculation and experiments it is necessary to have more complete decay schemes of fission products. The TAGS spectroscopy may be used for Qp measurement^^^'^) with accuracy up to 20keV and for decay schemes completeness testing. The combination of the TAGS with high resolution nuclear spectroscopy methods may be effectively used for more
508
complete decay schemes construction for fission products and understanding of the origin of the y-discrepancy in decay heat (Fig.7).
Figure 7. Gamma decay heat (multiplied by cooling time) for 239Pufission products as a function of cooling time after fission'). The lines represent r e s ~ l t s ~ . 'of ~ , calculations '~) using different database. There is y-discrepancy in the 300-3000s cooling period.
4. Neutron excess influence on Sp(E) structure
As the excitation energy increases, the nuclear level density grows rapidly and the wave functions of nuclear states acquire a quite complicated structure, science even a small residual interaction can lead to the mixing of the different configurations. In some cases it is assumed that the structure of these states is very complicated, and the coefficients of the wave function decomposition on the simple configurations follow statistical lows. The characteristics of different nuclear processes are rather simple to calculate by statistical model. Observations of the resonances in SB(E) for the Gamow-Teller beta decays indicate that there is no such strong mixing of the simple configuration populated in P-decay at sufficiently high excitation energy and non-statistical models must be used for beta strength function calculations'.'z). For Gamow-Teller transitions non-statistical effects are closely connected with spin-isospin SU(4) ~ y r n r n e t r y ' ~ ' ~However, ~ ' ~ ~ ' ~ )science . the nuclear interaction is observed to depend on spin, so that SU(4) may be only an approximated symmetry, non-statistical effects connected with the spin-isospin SU(4) symmetry will be less clearly manifested than effects connected with isospin SU(2) symmetry.
509
LF= [-39.6(N-Z)/A
+
9.5]MeV
0
Figure 8. Calculated location of the Garnow-Teller resonance E(GT) relative to the isobaranalog resonance E(1AR) as a function of neutron excess.
The width and fine structure spread of the peaks in Sp(E) for Gamow-Teller transitions (up to several MeV) will be much grater than for peak in Sp(E) for Fermi transitions or isobar-analog resonance (several tens of keV).The possibility of the spin-isospin SU(4) symmetry restoration with neutron excess (N-Z) increasing was pointed out in Ref.’). One of the spin-isospin SU(4) symmetry consequence is the equality of the isobar-analog resonance E(IAR) and the Gamow-Teller resonance E(GT) energie~”.’~). One may see from fig.8 that the differences A E essentially depends on the shell structure and described by a straight line only on the average. On the average AE decreases with neutron excess increasing, i.e. this indicates that the spin-isospin SU(4) symmetry and the related nonstatistical effects may be more clearly manifested’”) in neutronrich nuclei, for example in fission fragments. Experimentally the spin-isospin SU(4) restoration will be manifested as a decrease of the Sp(E) peaks widths. For correct calculations of the beta-delayed processes probabilities it is necessary to have experimental information and systematic on Sp(E) peaks width I.10,121
5.
Conclusion
For many fundamental and applied studies it is necessary to have enough complete nuclei decay schemes. The degree of the decay schemes incompleteness may be enough high at the excitation energies more than 2MeV3MeV in the medium and heavy nuclei. Only qualitative agreement between experimental fine structure and theoretical fine structure was obtained3).Theory predicts more absolute value of
510
strength than was experimentally observed. Beta decay strength and S,(E) fine structure measurements are important for symmetry of nuclear interaction study and proper analysis of delayed processes’”o*’2).The combination of TAGS spectroscopy and high resolution nuclear spectroscopy methods is very efficient for beta-decay strength fine structure measurements and for sufficiently complete decay schemes construction. The degree of the decay scheme completeness and energy regions where decay scheme is incomplete may be effectively estimated by comparison of the experimental TAS spectra with TAS spectra calculated from decay scheme and by comparison of the beta decay strength functions deduced from TAS spectra and decay scheme. For more complete decay scheme construction the combination of TAGS with high resolution gamma spectroscopy methods must be used. For using TAGS spectroscopy it is necessary to have both Z (element) and M (mass) separated sources.
References 1.
2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12.
13.
14. 15. 16.
17. 18.
Yu.V.Naumov et al. Physics of Particles and Nuclei. 14, 175 (1983) (in English, Translated by American Institute of Physics). G.D.Alkhazov et al. Nucl.Phys. A438,482 (1985). 1.N.Izosimov et al. Particles and Nuclei Letters, 4[101], 40 (2000). 1.N.Izosimov et al. Particles andNucleiLetters. 2[111], 36 (2002). 1.N.Izosimov et al. J.Phys. G24, 831 (1998). A.Algora et al. JYFL-I 77proposal. 2002. R.C.Greenwood et al. Nucl.Instr.hMeth. A390, 95 (1997). M.Karny et al. Nucl.Znstr.&Meth. B126, 41 1 (1997). Ph. Dessagne et al. IS370 ISOLDE and INTC-P-144 CERNproposals. 1.N.Izosimov Physics of Particles and Nuclei. 30, 131 (1999) (in English, Translated by American Institute of Physics). J.Wawryszczuk et al. Z.Phys. A357, 39 (1997). I.N. Izosimov and Yu.V. Naumov Bulletin of the Academy of Sciences of the USSR, Physical Series 42 ( l l ) , 25 (1978). (in English, Translated by American Institute of Physics,); I.N. Izosimov and Yu.V. Naumov Zmestya Akadernii Nauk SSSR, Seriya Fizicheskaya 42 ( l l ) , 2248 (1978). (in Russian). Ed. R.B.Firestone et al. Tables of Isotopes, Eighth Edition, (WileyInterscience, New York, 1996). T.Yoshida et al. Journal ofNuclScince and Technology. 36,135 (1999). H.V.Nguyen et al. Proc. Int. Con$ on Nucl. Data for Science and Technology, Italy, Trieste, p.835 (1997). 1.N.Izosimov et al. Physics ofAtomic Nuclei. 66(9), 1636 (2003). Yu.V. Gaponov and YuS. Lyutostanski, Sov. J.Nucl.Phys. 19,33 (1974). Yu.V. Gaponov and Yu.S. Lyutostanski, Sov.J.Part.Nuc1. 12,528 (1981).
511
BETA D E C A Y OF O D D - M A S S A S - G E ISOTOPES IN THE INTERACTING B O S O N - F E R M I O N M O D E L
L. ZUFFI Dipartimento di Fisica dell’Universith d i Milano and Istituto Nazionale d i Fisica Nucleare, Sezione d i Malano, Via Celoria 16, Milano 20133, Italy E-mail: [email protected]
N.YOSHIDA Faculty of Informatics, Kansai University, Takatsuki 569-1 095, Japan E-mail: [email protected] S . BRANT Department of Physics, Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia E-mail: [email protected] The structure and the beta decays of odd-mass As and Ge are studied in the interxting boson-fermion model. Reasonable agreement is obtained in the energies and moments. The calculated beta decays are stronger than the observed ones, indicating possible mixture of components outside the model space in actual nuclei.
1. Introduction
The study of beta decay of nuclei far from the stability line has two reasons of interest: (i) to test the model by the existing data because the decay rates are very sensitive to the wave functions, and (ii) to provide reliable information for astrophysics. The interacting boson-fermion model (IBFM)’ was applied t o the beta decay by Dellagiacoma and Iachello,’ and has been extendied t o wider region^.^^**^^^ In the present study, we analyze the beta decay from odd-mass As to Ge and other properties in the IBFM. 2. Calculations and R e s u l t s
The odd-mass isotopes are described by coupling a fermion to the even-even Ge core. The hamiltonian is H = HB+ HF+ VBF, where HBdescribes the
512
69As
71As 912
73As
-
....-
-:::
-
-.),,III-
,,,,
IBFM exp __ 71Ge
...' -
. . , __
IBFM
exp
73Ge 912
....... -. -._. :.
-
=:::
. .. ..
.... .. = __
-,.. ...'---
I::
0 512
Figure 1.
.-
-
, .. . -.........,
-
-.......-
312
-....... -
112
The energy levels of the negative-parity states in the As and Ge isotopes.
Ge core7 in the proton-neutron interacting boson model (IBM2).8 We ignore the intruder 0; states because they are outside of our model space. They are described in an extended IBM2 with s' and d' b o s ~ n s The . ~ fermion ~i ni. The single-particle energies taken hamiltonian is written as HF = from Ref. 10 are adjusted. The quasi-particle energies e j are used in HF. The boson-fermion interaction VBF is
xi
where p (p') denotes the like (unlike) particle of the odd fermion. For rij, I'ij and the same orbital dependence as in Ref. 1 has been assumed. The common factors I?, ,'?I A and A are varied. As seen in Fig. 1, reasonable agreement with the experiment'' is obtained.
hii,
513
. I
-1
I
AAS
N
Ln v
-
N
-2
Ln v
69
71 A
73
a
71
69
-
A
A
73
69
71
73
A
A
-1 -2
69
71
A
73
1 -
1 4
1 -
I N
Y31N
-IN
lOlN
-IN
t
1 mlN
t
I " -IN
t
t
t
I -
I -
1 -
I *
-IN
I # -IN
1 -
-IN
+IN
*IN
CIN
t
Figure 2. The calculated electromagnetic properties are compared with the experimental data. The symbol shows the experimental data while x shows the calculated value.
The electromagnetic transition operators are
We use eB = 0.0631 eb determined from B(E2; 0; + )2; in 70Ge. The strength ei,j is derived from e: = 1.5e (er = 0.5 e) for the proton (neutron). The boson g-factors are: g: = 0, 9," = 1 pp~.In ei:; the spin g-factors are quenched by 0.7. Some results are presented in Fig. 2. The beta-decay operators are written as
The last summation was introduced by Barea et al.," to reproduce correctly the matrix elements between the states with seniorities v = 2, 3. The factors K and K$ are determined from the expectation values of the particle numbers.' As seen in Fig. 3, the calculated log-ft values are appreciably
514
G 6 - 5 69
A
71
69
A
71
69
A
71
69
A
71
Figure 3. The log-ft values from As to Ge. The symbol shows the experimental value, while the symbol x (0)shows the calculated value without (with) the additional terms.
smaller than the data. The effects of the additional terms are small. The factor K i , which was introduced in order to compensate for the truncation of higher-order terms,13 may be reducing the effects of additional terms.
3. Conclusions We have studied the basic properties and the beta decay in odd-mass As and Ge nuclei. The energy levels and the electromagnetic moments reasonably agree with the experimental data. In beta decay, the calculation overestimates the decay rates. This may be due to the admixture of components based on intruder states in the even-even cores, that are outside our model space. The effects of the additional terms in the beta decay operator are small. References 1. F. Iachello and P. Van Isacker, The Interacting Boson-Fermion Model, (Cambridge Univ. Press, Cambridge, 1991). 2 . F. Dellagiacoma, Ph. D. thesis, Yale Univ., 1988; F. Dellagiacoma and F. Iachello, Phys. Lett. B218, 299 (1989). 3. G. Maino and L. Zuffi, in Proc. the 5th Int. Spring Seminar on Nucl. Phys., Ravello, 1995, ed. A. Covello (World Scientific, Singapore, 1996), p. 611. 4. N. Yoshida, L. Zuffi, and S. Brant, Phys. Rev. C66, 014306 (2002). 5. N. Yoshida, L. Zuffi, and A. Arima, Czech J. Phys. 52, suppl. C615 (2002). 6. L. Zuffi, S. Brant, and N. Yoshida, Phys. Rev. C68, 034308 (2003). 7. N. Yoshida and A. Arima, Phys. Lett. B164, 231 (1985). 8. F. Iachello and A. Arima, The Interacting Boson Model, (Cambridge Univ. Press, Cambridge, 1987). 9. P. D. Duval, D. Goutte, and M. Vergnes, Phys. Lett. B124, 297 (1983). 10. K. Langanke, D. J. Dean, and W. Nazarewicz, Nucl. Phys. A728,109 (2003). 11. M. R. Bhat and J . K. Tuli, Nucl. Data Sheets 90, 269 (2000); M. R. Bhat, ibid. 68, 579 (1993); Balraj Singh, ibid. 101, 193 (2004). 12. J. Barea, C. E. Alonso, and J. M. Arias, Phys. Rev. C65, 034328 (2002). 13. 0. Scholten, Prog. Part. Nucl. Phys. 14, 189 (1985).
515
ON ANALYSIS OF DATA OF RADIOACTIVE DECAYS UNDER CONDITIONS OF POOR STATISTICS AND SMALL OBSERVATION TIME
ZLOKAZOV V.B. FLNP, JINR, RUSSIA. E-mail: [email protected] The paper considers some aspects of the estimation of the exponential distribution parameter when the statistics of the sample and the observation interval are small.
1. P r o b l e m Let a set of radioactive atoms be given, which is equal to N at a time = 0. Events - decays of these atoms - are described by a random quantity E , subject to the distribution function
t
F'(( < t ) = 1 - e z p ( - t / T ) , t E [O,m),
(1)
Let us introduce an additional quantity n - the number of nuclei, which have decayed to the time t ; it will be described by a function of the multibinomial distribution F ( n = k , t ) , the parameter of which can be evaluated on the basis of (1). However, under very common, and, as a rule, holding conditions of radioactive decays, the function F will asymptotically tend to the Poisson one, the parameter of which also can be determined on the basis of (1): F ( n = k , t ) = (at)'/k! ezp(-at), k = 0,1, ..., 00, where at = N . (1 - e t ) , and et = e z p ( - t / T ) . As known [l],the quantity at is the mean (expectation) and the variance of the random quantity n at the same time. The Poisson distribution is more suitable for analytical operations. Let the interval of observation and registration of these decays be a time interval [0,B ] ,and Q = ( t l ,t z , ..., t m ) - a set of observed values of E in the interval [0, B] be given, i.e. a sample of time points from this interval, a t which the decays took place. We can introduce a quantity n - the number of events o f f in the sample Q. A specific feature of innovative experiments is the fact that often
516
the quantity B is much smaller than the quantity T . This diminishes the information volume of the data Q. The main problem of the analysis of samples of such events is the evaluation of parameters T and N . One can show that under the least favorable conditions of the experiment (poor statistics and/or small observation interval [O,B],B << T ) the chances of a successful solution of this problem are very small. First, let us find the mathematical expectation (mean) and the variance of the quantity [ and n. Omitting the details, we have
E[
=T -B
. e B / ( l - e B ) , (mean),
V< = T~ - B ~e g. / ( 1 - e g )
+ B 2 ( e g / ( l- e g ) ) 2 ,
(2) (variance).
(3)
It is obvious that En = v n = N . ( 1 - e g ) . If we consider the random quantity to be concentrated in the interval [0,B ] ,then the density of its distribution function is
<
e t / ( T .( 1 - e B ) ) if t E [O, B ] ; otherwise,
(4)
If B << T p ( t ) = e t / B . x p B ] ( t ) In . all the cases the likelihood function is m
j=1
and we can try to get the maximum likelihood estimates of the parameter T from maximum of (5) with respect to T . It is obvious, that the maximum of (5) is at T = cm,if, at least, one t j gets in the interval [O,B]. The distribution function of n on condition that the interval of the observation is [0,B ] ,does not depend explicitely on B:
P ( n = k) = ( a t ) k / k !e q ( - a t ) if t E [0,B] If the size of the sample Q for the time [O,B]is m, the equation of the maximum likelihood is as follows m/a - B = 0. jFrom this we get: ii =
m/B. One can try to use the moment estimator for the evaluation of T from (2) and (3). The mean of the sample Q will belong to [O,B]and will be strongly biased with respect to the true value T . The equation for the moment estimates is:
517
Already at T > 4B the function ( 6 ) of T is practically constant, and there are no chances again to find the root of ( 6 ) with the acceptable accuracy.
2. Estimate of lower bound We can proceed as follows. Let us denote the length of the observation interval as 2B, and introduce two random quantities: n1 and 122 - sums of registered decays in the intervals [0, B ] and [ B ,2B],respectively. It is obvious that
finl = N/(I - e B ) , En2 = N / ( e B - e2B) Let r = n1/n2. We have Er = e x p ( B / T ) . an estimator of T :
j h o m this one can build
? = B/ln(r).
(7)
The practical use of (7) is not successful in all the cases: the probability that n1 = 722 is not equal to zero, and it means that the mathematical expectation of (7) is not bounded. It is obvious that for the analysis only those samples are admissible which more or less look as exponential curves. For instance, we can make use of some criterion for testing the statistical significance of a n inequality 122 < n1, e.g. this one:
n1 > 722
+ k .u(n2).
(8)
where k is any number, and u - deviation function. For the Poisson distributed n2 we have u(n2) = &N(eB - e 2 ~ ) )and , using this from (8) we can derive the formula
N 2 k2 .e B / ( 1 -
eB)3,
(9)
and from it the restrictions on 0 0
the level of the statistics N at BIT given; the length of the observation interval B a t N given,
which provide for the success of the analysis of such data. Strictly speaking, the condition like (9) is necessary also for the above - considered estimation of T at the known N , since with a non zero-valued probability the sample can fail to contain any registered decays. But it is a theme of a special consideration [2]. If T >> B the formula 9) looks simpler: N 2 I c ~ ( ( T / B-) (~T / B ) 2 ) .
518
Thus, we see that for a successful estimation of the parameter T requirements on N and the ratio T I B are very severe. Here a n idea of an estimate of a lower parameter bound instead of parameter itself is very fruitful. The lower parameter bound is a quantity, which with a certain (calculable) probability is less than T , but greater than the length of the interval [0, B ] . In our case such an estimator can be obtained, e.g., from such a consideration: the estimator (7) is used only if the condition (8) with a given k holds. Such estimates of T are normally lower than the true parameter value since they are based only on data with a sufficiently stiff slope and will range between some minimal Tl and the true value of T. Just this Tl can be taken as the lower bound of the parameter T . Table 1. Test of the formulae (7 - (8)). A simulated series of decays in [0, Bo] subject to an exponential distribution P ( t ,To) 1.22 103.91 2.92 198 161 96.64 (Bo = 20, To = 200, N > 1050 was tested. For each sample (8) was checked 3.95 327 263 91.78 and, if satisfied, the estimate of TOwas 1.36 269.51 336 312 built with the help of (7). 2.89 136.46 389 336 The following Table 1 contains the re3.24 133.66 468 403 sults obtained. Average value of Test= 2.00 220.08 484 442 157, u = 163. The tests showed that if 3.64 139.43 644 558 B << T , and the statistics is small, the 5.26 96.44 641 521 1.81 284.59 practical chances to get a good estimate 664 619 3.81 141.96 of the parameter T are very small. 731 635 An alternative to this method is the regression analysis: building a distribution s ( t ) from the events t j , j = l , m and fitting it by the curve Nezp(-BIT), where N , T are parameters of interest. If the above conditions hold, this method gives very lowered estimates of the parameter T too. Besides, this method needs large statistics of the events t j .
3. Conclusion Summarizing one can say that the situation B << T is a bigger evil than the small statistics of data: even if the statistics is large, still the quality of the estimates of the parameter T will be very poor. In geology and other sciences often the estimation of half-lifes of the processes which last millions of years must use the apriori knowledge of the parameter No - some d u e
519 of N a t the time t = 0 and then if the observation time was B , which equals only to years one can get rather reliable estimate of T from
n = Noezp(-B/T) where n is the number of the events Tj in the interval (O,B]. Otherwise, the estimates of T will be completely unreliable.
References 1. S.S.Wilks. Mathematical statistics. Moscow,1967. (Translation from the English 1962 original.) 2. Zlokazov V.B. Particles & Letters, 2003, N2[117],Dubna, 2003.
520
COPLANAR TERNARY DECAY OF HYPER-DEFORMED NUCLEI OF MASS A=56* W. VON OERTZEN, B. GEBAUER, C. SCHULZ, S. THUMMERER, H. G. BOHLEN, TZ. KOKALOVA Hahn-Meitner-Institut-GmbH, Glienicker Strasse 100, 0-14109 Berlin, Germany C. BECK, M. ROUSSEAU, P. PAPKA Institut de Recherches Subatomiques. UMR7500, IN2P3-CNRS et Universitd Louis Pasteur, F-6703 7 Strasbourg Cedex 2, France
G. EFIMOV, D. KAh4ANIN Joint Institute for Nuclear Research, 141980 Dubna, Russia G. DE ANGELIS LNL, Legnaro. Italy Using a kinematic coincidence method the coplanarity of ternary fission events from %i compound nuclei formed in the 32S (164 MeV) + 24Mg reaction has been measured. Extremely narrow out-of-plane correlations are observed for two fragments emitted in either purely binary events or in events with a missing (ternary) mass consisting exclusively of a-particles. This observation is interpreted by a fission process through an elongated shape, where the lighter mass in the neck region remains at rest.
We have studied the fission events from the decay of the 56Ni compound nucleus at an excitation energy of E'm = 83.7 MeV formed in the 32S+24Mg reaction at Elab = 164 MeV. A previous study [ 1, 21 has been reported for the 60 Zn CN at the same excitation energy and a similar angular momentum range with the 36Ar+ 24Mgentrance channel. The system 32S+24Mghas been studied extensively by Sanders et al. [3], with the emphasis on the binary fission process. With the present result and those of Refs. [ 1, 21 we have observed the ternary fission decay of these nuclei, which competes with the binary fission due to the formation of hyper-deformed configurations. The present experiment was performed at the VIVITRON Tandem accelerator of the IReS (Strasbourg) with the BRS-EUROBALL setup [ l , 41 aimed at particle y-spectroscopy. Two heavy fragments are registered in *
This work is supported by the m i n i s t r y of research (BMBF) Germany under contract Nr. 06-OB-900
521
kinematical coincidence and identified by their charge (Z3, Z4) in two large area position sensitive Bragg-ionisation chambers [ 11. Correlations have been measured of two heavy ejectiles with respect to in-plane and out-of-plane scattering angles 6 and cp, respectively. For the inclusive detection of “binary” exit channels with two heavy fragments, but with a definite choice of the missing mass and charge, very broad distributions are expected in the 0 and cp correlations, because of missing informations on the momenta of the unobserved thud (and/or more) particles. Reaction channels are in our case defined by the sum of the observed charges of the fragments Ztotal = Z3 -I-Z4 with a well defined missing charge AZ = ZCN- (Z3+Z4) varying from 0 to 8. The coplanarity condition is defined by the sum (p3+’p4=18Oo (the reaction plane is spanned by the beam axis and the vectors of the two detected fragments). The out-ofplane correlation N(cp3-cp4) of the selected events is uniquely determined with the BRS-detector system over a very wide angular range in the reaction plane (A6 = 12’ - 46’, Acp = 2 ~17.4’).The fragment yields N(cp3-cp4) are plotted in Fig. 1 for different combinations of the charges and different missing charge AZ but for even total charge Ztotal.The coplanarity condition is well fulfilled for binary events in form of a narrow peak at 180°, no narrow correlation is observed for AZ = 2, as expected, the corresponding recoil widens the angular correlation (shown in the second column of Fig. l), however, we observe that a narrow correlation peak appears consistently for larger missing charges with AZ = 4 and 6. For the binary decay processes (AZ = 0) for different exit channels corresponding to a transfer of charges or different mass splits but with the same total charge (shown in the first column of Fig. l), the out-of-plane angular correlations are sharp, with a small broader component, which must result from neutron evaporation. The expectation that for a sequential emission of several charged particles from the highly excited fragments, the (cp3-cp4)-correlations have to reveal increasing width with larger charge loss AZ > 0, is only partially fulfilled with a broad component. However, for AZ = 4 (which corresponds to two missing a-particles) depicted in the third column, a very narrow component as sharp as in the binary cases, is observed together with a broad component. The latter is easily interpreted as resulting from uncorrelated sequential emission of typically two a-particles or several nucleons, from either of the two fragments. Surprisingly the pattern of narrow correlations continues to appear with the cases of three missing a-particles, where two components, a narrow peak (as in the case of a binary event) and a wider distribution (with increasing width for increasing missing mass), are observed.
522
Figure 1. Out-of-plane angular correlations for binary decay and the respective non binary emission channels with missing la, 2a, and 3a-channels in the reaction 32S (164 MeV) +24Mg.X-scale gives for each panel (p3-(p4 from 160" to 200". The numbers (like 10 + 12) indicate the charges of the both fragments.
On the other hand no such narrow peak appears for odd total charges. We can state that thls indicates that the missing particles created in the neck zone appear as multiples of a-clusters @Be, 12C*0f).Such behavior is predicted by the a-cluster model for the hyper-deformed %i at high angular momenta [ 5 ] . The narrow width components around cp3-cp4 = 180' can originate from different fission mechanism assuming either: i) a fission process after a fast emission of four, or even more, charges (plus neutrons), ii) a process where particles must be emitted correlated in-plane by a process involving two primary heavy ejectiles, iii) ternary fission with the missing mass from the neck (several a-particles) remaining at rest in the centerof-mass frame. This process produces a narrow (cp3-cp4)-correlation as in a standard binary decay, because the neck-particles carry no momentum in the centre of mass, and the emission angle (cp3+(p4) remains 180'. For thefirstpoint binary fission to occur after emission of a first particle, we find that the fission probability has decreased drastically with decreasing excitation energy, and no second chancefission can be expected. Indeed, no significant contribution from a narrow peak in the (cp3-cp4)-correlations can be observed for the fragment-fragment coincidences with one missing a-particle (Ztotal- 2) (see second column in Figure 2). For the second scenario involving a
523
correlated emission from the two fragments, the fact that the narrow correlations appear as strong for AZ = 4, 6 makes it rather unlikely that such a very special correlation persist through all decays. For the third scenario we conclude that in ternary fission with the third clustered fragment in the neck will consist of a-particles which possess zeromomentum in the center-of-mass frame. The particles from the neck are expected to travel with the center-of-mass velocity in the beam direction (towards 0'). A corresponding measurement, showing this phenomenon with one a-particle from the neck, in the decay of 28Siinto I2C + a + 12Chas been reported by Scheurer et al. [6]. Whereas the study of the 28Si + 24Mg deep-inelastic scattering [7], leading to the three-body final states such as a + 24Mg+ 24Mgand a + 2%e + 28 Si has shown that decay of unbound states of the fragments are involved, originating from a binary decay process. We can conclude that the observation of the very narrow coplanar fission fragment coincidences in the present 32S + 24Mgdata, in conjunction with the earlier work on the same phenomenon in Ref. [l], is a unique feature, whch gives clear evidence for the occurrence of ternary decay processes of a hyperdeformed '6Ni nucleus as predicted by Zhang et al. [5]. The competition of binary and ternary fission can be explained to occur for high angular momentum because of the large difference between the moments of inertia, giving the saddle points of almost equal height [8]. '
References 1. S. Thummerer et al., Nuovo Cimento 111 A (1998) 1077, and Dr.Thesis Freie Universitaet, Berlin, 1999. 2. V. Zherebchevsky, S. Thummerer, W. von Oertzen, D. Kamamin, et al., in preparation. 3. S.J. Sanders, A. Szanto de Toledo, and C. Beck, Phys. Rep. 311 (1999) 487; and references therein. 4. C . Beck et al., Nucl. Phys. A734 (2004) 453. 5. J. Zhang, A. C. Merchant, and W.D.M. Rae, Phys. Rev. C49 (1994) 562; see also W.D. M. Rae in Proc., 51h Intern. Conf on Clustering Aspects in Nuclear and Subnuclear Systems (1988), Kyoto, Prog. Theor. Phys. (Jap.) ed. K. Ikeda, 1989, p.80. 6 . J.N. Scheurer et al., Nuc2. Phys. A 319 (1979) 274. 7. A.H. Wuosmaa et al., Phys. Rev. C40 (1989) 173. 8. G. Royer, J.Phys. G 21 (1995) 249.
524
NUCLEAR MOMENT MEASUREMENTS OF SPIN-ALIGNED ISOMERIC FRAGMENTS
J. M. DAUGAS, G. BELIER, M. GIROD, H. GOUTTE, CEA/D IF/DP TA/SPN, B P 12, 91680 Bruykres le ChBtel, France
v. MEOT,
0 . ROIG
I. MATEA, G. GEORGIEV, M. LEWITOWICZ, F. D E OLIVEIRA SANTOS GANIL, B P 55027, 14076 Caen Cedex 5, France M. HASS, L. T. BABY, G. GOLDRING The Weissman Institute, Rehovot, Isorel G. NEYENS, D. BORREMANS, P. HIMPE Ih’S, KULeuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium R. ASTABATYAN, S. LUKYANOV, YU. E. PENIONZHKEVICH FLNR- JINR, Dubna, Russia D. L. BALABANSKI Faculty of Physics, St. ICliment Ohridski University of So$., Bulgaria
1164 Sofia,
M. SAWICKA IFD, Warsaw University, Hoia 69,00681 Warsaw, Poland
Electromagnetic moments measurement of isomeric states produced and spinoriented in projectile fragmentation reactions a t intermediate energies have been performed using the Time Dependent Perturbed Angular Distribution (TDPAD) method. This allows the study of neutron-rich nuclei unaccessible by other kind of reaction. An important experimental achievement is presented.
525 1. Introduction
Magnetic moments are very sensitive probes to the detailed composition of the nuclear wave function. Measurement of the g-factor of a nuclear state provides unique information on its single particle structure and is a test ground for nuclear models. Due to its dependence of the spin and orbital angular momentum of the involved valence nucleons, magnetic moment is a rigorous probes for the spin and parity assignment of the nuclear states. It is especially interesting to test nuclear models for nuclei far from stability in the vicinity of a spherical shell closure, where the nuclear wave functions are expected to be rather pure, and in regions where intruder states are expected, reflecting a large deformation. One current region of interest is the neutron rich species around N = 40 and Z < 28. We focus here on the role played by the ~ g 9 / 2orbital in the low-energy level structure of nuclei below N = 40 in particular case where this orbital manifests itself as an isomeric state. We repport here an important experimental achievement in the study of magnetic moments of isomeric states produced by projectile fragmentation reactions at intermediate energy. 2. Experimental detail 2.1. The TDPAD-method Magnetic moment measurements have been performed using the Time Differential Perturbed Angular Distribution (TDPAD) method in combination with ion-y correlations. The isomers are stopped in a perurbation free environment by choosing an appropriate stopper material that has cubic lattice structure. The implantation foil is placed between the poles of an electromagnet delivering an external magnetic field B causes a Larmor precession of the initially aligned isomeric nuclear spins with a frequency
where g is the nuclear gyromagnetic factor; ,LLN the nuclear magneton and ti Plank's constant. The monitoring of y radiations is performed using 4 high-purity Ge detectors. For each Ge detector, energy and time signals are stored in an event by event mode. y-ray time spectra are started by the ion implantation time, and stopped by the detection of a delayed yray. To extract the precession pattern out of the individual time spectra, detectors at 90' with respect to each other are combined to generate the
526
R(t)-function R(t) =
+ + 4,
Il(e, t ) - E I ~ ( ~,; t ) Il(e,t) e ~ ~ ( e t )
+
+
A ~ B ; (= ~ 0 ) ~ ~ ~ ( 2 -( e)) ~ ~ t(2)
where I1 and I 2 are the summed intensities of detectors placed at 180'; E the relative efficiency between detectors; A2 the radiation parameter of the transition; B! the second component of the orientation tensor describing the initial alignment; 0 the angle between the beam axis and the first detector and a the angle between the beam and the symmetry axis of the spinaligned ensemble 2 :
where €Jcis the rotation angle in the spectrometer; A the mass of the implanted ion and 2 its charge. 2.2. Secondary isomeric beam production
In experiments described below we aim to measure the g-factor of neutronrich exotic isomers. The TDPAD is a very powerful methods which has been widely used in fusion-evaporation reactions '. This kind of reaction does not allow the production of the request nuclei. The neutron-rich part of the nuclides chart can be reach using projectile fragmentation reaction. The first application of the TDPAD method on isomeric states produced in high-energy projectile fragmentation has been performed on the case of 4 3 m 3, S~ where a significant amount of alignment has been observed. Application of the TDPAD method to measure the g-factor of isomeric states of neutron-rich nuclei produced in intermediate energies projectile fragmentation was performed at GANIL using the LISE spectrometer. 2.3. Pioneering experiment
The pioneering experiment 4,5 in which the TDPAD technique was applied on an unknown case after projectile fragmentation at intermediate energies was performed. The aim of this experiment was to measure g-factors of 6 9 m Cand ~ 67mN i . Spin-alignment and isomeric ratio calculations as function of the momentum distribution have been performed in the framework Thus, measurements have been of a kinematical fragmentation model performed in the outer wing of the momentum distribution. Dispite of the expected one, a low amount of alignment was observed, 2.6(10)% and ~ 67mNi respectively. The extracted absolute value 1.7(5)% for 6 g m Cand 6t7,8.
527 XY
b &tcccor
scwsdary hain
50 pin plastic scintillabr
Figure 1.
Schematic drawing of the experimental setup.
for the g-factors are 0.225(25) and 0.125(6). The g-factor of the 6 9 m Cis~in agreement with 13/2+ spin whereas the 67mNidiffers from 9/2+ g-factors in this region which are around -0.27. A new measurement is planned. It has been found out that the low amount of alignment is mainly due to electron pick-up and high implantation frequency generating non-correlated events, and no measurement of a known g-factor for validation and calibration of the method was performed. In the forthcoming experiment, a significant achievement of the experimental procedure was done '. In the improved experiment we aim to measure the g-factor of the 61mFe ( E = 861 keV, I = (9/2+), tllz = 250(10) n s ) . 3. Improved experiment 3.1. Experimental set-up
The experimental set-up is in figure 1. Fragments identification was performed using a removable Si-detector. A 50 pm plastic scintillator was used as t = 0 signal for the time-decay curves. This allows to have a significant decrease of the electron pick-up of the ions passing through it. We estimated the pick-up less than 2 %, instead of about 60 %. The implantation foil was surrounded by 4 high purity single crystal Ge detectors. The delayed y-rays coming from implanted isomers have been registered within a time window of 3 ps. In order to reduce random coincidences we have used a package suppresser (P.s.) allowing 1out of 10 packages provided by the GANIL accelerator. The implantation frequency was then 950 ns. 3.2. g - factor measurements
The measurement of a known case, the 54mFe (E = 6527 k e V , I = l o + , l o , have been performed. We have extracted the value of the applied magnetic field B = 0.680(4) T including systematical errors:
g = +0.7281(10))
528
Figure 2 . The left picture represents the R ( t )function for 5 4 m F e and the right one the comparison between measured and calculated alignment, theoritical curve is scaled by a factor of 1.8.
distribution of the magnetic field over the beam spot, its paramagnetic amplification and Knight shift. In the figure 2 are shown the R(t)function (left) and the comparison between the experimental and calculated spinalignment (right) of the 54mFe. The measurement of the ‘ l m F e is presented in figure 3. A g-factor value of -0.229(2) l 1 was extracted. Error on the fitting procedure includes errors on the effective field. The alignment was +6.2(7)% in the center and -15.9(8)% in the outer wing of the momentum distribution. The opposite phase of the R(t) function for the 2 different 7-rays indicates different multipolarities, reflected a level sequence of 9/2+ -+5/2- -+ 3/2-. 4. Summary
Magnetic moments of neutron-rich nuclei have been measured using projectile fragmentation reactions. The large spin-alignment shows that fragmentation provides a powerful tool for measuring gyromagnetic factors and quadrupole moments in neutron-rich exotic nuclei using the TDPAD method. This allows investigation of nuclear structure away from stability. The measured g-factor for ‘ l m F e is in agreement with the assigned spin and parity of the isomeric state. Large Scale Shell Model and HFB calculations have been performed and both indicates that this state is characterised by a deformed potential. The next step consists to measure the quadrupole moment. Acknowledgments
This work has been partially supported by the Access to Large Scale Facility program under contract nr. HPRI-CT-1999-00019, the INTAS project nr.
529
I
Figure 3. In the top are shown the R ( t ) function for the 207 k e V y-ray f o r different selections in momentum distribution. Below are the same pictures for the 654 k e V y-ray. In the bottom is the comparison between measured and calculated alignment with and without packages suppression, theoritical curve is scaled by a factor of 1.8.
00-0463 and the IUAP project P5/07 of the BSPO. We are grateful to the IN2PS/EPSRC French/UK loan pool for providing the Ge detectors. The Weissman Institute group was supported by the Israel Science Foundation. G.N. and D.B. acknowledge the FWO-Vlaanderen. References 1. G. Goldring and M. Hass, Treaties in Heavy Ion Sciences, D. E. Bromley ed. Plenum Press, Vol. 3, p.539. 2. G. Neyens et d., Nucl. Instr. and Meth. A340, 555 (1994). 3. W. D. Schmidt-Ott et d.,Z. Phys. A350, 215 (1994). 4. G. Georgiev et al., Journ. of Physics G 2 8 , 2993 (2002). 5. G. Neyens et ~ l . Nucl. , Phys. A701, 403 (2002). 6. K. Asahi et d.,Phys. Rev. C43, 456 (1991). 7. H. Okuno et d.,Phys. Lett. B335, 29 (1994). 8. J. M. Daugas et d.,Phys. Rev. C63, 064609 (2001). 9. G. Georgiev et d.,A I P Conf. Proc. 701, 169 (2004). 10. M. H. Rafailovich et ~ l . Phys. , Rev. C27,602 (1983). 11. I. Matea et d.,Phys. Rev. Lett., in print (2004).
530
UNEXPECTED RAPID VARIATIONS IN ODD-EVEN LEVEL STAGGERING IN GAMMA-VIBRATIONAL BANDS
E. F. J O N E S ~P. , M. GORE^, J. H. HA MILT ON^, A. v. RAMAYYA~,x. Q. Z H A N G ~J. , K. H W A N G ~Y. , x. L U O ~ ,J.~ KORMICKI~, , K. L I ~ s. , J. Z H U ~ , W . C. MA3, I. Y. LEE4, J. 0. RASMUSSEN4, P. FALLON4, M. STOYER5, J. D. COLE', A. V. DANIEL7, G. M. TER-AKOPIAN7, YU. TS. OGANESSIAN7, R. DONANGELO', AND 3. B. GUPTA' Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA 2Physics Dept., Tsinghua University, Beijing 100084, Peoples Republic of China Department of Physics, Mississippi State University, Mississippi 39762, USA Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Lawrence Livermore National Laboratory, Livermore, California 94550, USA 61daho National Laboratory, Idaho Falls, Idaho 83415, USA Flerov Laboratory for Nuclear Reactions, J I N R , Dubna, Russia Universidade Federal do Rio de Janeiro, C P 68528, R G Brazil Ramjas College, University of Delhi, Delhi 11 0 007, India
'
The even-odd spin energy-level splittings, e.g. A E = E3+ - E2+, E4+ - E3+, ..., show striking and rapid variations with spin in 1 0 4 - 1 0 8 M ~108-11zRu , , and 112-116Pd. The A E values for known y-vibrational bands in even-even 152,154Sm, 152-160~d 1 5 4 - 1 6 6 ~ ~1, 5 6 - 1 7 0 ~ ~162-174yb 1 6 8 - 1 8 0 ~ f 1 7 0 - 1 8 6 ~ 1 7 2 - - 1 9 2 0 ~ and 1s0-196Ptwere calculated. In general, Sm, Gd, Dy, and E r exhibited little staggering in AE, with differences in A E 40 keV up t o spin lo+, except for N = 88 152Gd,154Dy,Is8Er, where the differences are about 200 keV. In 162,166Yb,the differences in adjacent A E values above 9+ reach 200 and 640 keV, respectively. For 180-186Pt,there is a strong decreasing oscillation to the maximum known spin 7+. In 18s-196Pt, only in lg2Ptare the levels known above 6+ and staggering sets in at 5+ with an increasing difference to 220 keV at 8+. In all the Sm to P t nuclei, only between 7+ and 12+ in 170Er compared to 162!164Eris there a reversal in AE as found in the Mo, Ru, and P d nuclei.
<.
1. Introduction
Gupta and Kavathekar [l]have recently investigated the systematics of the KX = 2+ y-vibrational bands and odd-even staggering from Ba to Pt nuclei. They have analyzed the data within the framework of the unified
531
Bohr-Mottelson collective model, where it represents the axial- symmetrybreaking quadrupole vibration, the rigid triaxial rotor model of Davydov and Fillipov, the Wilets and Jean model, and the interacting boson model. They conclude that "the sign of the odd-even energy staggering (OES) index in the y bands distinguishes between the rigid triaxial rotor shape and the y-soft vibrator or the O(6) symmetry. Its absolute magnitude indicates the degree of deviation from an axial rotor. The coefficient of the OES term falls sharply towards the midshell. This OES index S(4) is large for the shapetransitional nuclei and is much reduced for well-deformed nuclei. The OES is related to the split multiplets of the anharmonic vibrator, wherein the 3+, 4+ states belong to the n = 3 quintuplet. Hence, these states lie closer as compared t o the 2+ state which belongs to the n = 2 phonon triplet. The same is true for the (5+, 6+) and (7+, 8+) in the higher n-phonon multiplets [l]."We have analyzed the y-vibrational bands in Sm to Pt nuclei and come to similar conclusions. In our experimental studies, y-vibrational bands in 104-108M0, 108-112R~, and '12-'16Pd have been extended to considerably higher spins by using y-y-ycoincidence data ( 5 . 7 ~ 1 triples 0 ~ ~ and higher folds) from the spontaneous fission of 252 Cf taken at Gammasphere. Experimental details are found elsewhere [2]. The y-bands are seen to 13+ in 104-106M09 t 0 17+ in 'l2Ru, and to 15+ in 'I4Pd. Several papers have reported evidence for triaxial shapes in the neutron-rich Mo and Ru isotopes [2-51. Also in lo6Mo,low-lying one- and two-phonon y-vibrational bands are reported t o indicate softness with respect to triaxial deformation [6]. In lo4M0to 12+, the odd spin is pushed up closer to the even spin with 106i108M~ having little staggering. There is a reversal in patterns between lo8Ru, '12Pd and '12Ru, '14Pd, with the differences in adjacent AE values increasing to a difference of 570 and 480 keV in '"Ru and '14Pd, respectively.
2. Results and Analysis
For the Mo, Ru, and P d nuclides studied in this work, the minimum E(2+g) is 171.8 keV for lo6Mo,where deformation is a maximum. The E(2+g)values rise from there and the deformation decreases with N and Z through Ru and Pd. The minimum E(2+y) is 523.6 keV for l12Ru. As an example of our high-statistics data, Figure 1 shows a double gate on two high-spin transitions in the '12Ru y band, 13+ to 11+ and 11+ to 9+. In this spectrum, one can see other y-band transitions and lower-lying transitions of the ground band as well as transitions in the Xenon fission partners. Ex-
532
13-11
11-9
'l*Ru y-band 5-3
r n ~
107,s
I I
3-2
7-5
::
124.a'i
1 j/
Figure 1. Spectrum double-gated on the 13+ to 11+ and 11+ to 9+ gamma-band transitions in '12Ru.
cept where noted, the transitions shown are in the y band. We see the lower-lying 3+ t o 2+, 5+ to 3+, 7+ to 5+, and 9+ t o 7+ transitions, and the higher-lying 15+ to 13+ and 17+ to 15+ transitions. The transitions of 756.3 keV and above are added in our work. Note the odd-spin level energies are closer to the next higher even-spin level than to the next lower even-spin level. Now look at the systematics of the y-vibrational bands in Mo, Ru, and P d nuclei. In lo4Mo (see Figure 2a), we see a clear staggering pattern where the odd-spin members are pushed up compared t o the even-spin members until the last transition. In losMo AE increases smoothly until the 14+ to 13' transition. Here, simple differences in level energies are plotted, e.g. AE = E3+ - E2+, E4+ - E3+,.... One can see a clear difference between lo4Mo and lo6Mo, with lo4Mo showing a marked staggering to spin 12+. One can see a small staggering at low spins in losMo but the staggering smoothes out at higher spins until spin 13+. In "*Mo, AE shows a jump in the 5+ t o 4+ value compared t o the 4+ to 3+ one, but then smoothly increases to 8+ as in "'Mo. In losRu (see Figure 2b) the odd-spin members are pushed up compared to the even-spin members in a staggering pattern similar to lo4M0. In "'Ru the first two AE values stagger like lo8Ru then smoothly increase to l o+, as in losMo. But above 10+ there is the opposite staggering t o 1 0 8 R ~In .
533
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Spin of Lower Energy Level
Figure 2.
Level-energy differences in (a) 104,106,108M 0, (b) 108,110,11zRu, and ( c )
112,114,116pd,
'12Ru, AE starts out smoothly increasing but then above 5+ exhibits an opposite staggering to losRu with the even-spin levels pushed up to the odd-spin levels. This is the largest staggering seen in these nuclei. In going from lo8Ru to '12Ru, we see a clearly changing pattern that is not easily reproduced within any theoretical model. In 1996, Troltenier, et al. [4], based on the low spin data, calculated that these Ru isotopes would have
534
the same pattern as each other - that the even-spin levels would always be pushed up to the odd-spin levels as a characteristic of a triaxial nucleus and that is not what we find in our extended y-band data. In our data, '"Ru has staggering that follows the calculations, but lo8Ru does not. The y band in '12Pd (see Figure 2c.) has A E staggering like that of lo8Ru. In the '14Pd y band A E starts out with similar staggering t o '12Pd, lo4Mo, and lo8Ru, but a t 6+ reverses the pattern and has a staggering as in '12Ru to its highest spin 14+, where the even-spin energy levels are pushed up with respect to the odd-spin levels. The '16Pd energy differences are smoothly increasing to 7+ followed by some staggering. Note lo4Mo, lo8Ru, and '12Pd, which are separated by an a particle, have the same staggering pattern. However, "'Ru and 'l4Pd, which have similar but the reverse pattern from lo4Mo to '12Pd, are separated by only two protons. These rapid changes that we see in the AE patterns as you go across these Mo, Ru, and P d nuclei clearly show the influence of triaxial shapes, but they also call for a more microscopic description of the interplay between triaxiality and prolate deformation in this region. To better understand the behavior of y-vibrational bands, the AE values for known y-vibrational bands in even-even 152,154Sm,152-160Gd, 154-166~ 1 5 6 - 1 7 0 ~ ~ 162-174yb 1 6 8 - 1 8 0 ~ f 1 7 0 - 1 8 6 ~ 172-1920s a Y? Y 7 7 7 7 nd 180-196Ptwere calculated. In general, Sm, Gd, Dy, and Er exhibited little staggering in AE, with differences in AE 5 40 keV, up to spin 10+ except for N = 88 152Gd, 154Dy,188Er, where the differences are about 200 keV. Note these nuclei are outside the region where sudden deformation sets in at N = 90 (see Figure 3). At higher spins, differences up to 100 keV are seen in other isotopes of Sm, Gd, Dy, and Er. Only in 162i166Yband 176Hfare y bands known above 6+ and in 162,166Ybthe differences in adjacent AE values reach 200 and 640 keV, respectively. Only in Ig2Osbetween 5+ and 8+ are significant differences seen. For 180-186Pt,there is a strong oscillation t o the maximum known spin 7+ with a reverse pattern where AE goes from 200 - 300 keV t o 100 - 200 keV as the spin increases. In 188-196Pt, only lg2Pt levels are known above 6+ and staggering sets in at 5+ with AE increasing to 220 keV. These data show the smooth behavior noted by Gupta and Kavathekar [l]with very little staggering in well-deformed nuclei and more pronounced staggering outside those nuclei. Only in 17'Er compared to 1623164Eris there a reversal in the staggering pattern like we observed in the Ru and P d nuclei. Work at VU, MSU, INL, LBNL, and LLNL is supported by DOE Grants and Contracts DE-FG05-88ER40407, DE-FG05-95ER40939, DE-
535
. I
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Figure 3.
Level energy differences in 154--166Dy and 156--170Er.
AC07-99ID13727, W-7405ENG48, and DE-AC03-76SF00098 and work at JINR by the U.S. DOE Contract DE-AC011-00NN4125, BBWl Grant 3498 (CRDF Grant RPO-10301-INEEL) and by the joint RFBR-DFG Grant [RFBR Grant p2-02-04004, DFG Grant 436RUS 113/673/0-1(R)]. Work a t Tsinghua was supported by the Major State Basic Research Development Program Contract G2000077405, the National Natural Science Foundation of China Grant 10375032, and the Special Program of Higher Education Science Foundation. Grant 200300090. References 1. 2. 3. 4. 5. 6.
J. B. Gupta and A. K. Kavathekar, Prumuna - J. Phys. 61, 167 (2003). Y. X. Luo et al., Phys. Rev. C69, 024315 (2004). A. G. Smith et al., Phys. Rev. Lett. 77,1711 (1996). D. Troltenier et al., Nucl. Phys. A601, 56 (1996). H. Hua et al., Phys. Rev. C69, 014317 (2004). A. Guessous et al., Phys. Rev. Lett. 7 5 , 2280 (1995).
536
MATTER RADII OF PROTON RICH GA, GE, AS, SE AND BR NUCLEI
A. LEPINE-SZILY, G. F. LIMA AND R. LICHTENTHALER Instituto de Fl'sica- Universidade de S i o Paulo, C.P. 66318, 05315-970 5'60 Paulo, Brazil E-mail: alinkaOaf.usp.br
A. C. C. VILLARI AND W. MITTIG Grand Accelerateur National d 'Ions Lourds GANIL, Boulevard Henry Becquerel, BP 5027, 14021 Caen Cedex, fiance, M. CHARTIER University of Liverpoo1,Department of Physics, Liverpool, L69 7ZE, UK
N. A. ORFt LPC, IN2P3-CNRS, ISMRA et Universit de Caen 14050, Caen Cedex, h n c e Proton-rich isotopes of Ga, Ge, As, Se and Br had their total reaction cross sections measured. Root-mean-squared matter radii were determined from Glauber model calculations, which reproduced the experimental CTRvalues. For all isotopic series a decrease of the r T m s with increasing neutron number was observed. A clear correlation with deformation was also observed for Ga, Ge, As and Se istopic chains.
1. Introduction
The discovery of extended neutron distributions in light nuclei, close to the neutron drip-line, as e. g. 'lLi and l1Be, also called neutron halo' is one of the most interesting recent results in nuclear physics. This phenomenon was first observed in the interaction radii obtained from reaction cross section measurements by Tanihata and collaborators 2 . With new radioactive beam facilities the quest for halos and skins of neutrons and also of protons has gained much evidence. The comparison of charge and matter radii of the Na isotopic chain by Suzuki and collaborators has allowed the observation of neutron skin increasing with isospin3. Proton halos are expected to be
537 less pronounced due to the Coulomb barrier. The first observed proton halo was for the 'B proton drip-line nucleus4. We have recently measured the root-mean-squared matter radii of proton-rich isotopes of Ga, Ge, As, Se and Br5. In this paper we compare the matter radii with nuclear structure informations about these nuclei and also with existing charge radius values of stable isotopes. 2. Experimental set-up
The radioactive ions were produced at the Grand Accklkrateur National d'Ions Lourds (GANIL), Caen, France, through the fragmentation of a 73 A.MeV primary beam of 78Kr, hitting a 90 mg/cm2 thick natNi target, located between the two superconductor solenoids of the SISSI device. Details of the experiment were described in recent paper5. After the selection of the reaction products by the a-shaped spectrometer, they were driven to the high-resolution energy-loss magnetic spectrometer SPEG. They were detected in the focal plane of SPEG by a cooled silicon telescope formed by three transmission (4E) detectors followed by a thick Si(Li) detector where all ions of interest were stopped. The reaction target was the whole Si telescope behind the thin AE1 detector used for identification purpose. We used two methods in our measurement: one based only on reactions in the thin 4Ez detector at a well defined energy Eo,thus allowing the determination of the reaction cross-section at this energy. The other is based on reactions in the whole target/telescope system until the complete stopping. In this case the energy integrated average reaction cross-section is determined. 3. Data analysis
3.1. Reaction cross sections and reduced radii
The reaction cross section was obtained from the ratio between the number of events in the low energy tail of the energy spectrum (corresponding to events undergone nuclear reaction) divided by the total number of events. A phenomenological formula developed by Kox' relates the reaction crosssection U R with a reduced strong absorption radius T O . For stable nuclei the formula gives a good description of a wide variety of target and projectile systems at different energies with a constant value of ro=l.lfm. We have deduced two independent sets of values for T O respectively from reactions in the thin 4E detector, and from the whole target/telescope
538 system. The agreement between them is good within the uncertainties, indicating that the Kox method is also adequate for the radioactive nuclei. Then we inverted the problem and used the Kox formula to obtain reaction cross-sections at energy EOfrom the average values of the reduced strong absorption radii. 3 . 2 . Glauber model calculations
The optical limit of the Glauber model was used to deduce matter distributions from the measured reaction cross-sections. Two-parameter Fermi type density distributions were used for protons and neutrons, assuming them as dimensionless points, and the elementary N-N cross-sections from the literature. The point proton distributions can be deduced from measured charge distributions deconvoluting the proton size. For the stable N=Z 28Sitarget nucleus we have assumed that the proton and neutron distributions are equal, and known from the measured charge distribution7. Unfortunately, the stable nuclides of the isotopic chains of our interest were observed with very low statistics in this experiment, due to production cross-sections and to the purification method used. Only one (70Ge)of the proton-rich radioactive projectiles of this work had the charge distribution measured previously and we had to make several assumptions to infer the proton and neutron distributions. 3.2.1. Proton and neutron distributions
Thus, our measured quantities are the reaction cross-sections and the proton distribution of the stable isotopes not observed in our measurement. We cannot expect to determine unambiguously the half-density radii and diffuseness ( R p lup, R, and a,) of both, proton and neutron distributions from the reaction cross-section and the information on stable isotopes. We adopted a procedure3, in which two different assumptions were applied to allow the extraction of the density distributions. The assumptions were: a) The half-density proton radii for all members of a given isotopical chain were constant and the half-density neutron radii increase with N1I3 and the T, = 0 or 1/2 nuclei had the same neutron and proton half-density radii: After fixing these criteria for the half-density proton and neutron radii, the proton and neutron diffusenesses up and a,, were free parameters, in order to reproduce the measured reaction cross-sections through the Glauber model calculations. b) we fixed as equal the diffusenesses of the proton and neutron distributions, using the systematics or the mea-
539 surement of stable isotopes, and varied Rp and R, independently, in order to reproduce the reaction cross-sections. We included our Glauber theory calculation into a search routine, where the parameters were varied between given limits and the reaction crosssection was calculated for every ensemble of parameters. We have performed many searches and calculated the average of all r.m.s. matter radii for each projectile nucleus. We could reproduce the reaction cross-sections with several, fairly different proton or neutron distributions and r.m.s. proton and neutron radii. However, the r.m.s. matter radii, which were calculated from these different distributions using a simple averaging formula3 < TL >= ( Z I A ) < T ; > + ( N / A ) < T: > were very similar, the difference being always less than the uncertainties. The uncertainties in the r.m.s. matter radii were scaled by the uncertainties of the total reaction cross-sections, adopting the same relative errors for both quantities.
4. Discussion of results
We present on Fig.1 and 2 our results for the Ga, Ge, As and Se, Br isotopic series as a function of the neutron number N . The r.m.s. matter radii we obtain from the Glauber calculations are calculated from point distributions and before comparing them with measured r.m.s. charge distributions they should be folded with the nucleon matter distributions. However they can be directly compared to the r.m.s point proton radii (assuming the proton as a point particle). We also include in Fig.1 and 2 the r.m.s proton radii of the stable isotopes of the cited isotopic chains, as well as the r.m.s. charge radii for the Kr isotopes8. We calculate the r.m.s. point proton radii from the measured r.m.s. charge radii, by using the formula < r: >=< r,"h > - < r:hp >, where we use the more recent value for the r.m.s. charge radius of the proton7 < r:hp >1/2=0.8791(88)fm. We also show on these figures, presented by dotted lines, the values of the nuclear radius given by R = 0.95A1I3. A decreasing tendency of the radii with increasing neutron number N can be observed for most of isotopic chains, inverse to the expected behaviour . In Fig.1 and 2 we also compare the N dependence of the radius and the deformation. We use as a relevant parameter the excitation energy of the first 2+ state for even-even nuclei, or the excitation energy of the first excited state with J = Jgs 2 for odd-even nuclei. It is well known that, the higher this excitation energy, the less collective or the less deformed is
+
540
Figure 1. Lower pane1:The r.m.s. matter radii (full squares) of the Ga, Ge, As isotopes as a function of the neutron number N . We also show the r.m.s. proton radii (stars) of the stable isotopes. Upper panel: the excitation energies of the first 2+ or J = J9. + 2 state as a function of N .
the nucleus. The Kr r.m.s. charge radii present a minimum at the magic number N = 50, where the excitation energy presents a strong peak. The increase in radius with decreasing N between N = 50 and 40 is correlated with the deformation, the maximum of deformation occurring for N = 40. For N 2 5 0 the charge radii increase in the same rate as All3. Similar behaviour can be observed for the Ga, Ge, As and Se chains, with minima in the r.m.s. radii respectively at N=36, 37, 38, where the excitation energies present a strong peak. The comparison with the r.m.s. proton radii of the stable isotopes is useful to visualize the existence of minima. The proton radii seem slightly higher than the r.m.s. matter radii, a feature also observed for the Ar isotopic chaing. 5 . Conclusion
Thus we observe a clear correlation of r.m.s. matter radii with deformation for the Ga, Ge, As and Se isotopic chains. For N values where the nuclei are less deformed the radii present clear minima. For the Br chain the error bars are too large and any conclusion is difficult. For N 536-38,
541 1.2
~
,
1.6,
+-
4.44
Figure 2. Lower pane1:The r.m.s. matter radii (full squares) of the Se, Br isotopes as a function of the neutron number N . For the Kr isotopes the r.m.s. charge radii were plotted for comparison. Note the difference in scale. We also show the r.m.s. proton radii (stars) of the stable isotopes. Upper panel: the excitation energies of the first 2" or J = J,, 2 state as a function of N .
+
we observe a4 increase in the matter radii for all isotopic chains as the neutron number decreases. This increase in the radii is not correlated with deformation since the excitation energy increases (Se) or remains quite constant (Ga,Ge). This surprising behaviour is still to be explained. References 1. P. G. Hansen, A . S. Jensen and B. Jonson,Ann. Rev. Nucl. Part. Sci.45, 505 (1995). 2 . I. Tanihata e t al., Phys. Lett. B 1 6 0 , 380 (1985). 3. T. Suzuki et al., Phys. Rev. Lett. 7 5 , 3241 (1995). 4. W. Schwab e t al., 2. Phys. A 3 5 0 , 283 (1995). 5. G.F.Lima et al., Nucl. Phys. A 7 3 5 , 303 (2004). 6 . S. Kox e t al., Phys. Rev. (335, 1678 (1987). 7. I. Angeli At. Data Nucl. Data Tabl. 87, 185 (2004). 8 . M. Keim e t al., Nucl. Phys. A 5 8 6 , 219 (1995). 9. A. Ozawa e t al., Nucl. Phys. A 7 0 9 , 6 0 (2002).
542
SHELL MODEL TREATMENT OF NEUTRON-RICH NUCLEI NEAR 7 8 ~ 1 *
A. F. LISETSKIY AND B. A. BROWN National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824 E-mail: [email protected] M. HOROI
Physics Department, Central Michigan University, Mount Pleasant, Michigan 48859 E-mail: [email protected] H. GRAWE Gesellschaft f i r Schwerionenforschung mbH, 0-64291 Darmstadt, Germany E-mail: H. [email protected]
The shell model predictions for spectra and transition rates of even 68-76Ni isospace topes with newly derived effective interaction for the fs/zp3/2~1/2g9/2mode1 are presented. New results indicate the differences in the structure of 68-78Ni isotopes and corresponding valence mirror symmetry partners, A= 90 - 98 N= 50, isotones. The issues of the magicity of 6sNi nucleus, disappearance mechanism of seniority isomers in 72,74Niisotopes and signatures of collectivity are discussed.
1. Introduction Atomic nuclei a t the upper part of the pf-shell with mass number A = 58 - 98 exhibit a variety of interesting phenomena that are in the focus of modern nuclear physics and astrophysics. Properties of exotic neutronrich nuclei in a vicinity of doubly-magic ZtNi50 is one of the challenging problems 1,2,3,4,5,6,7. The enormous interest in this region is motivated by several factors. The primary issue questions the doubly magic nature of EtNi as compared *This work is supported by the NSF grant PHY-0244453
543
t o loOSn,for example. Since the shell-model orbitals for neutrons in nuclei with 2 = 28 and N=28-50 (56Ni-78Ni)are the same as those for protons in nuclei with N=50 and Z=28-50 (78Ni-100Sn),the comparison of these two groups of nuclei within the concept of valence-mirror symmetry (VMS) promises t o be very helpful. The astrophysical importance of the region is related t o the nuclear mechanism of the rapid capture of neutrons by seed nuclei through the rprocess '. The path of this reaction network is expected in neutron-rich nuclei for which there is little experimental data, and the precise trajectory is dictated by the details of the shell structure far from stability. Last decade of the great advance of the experimental investigations requires also reliable theoretical shell model studies of the neutron-rich nuclei in the region near XiNi. the most important orbitals for neutrons in the region of 68Ni t o 78Ni are ~ 3 1 2 ,f5/2, pll2 and g9/2 (referred t o from now on as the pf5/2g9/2 model space). Recently we have derived the T = 1 part of the effective interaction for the pf5/2g9/2 model space lo. This provides a part of the input for the larger model space of both protons and neutrons in these orbits where the maximum m-scheme dimension is 13,143,642,988. This proton-neutron model space is computationally feasible with conventional matrix-diagonalization techniques for many nuclei in the mass region A=56-100, and Quantum Monte Carlo Diagonalization techniques l5 or Exponential Convergence Methods l6 can be used for all nuclei. Present contributions reports first shell model results obtained with new interaction for nickel isotopes near 78Ni. Analysis of the spectra and B(E2) transition strength for even Ni isotopes is performed and new nuclear structure features that can be tested experimentally are highlighted.
2. Effective interaction The effective interaction is specified uniquely in terms of interaction parameters consisting of four single-particle energies and 65 T = l two-body matrix elements (TBME). The starting point for the fitting procedure was a realistic G-matrix interaction based on the Bonn-C N N potential together with core-polarization corrections based on a 56Ni core 17. The values of the neutron interaction parameters are adjusted t o fit 15 experimental binding energies for 57-78Ni and 91 energy levels for 60-72Ni. For protons 19 binding energies and 113 energy levels were used in a fit (this data set is similar to that used in Ref. 19). The average deviation in
544
binding and excitation energies between experiment and theory is 241 keV and 124 keV for neutrons and protons, respectively. Proton, neutron and original G-matrix interactions are summarized in l o .
3. N = 40 shell gap and magicity of iiNi40 The excitation energies of the 2; and the 4: states in valence neutron (nickel isotopes) and valence proton ( N = 50 isotones) spaces were analyzed in details in lo. The systematic shows good agreement between shell-model calculation and experiment. There is some similarity in the trends for the nickel isotopes with A = 68 - 76 and N = 50 isotones with A = 90 - 98, that is referred t o as the VMS '. This is because of the dominance of the g g / 2 orbital for both groups. However, one notes drastic differences for nickel isotopes with A = 60 - 68 and for N = 50 isotones with A = 82 - 90. This is caused mainly by the different ordering of proton and neutron single-particle orbitals: the proton f5/2 orbital is 1.5 MeV below the p 3 / 2 orbital while for neutrons the p312 orbital lower than the f5/2 one by 0.8 MeV. Two nuclei, 66Ni and "Sr, show the most profound differences
80
60
40
20
-
88Sr 66Ni 90Zr 68Ni 9 2 W 70Ni 94Ru 72Ni 96Pd 74Ni 98Cd 76Ni
n=O
n=2
n=4
n=6
n=8
n=lO
component t o the ground Figure 1. Contribution of the ( f ~ / 2 p ~ 1 2 ) o + state of the Ni isotopes with A = 66 TZ and N = 50 isotones with A = 88 n.
+
+
Ot
in the location of the 4: state: it is 4.3 MeV for "Sr and only 3.2 MeV for
545
the 66Ni. The structure of the corresponding ground O+ states is also very different (see Figure 1): the contribution of the closed shells ( f ~ / z p ~ 1 2 ) o + component is 20 % and 60 % for 66Ni and 88Sr, respectively. The small (20 %) contribution of the (f56j2pi/,)0+ component for the 66Ni determines the structure of the Ni isotopes with larger number of neutrons. Thus we find that for the 68Ni the (f56/2pt,2p$2)o+ component ( N = 40 shell closure) in the ground O+ state constitutes only 42 %, while an assumption of N = 40 magic number requires 100 %. Furthermore, the weight of the ( f ~ / 2 p ~ l z ) ~ + ( p 1 / 2 g 9 component /2)~+ (Figure 1) is below 50 % as well that disapproves the N = 40 as a good magic number for this region. However, some of the observables (like single-particle like magnetic moments in 69Ni or 69Cu ) may be misinterpreted as a n indication of the magicity. For example, magnetic moments of 6gNi or 69Cu are calculated to be very close to corresponding single-particle values. But this is caused by the fact that the first excited state at 1.770 MeV in ggNi40 has J" = O+ and coupling of the single-particle state t o it will not change the magnetic moment of the final state as compared t o the single-particle value. Another interesting issue is the smallness B(E2;2; + 0;) value which in contrast t o doubly magic 56Ni, for example, constitutes only a small part of the summed E2 strength and therefore is not a good indicator of magicity as well.
4. Properties of neutron
configurations
The weight of correlations in p1/2~9/2subspace becomes larger for the ground state with a n increase in a number of valence neutrons (protons). For excited states in nuclei with A = 66 -t n ( A = 88 n)the (g9/2)? configurations with n 2 4 play dominant role for neutrons (protons) starting from J" = 6+. However there are some differences in the spectra of 72174Niand 9 4 R ~ , 9 6 P drespectively. , Shell model predicts that both the 6: and 6; states are below the 8; state in 72174Ni,while in the case of 9 4 R ~ and 96Pd only the 6; state is below the 8: state. Furthermore, because of the dominance of the (g9/2)nJ,6 configuration (n=4 and 6 for 72Ni and 74Ni, respectively) the 6: state is characterized by seniority s = 4. This leads t o enhanced seniority-changing E2 decay of the 8+s = 2 state, significantly reducing its lifetime (6 ns for 72Ni) comparing t o N = 50 isotons and is sup(3ps for 96Pd). The above mechanism was suggested in ported by our present shell model calculations. To understand which part of the effective interaction responsible for this effect one can write down the analytical expressions for the energies of the 6+ states with s = 2 and
+
546 s = 4 for the gi12 configuration as a function of two-body matrix elements VJ = (gg12; J+IV,,,1gg12; J+). The difference of the excitation energies is given by the following expression:
+
+
E, (6$=4) - E, (6:==,) = -0.6. Vo 1.3.& - 0.69. V4 - 0.18. Vs 0.22. Vg . (1) One concludes from the Eq. 1 that it is most sensitive t o V2 and V4 matrix elements. Plotting the excitation energies of the shell model states as a function of the V2 (Figure 2) we see how the energy of the 6; evolves if we move from its proton value (dashed vertical line) to neutron one (solid vertical line). The Vz tbme enhances by 0.6 MeV when we switch from
MeV
3
*+----+
2
1
-1.4
-1.2
-1.
-0.8
-0.6
-0.4
V2,MeV
Figure 2. Excitation energies of the shell model 2,: 6:, 6; and 8; states for 16 particles in pf5/2g9/2 space as a function of Vz tbme. Vertical solid line marks the value of Vz for the neutron interaction and dashed line for proton one.
protons to neutrons causing a critical change in the location of the 6; state. We find that the V4 plays important role too. Thus it would of
547
great experimental interest to find both 6+ states in 72974Niisotopes. The situation with the 4+ states is also very interesting but more complicated: the higher seniority s = 4 state mixes with the s = 2 state and becomes below it. This indicates transition to collective vibrational picture 2o for the 72174Niisotopes and, in turn, larger softness of the 78Ni comparatively looSn.
References 1. R.Broda et. al., Phys. Rev. Lett. 74,868 (1995). 2. R. Grzywacz et. al., Phys. Rev. Lett. 81,766 (1998). 3. H. Grawe, Nucl.Phys. A704, 211c (2002). 4. M.Sawicka et. al., Pbys. Rev. C68,044304 (2003). 5. T. Ishii et. al., Phys. Rev. Lett. 84,39 (2000). 6. 0. Sorlin et. al., Phys. Rev. Lett. 88, 092501 (2002). 7. K. Langanke et. al., Phys.Rev. C67,044314 (2003). 8. R. Wirowski et al., J. Phys. G: Nucl. Phys. 14,L195 (1988). 9. H.Schatz, Phys.Rep. 294,167 (1998). 10. A. F. Lisetskiy, et. al., Phys. Rev. C70,034321 (2004). 11. B. A. Brown and B. H. Wildenthal, Ann. Rev. Nucl. Part. Sci. 38, 29 (1988). 12. A. Poves, et al., Nucl. Phys. A694 157 (2001). 13. M. Honma et al., Phys. Rev. C65,061301(R) (2002). 14. M. Honma et al., Phys. Rev. C69, 034335 (2004). 15. T. Otsuka et al., Prog. in Part. and Nucl. Phys. 47,319 (2001). 16. M. Horoi, B. A. Brown, and V. Zelevinsky, Phys. Rev. C67,034303 (2003). 17. M. Hjorth-Jensen, private communication. 18. H. Grawe, Prog. Part. Nucl. Phys. 38, 15 (1997). 19. Xiangdong Ji, and B. H. Wildenthal, Phys. Rev. C37,1256 (1988). 20. J. J. Ressler et al., Phys. Rev. C69 034317 (2004).
548
PARTICLE-NUMBER PROJECTION IN THE T=l NEUTRON-PROTON PAIRING N.H. ALLAL" 2 ) , M. FELLAH".'), M.R. OUDIH'') AND N. BENHAMOUDA(l) "'Laboratoire de Physique The'orique, Faculte' de Physique, USTHB BP32, El-Alia, 161I I Bab-Ezzouar, Alger, ALGERIA '2iCentrede Recherche Nucliaire dillger, COMENA BP399, Alger-Gare, Alger, ALGERIA A particle-number projection method for the study of the T=l neutron-proton (n-p) pairing is presented. The usual n-p BCS wave-function is simultaneously projected on both the good neutron and proton numbers and an explicit projected wave-function is obtained. The influence of the projection on the particle-number fluctuations and the energy of the system is studied within the Richardson schematic model. It is shown that the convergence of the method is very fast and that the projection significantly reduces both the particle-number fluctuations and the energy value.
1.
Introduction
A method which is often used to study the neutron-proton (n-p) pairing correlations is a generalized BCS treatment [l]. However, it is well known that
the main shortcoming of such an approach is the non-conservation of the particle-number. The usual techniques used in order to remedy this shortcoming are those already used for the pairing correlations between like particles, i.e. the Quasiparticle Random Phase Approximation [2], the Generator Coordinate Method [3] or the Lipkin-Nogami method [4]. However, in these methods, the particle-number symmetry is only approximately restored. The aim of the present work is to present an exact particle-number projection method in the T = l case. The method is based on a discrete form of the projection operator that allows one to derive an explicit form of the projected wave-function. 2. Projection Method In the second quantization and isotopic spin formalism, a system of N neutrons and Z protons is described by the Hamiltonian:
where: t corresponds to the isotopic spin component ( I = N , P), U; and a, respectively represent the creation and annihilation operators of the particle in the state v I ) , of energy E , ; t ) is the time-reverse of v t ) and has the same energy.
I
I
I
549
G f fcharacterizes f the pairing-strength. The neutrons and protons are supposed to occupy the same energy levels.
The standard procedure is to use the generalized BogoliubovValatin transformation approach [ 11 where the BCS state I W > is given by:
Iiy) =
n
[ B y a $ ~ ; ~ u ; ? ~ a+&B ; U & ~ ; + ~ BJu&p&, + BJ"(a&&,
+ a&u;j,)+
B l ]0) (2)
V>O
BY
being variational parameters.
The projector that allows one to obtain the state corresponding to both the good proton and neutron numbers (and hence the good isospin) is of the form: [5]
where
1/2
I
ifk=Oork=n+l , zk = e x p ( i k ) , n is a non-negative if I l k l n n+l
integer, cc means the complex conjugate with respect to Z k and Pf refers to the neutron (respectively proton) pair number. The projected wave-function is then:
where
c,,*
is a normalization factor, Wkkt
= z-81 k ' k-p''
' Ykk'
--eIz-PP, k'
='k
Zk
is the complex conjugate of Z k and:
I Y ( z ~ .zp)) =
n
[B~z~zpappavpa;,+& + + +
+ B;zp&&
v>o
+ E T Z ~ U & U ;+~ B J G ( a $ p a : N
(5) +a&&)+
B:]O)
The integers n and n' measure the extraction degrees of the false components of Iy). As soon as the condition: 2 ( n + l ) > M a x ( p ~ , R - P ~ ) and 2(n'+l) > Max(Pp, 0 - Pp ) , Q being the total degeneracy of pairs, is satisfied, the state (4) coincides with the physical component. We have thus obtained an explicit expression of the projected wave-function.
550
The normalization condition of
where:
and that of N 2 is given by:
Iv,,I) leads to:
55 1
with:
the particle-number fluctuations will be measured by m n n t = ( ~ n n1f i 2 1 ~ n n t ) - ( ~ n nI N t lvnn')2
3.
Numerical Test and Discussion
The previously described method is applied within the Richardson model. and We consider here a system such that: N = Z = R = 8 , GNN=GPP=0.8 GNp=0.64. Table 1 shows the variations of the N and N2expectation values, the particle-number fluctuations m n n f , as well as those of the projected energy Ennl as a function of n and n'. It clearly appears that all the quantities rapidly converge (the convergence is reached as soon as n and n ' 2 2 ). The spurious components are thus completely eliminated since the particle-number fluctuations change from 3.528 with the BCS theory, to 0 with the present method. Moreover, the energy value is significantly reduced with regard to the BCS one (38.21 1 MeV). Table 1. Variations of the fi and fi2 expectation values, the particle-number fluctuation and the projected energy as a function of n and n'. n Ennl mnnl 0 37.31615 1.00076 37.15975 0.50055 0 16.00000 256.50055 1 37.15970 0.50038 256.50038 2 0 0 3 37.15940 256.49800 15.99992 1 0 0.50055 37.00282 16.00000 1 0.00034 256.00033 1 37.00278 2 0.00017 256.00017 1 3 1 37.15936 0.50038 256.49783 15.99992 2 0 37.00278 256.00017 16.00000 2 1 0.00017 37.00273 256.00000 2 0.00000 2 2 3 37.15936 0.50038 256.49783 0 15.99992 3 37.00278 256.0001 7 0.00017 16.00000 1 3 37.00273 0.00000 256.00000 2 3 a 3
(9)
552
References 1. A. Goodman, Adv. Nucl. Phys.l1,263 (1979). 2. G. Pantis et al., Phys. Rev. C53,695 (1996). 3. M. Kyotoku and H.T. Chen, Phys. Rev. C36, 1144 (1987). 4. W. Satula and R. Wyss, Nucl. Phys. A676, 120 (2000). 5 . M.R. Oudih, M. Fellah and N.H. Allal, Int. J. Mod. Phys. E12, 109 (2003).
553
LASER SPECTROSCOPY OF TRANSURANIUM ELEMENTS YU. P. GANGRSKY, D. V. KARAIVANOV, K. P. MARINOVA, B. N. MARKOV, YU. E. PENIONSHKEVICH, S. G. ZEMLYANOI Joint Institute for Nuclear Research, Dubna, Russia The present paper aims to discuss the prospects for nuclear structure investigation of the transuranium elements by laser spectroscopy. The authors lay stress on two peculiarities of the nuclear structure in this region: the deformed shell closure at neutron number N = 152 and the appearance of superdeformed isomeric states. A laser spectroscopic experimental method is proposed for studying these features.
1 Nuclear Structure Peculiarities of the Transuranium Nuclear structure investigation is one of the basic directions of low energy nuclear physics. It is known that the different regions of the chart of nuclides show various characteristic features. In the area of the transuranium elements they are the following: 1. Deformed shell closure with neutron number N = 152. The dependence of the energy of the a-decay on the neutron number with a characteristic kink at N = 152 [l] (see Fig. 1) can be interpreted as an indication of a closed shell. This behaviour is consistent with the well-known effect at other spherical shell closures, for example N = 126 (see Fig. 1). In addition, nuclei with N = 152 are the most stable isotopes toward spontaneous fission ( e g the corresponding Cm, Cf and Fm isotopes have the longest spontaneous fission half-lives) [2]. Of course, it is of great interest to obtain more data on other parameters characterising nuclei with closed shell, e.g. nuclear charge radii, magnetic dipole and electric quadrupole moments, and scheme of the nuclear states. 10
:Po (2= 84)
E
Fm (Z = 100.)
1 -
5 -
4
'
1
"
1
1
'
1
1
1
1
1
1
1
1
1
'
1
'
118 122 126 130 134 138 142 146 150 154 158
Neutron number Figure I Dependence of the a-decay energy on the neutron number for the Po, Cf and Fm isotopes.
554
2.
Shape isomers in the Z = 92 - 97 region: U - Bk. In the nuclei of these elements isomeric states have been observed which decay predominantly by spontaneous fission (spontaneous fissioning isomers) [3]. These states have been interpreted as lower levels in the second potential minimum of the fission barrier (Fig. 2) [4]. The isomeric states have an unusually large quadrupole deformation (B = 0.6) which has been deduced from measurements of the rotational level lifetimes [5] and from the nuclear charge radii of the isomeric state [ 6 ] .
U
Figure 2 The nuclear potential energy in dependence on the deformation parameter.
The determination of the nucleon configurations of the isomeric states and their static quadrupole moments is of particular importance. The corresponding data would be able (i) to give more detailed insight into the structure of these states and (ii) to clarify more definitely the regions where they can occur. In a number of nuclei, e.g. in the odd U and Np isotopes (with an exception of '"Np), no spontaneous fission isomers have been observed and this fact remains so far unexplained (either there are no isomers of these isotopes or the isomers exist but decay in another way). For a rigorous and meaningful test of the nuclear structure of the transuranium elements a set of data as complete as possible should be available. This determines the steadily increased interest in studying the transuranic elements by different experimental techniques. 2 Nuclear Structure Investigations by Laser Spectroscopy
Nowadays a great deal of systematic experimental information on nuclear properties obtained by laser spectroscopy is available. The laser spectroscopic methods are based on the electromagnetic interaction between nucleus and the electron shell. Optical isotope shifts are directly related to differences of nuclear mean square charge radii, and hyperfine structures of spectral lines contain information about nuclear spin, magnetic dipole moments and electric quadrupole moments [7]. The phenomena observed in the systematics of these quantities include collective properties and deformation, nuclear shape coexistence and shell effects.
555
34.4
I
34.3
+I
34.2
~ ~ 3 4 . 1 c'1
rs 34.0 33.9 33.8
I
140
142
I
I
I
I
144
146
148
150
152
Neutron number Figure 3 Dependence of the nuclear charge radii on the neutron number for the uranium and thorium isotopes. Unfortunately, no data is available at the shell closure N = 152.
A set of such parameters have been determined for the transuranium nuclei, too. For example, data is available on the charge radii changes in isotope sequences of U and Pu [8,9] (Fig. 3), magnetic dipole and electric quadrupole moments of several odd U, Pu and Am isotopes [lo], deformation parameters of the spontaneous fission americium isomers [6]. However, this information is not sufficient for detailed explanation of the above mentioned peculiarities of the nuclear structure of the transuranium elements [2]. Thus, the necessity of firther investigations is evident. The investigation of nuclear properties of heavy elements is very difficult. A large variety of transuranium element can only be produced in heavy ion reactions with very low cross-sections, usually smaller than several mb. The experimental situation is very challenging because extremely high sensitivity of detection is required. For this reason new generation of high sensitive experimental methods has to be developed suitable for laser spectroscopic investigations of nuclides accessible in very low amount. For example, highly improved experimental technique is initiated already during the last years at GSI, Darmstadt. In order to search for optical transitions and to determine the nuclear ground state properties of transuranium elements, a new facility, called SHIPTRAP, is presently being build up and tested there [l 13.
556
3 Light Induced Drift of Atoms in a Buffer Gas An improvement of the sensitivity can be achieved if one detects radioactive decay instead of photons or ions. A way to do this may be the use of the wellknown method of light induced drift of atoms in a buffer gas [12]. The effect is based on the difference between the diffusion coefficients of excited and unexcited atoms. As a rule, excited atoms are bigger than the ground-state atoms, and for this reason their collision cross sections with the buffer gas is larger and their diffusion coefficient is smaller. The light induced drift appears when the atoms of a selected isotope are excited resonantly, accorcky to their velocity, by laser radiation. Due to the Doppler shift, the resonance frequencies for atoms moving in different directions will be different. The frequency shift can be expressed by the relation U
Av =v-cosa C
(1)
Here u and c are the atom and light velocity, respectively; a - the angle between the directions of laser light and atom velocity. In the cases of high resolution laser spectrometers the line width is 10 to 20 times smaller than Av. Thus, the laser light acts as a mirror which stops the drive of the atoms in a given direction and favours their movement in an opposite direction. The effect has already been demonstrated on the example of the radioactive isotopes "Na and 24Na [13]. The isotope shifts of these isotopes at the D2 line relative 23Na are 750 MHz and - 706 MHz respectively and the Doppler shift at the room temperature is about 700 MHz for a = 0' or 180". Tuning the laser frequency in the resonance frequencies for 23Naresults in the drift of both isotopes in opposite directions. This has been observed experimentally in a long tube with a buffer gas (neon at a pressure of about 30 Torr) superimposed collinearly on a cw laser beam. The difference between the concentrations of "Na and 24Nadetermined by the intensity of their y-radiations was about two orders of magnitude. The analogous principle can be applied for determination of the resonance frequencies of the isotopes of the transuranic elements. A light induced drift of a selected isotope can be produced taking advantage of the appropriated choice of the resonance frequency shift. Thus, the investigated isotope can be transported to the a-detector at the end of the tube and an increase of the a-counts rate will indicate the optical resonance. Of course, this implies further investigation of the optical properties of the transuranium elements to determine the changes of the atomic sizes in different excited states. The experimental apparatus we propose here (Fig. 4) is well suited for off line as well for on line measurements. In the first case, a thick target must be used and the investigated element has to be chemically separated. Further the sample containing a given transuranium
557
a-detector laser beam
2
crucible with oven
Figure 4 Principal scheme of the setup based on the laser light-induced drift
element is inserted in a heated crucible and the atoms evaporate into a buffer gas of the tube illuminated by the laser light. In the second case, the recoils of reaction leave the target and are slowed in a buffer gas where the light induced drift can be produced. The proposed method is sensitive enough to be applied to transuranium isotopes with very low production rates (> 10 s-’)and with sufficiently short halflives (the drift time at distances of the order of 10 cm is < 10” s). It is well known, that such parameters have already been obtained in experimental setups which combine multi-step resonance ionization of atoms (or resonance neutralization of ions) and detection of radioactive decay. However, the method discussed here has an essential advantage: it is easily accessible and technically simple. To obtain a drift of selected isotope it is sufficiently to have only one laser frequency corresponding to a given atomic transition from the ground or low lying atomic states. This requirement is easily met. Thus, it is not necessary to use a set of tunable lasers and to know in detail the atomic level scheme up to the ionization limit. The latter is from especially importance in the case of the heavy actinide (trans-einsteinium elements) with their generally not known atomic level schemes. 4 Future Experiments
At the Laboratory of Nuclear Reaction (LNR) of JINR, Dubna an extended programme is planned for nuclear structure investigation of transuranium elements by laser spectroscopy. The investigations in this field of nuclear physics are traditional for the LNR. The programme includes study of elements heavier than plutonium. Isotopes of such elements can be produced (i) in nuclear reactors of JMR by irradiation with highly intensive neutron beams and (ii) in the LNR accelerators by heavy ion reactions. The investigated elements will be chemically separated. The mass separation will be used in some cases. The laser
558
spectroscopic studies are planned to be performed using two different methods: laser-induced resonance fluorescence in a collimated atomic beam (in the case of long-lived isotopes) and laser light-induced drift in a buffer gas ( in the case of short-lived nuclei). The first experiments will include measurements of the nuclear charge radii and nuclear moments of the Cm, Bk and Cf isotopes in the region of the neutron shell closure N = 152.
References
1. A. Rytz, ADNDT, 47(1991)205. 2. R. Vandenbosch, J.R. Huizenga, Nuclear Fission, N.Y., Acad. Press, 1973. 3. H. Britt, ADNDT, 12(1973)407. 4. V.M. Strutinsky,Nucl. Phys. A95( 1967)420. 5. D. Habs, V. Metag, H. Specht, G. Ufert, Phys. Rev. Lett. 35(1977)387. 6. H. Backe, M. Hies, H. Kunzer et al., Phys. Rev. Lett. 80(1998)920. 7. E. Otten, Treatise on Heavy Ion Science. Nuclei Far from Stability, 8(1989) 517. 8. Yu.P. Gangrsky, S.G. Zemlyanoi, B.K. Kuldzhanov et al., Izv. AN USSR, ser. fiz., 54(1990)830. 9. P. Aufmuth, K. Heulig, A. Steudel, ADNDT, 37(1987)445. 10. P. Raghavan, ADNDT, 42(1989)189. 11. J. Dilling et al., Hyperfine Interact. 127,491 (2000). 12. F.Kh. Gelmukhanov, A.M. Shalagin, Pisma Zh. Eks. Teor. Fiz. 29(1979) 173. 13. Yu.P. Gangrsky, C. Hradecny, S.G. Zemlyanoi, Zh. Eks. Teor. Fiz. 106 (1994)825.
559
SPECTROSCOPY AT N=28 NEW EVIDENCES OF DEFORMATION
S. GREVY
',J. C. ANGELIQUE, F. R . LECOLLEY, J. L. LECOUEY, E. LIENARD,
N. A. ORR, J.PETER, S. PIETRI, I. STEFAN Laboratoire de Physique Corpusculaire de Caen, IN2P3-CNRS, 6 bd du Ma1 Juin Caen, F-14050 Cedex, France
C. BORCEA, A. BUTA, F. NEGOITA, D. PANTELICA Institute of Atomic Physics, IFIN-HH , Bucharest-Magurele, P.O. Box MG6, Romania P. BAUMANN, G. CANCHEL, S. COURTIN, P. DESSAGNE, A. KNIPPER, G. LHERSONNEAU, F. MARECHAL, C. MIEHE, E. POIRIER Ires, IN2P3/ULP, 23 rue du Loess, BP 30, F-67037 Strasbourg, France
Y. PENIONZHKEVICH, S. LUKIANOV FLNR, JINR, 141980 Dubna, Moscow region, Russia J. M. DAUGAS, F. DE OLIVEIRA, M. LEWITOWICZ, M. STANOIU, C. STODEL GANIL, CEA/DSM-CNRS/IN2P3, BP.5027, F-14076 Caen Cedex, France D. GUILLEMAUD MUELLER, F. POUGHEON, 0. SORLIN Institut de Physique Nuclkaire d'Orsay, IN2P3-CNRS. F-91406 Orsay Cedex, France W. CATFORD, C. TIMIS Department of Physics, University of Surrey, Guildford, Surrey.GU2 7XH, UK Z. DLOUHY, J. MRAZEK Nuclear Physics Institute, AS CR, CZ-25068 Rez, Czech Republic K. -L. KRATZ, B. PFEIFFER Institut fur Kernchemie, Universitat Mainz, 0-55128 Mainz, Germany
Corresponding author: [email protected]
560 The spectroscopy of neutron-rich nuclei located at N=28 below 48Cais of importance in order to obtain information on the structural changes occurring in this region. In particular, one question concern the importance and the origins of the deformation observed in the Sulfur isotopes. We are going to report on this paper about an experiment of beta-decay performed at GANIL in which we have obtained new periods below 48Ca and, in particular, for the most neutron-rich Si isotopes (Z=14) which suggest strong deformations for these isotopes. We also obtained decay schemes of M945s46Ar (Z=18). Together with the results of an in-beam y spectroscopy experiment in the S isotopes (Z=16), we propose a coherent description of this region. We are also reporting preliminary results about an electron-gamma spectroscopy experiment dedicated to the observation of the isomeric 2 ' 0 state in 44S.
1.
Introduction
Since more than ten years it is known that the deformation plays an important role in the neutron-rich nuclei below 48Ca.In particular, the half-lives [l], the energies of the first 2' states and the corresponding B(E2) [2] have been measured in the S isotopes (Z=16) and have been well reproduced by different theoretical calculations [3,4,5]. However, the importance of the deformation and the mechanisms involved are still not well understood mainly due to the lack of precise spectroscopic information. In particular, is the deformation already present in the Ar isotopes (Z=18) and what is be the situation in the Si where the proton sub-shell closure Z= 14 could stabilize these isotopes again the deformation ? In the following, we are going to explore the isotones N=28 taking the well spherical doubly magic nucleus 48Ca (Z=20) as a reference. By removing successively two protons to reach the Argon (Z=18), the Sulfur (Z=16) and finally the Silicon (Z=14) isotopes, we are going to report on the changes observed in the structure of these nuclei through experimental spectroscopic information and compare them mainly to the results of shell model calculations. 2.
Experimental results
2.1. Z=18 - Ar isotopes
The excited states in the neutron-rich 44Ar(N=26), 45Ar(N=27) and 46Ar(N=28) have been extracted from the beta-decay of 4494s,46Cl [ 6 ] in an experiment performed at GANIL on the LISE3 spectrometer. On the figure 1 are displayed the level scheme obtained for the 45Ar and the corresponding shell model calculations for negative parity states presented in [7]. We can observe the very good agreement between experimental and calculated levels up to approximately 2 MeV. In particular, the first 312- excited state corresponding to the promotion of one neutron from the f712 to the p312 shell is located at 543 keV whereas the
561
same state in 47Cais located at higher energy (2.47 MeV). This reduction of approximately 2 MeV is partially due to a reduction of the N=28 shell gap itself but also to a gain in the correlation energy due to the opening of the proton d3/2 orbital. In the shell model calculations, the reduction of the gap itself is limited to 900 keV (4.73 MeV in Ca and 3.84 MeV in Ar). Moreover, the first 312- state is dominated at more than 85% by normal configurations (Ohw) whereas the intruder configurations (2hw) dominate the second 312- state around 1.3 MeV. Then, at Z=18, the intruder strength is located at relatively high excitation energy. This results agree well with the conclusions of in beam spectroscopy experiment in which the 4' and the second 2' states have been measured in 46Ar(see below - S isotopes).
2.2.2=16
177001 17357 14167 IYOI
45Ar
1790-11/2 1330 1240
-
512 312
SM
Figure 1 . Deduced level scheme of 45Arwith intensities of transitions. Values in oval indicate transitions observed also in P-n decay [6]. Corresponding shell model calculations are taken from [7].
- S isotopes
Detailed information on the shell structure of the S isotopes was obtained from in beam gamma spectroscopy experiment employing high-energy fragmentation performed at GANIL on the SPEG spectrometer using the "Chateau de Crystal" [8]. In particular, the energies of the 4' in 40342S as well as those of the second 2' states in 42,44Swere determined. It was concluded that 40S and 42S are deformed, y-soft nuclei, while 44S exhibit shape mixing in the low lying states. The role of the proton has also been pointed out as a major contribution to the quadrupole collectivity in these neutron-rich isotopes. In the shell model calculations, which again reproduce very well the experimental results, the reduction of the gap is limited (gap = 3.23 MeV). Interesting also was the prediction in recent shell model calculations [9] concerning the second 0+2state in '% located at relatively low energy (1260 keV), just few tens of keV above the first 2' state (1220 keV) and which was interpreted as a signature of shape coexistence. We performed at GANIL an experiment of electron-gamma spectroscopy dedicated to the observation of this 02' state which has been then observed at
562
1365 keV, just 36 keV above the first 2+, in perfect agreement with the shell model predictions (see Table 1). Table I. Experimental and calculated [9] energies for the 2' and 0 ' 2 states in 44S.
2+ 0+*
1329f1 1365f1
1220 1260
The decay of this 0 ; state has been observed through both the EO (0'2 + 0+1by conversion electron and pair creation) as well as the E2 (0+2+ 2+1+0+1) channels with a decay time of 2.3k0.3 psec. Experimental spectra are displayed on the figure 3 for the EO decay through conversion electrons. 45 40
aa
10
9
~~~~~
Electron energy (keV)
Figure 3. Decay through conversion electrons of the 0+2state in "S, On the upper panel is displayed the decay curve (2.3*0.3 p e c ) whereas the energy spectrum is displayed on the lower panel (1365*1 keV).
2.3.2=14
- Si isotopes
From the above analyses, it results that the reduction of the N=28 shell closure below 48Cais relatively limited, at least for Z=18 and 16. The deformation and the shape coexistence observed in- the S isotopes seems to be the result of an interplay between neutron excitations above N=28 and a collectivity driven by the protons. Then, the question of the persistence of the deformation in the Si isotopes is pertinent since the proton configuration is expected to be more stable
563
due to the Z=14 sub-shell closure. Indeed, experimental data from Ca(d,3He) reactions suggest that the gap Z=14 is even larger for N=28 than for N=20. We can then believe that the protons would not contribute significantly to the deformation in the Si isotopes. Because of the very low production rate for such exotic nuclei (42Si N/Z=2), obtain spectroscopic information is an experimental challenge which has not been reach up to now. However insights about the deformation can be derived from half-lives and QRF'A calculations as we have done in ref. [lo]. The half-lives obtained for 3942Siat GANIL (Tl12 going down from 47.5f2.0 to 12.5k3.5 msec) cannot be reproduced by QRF'A calculations without involving important deformation (IpI 2 0.4 in 42Si).Nevertheless, these conclusions have to be consider carefully since such calculations strongly depend of mass excess which are not known with high precisions in this region. We should also point out that the different theoretical predictions for the Si isotopes do not agree very well each other concerning the importance of the deformation. If the shell model calculations predict a rather limited deformation (the 42Si behaves more like a 'doubly magic nucleus'), different mean field approaches predict a well prolate 42Si.It is an illustration of the importance to obtain precise spectroscopic information like the position of the 2' state for the most neutron-rich Si isotopes.
References
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
0. Sorlin et a]., Phys. Rev. C47,2941(1993). T. Glasmacher et al., Phys. Lett. B395, 163(1998). E. Caurier et al., Eur. Phys. J. A15, 145(2002). P. G. Reinhard et al., Phys. Rev. C60,0143 16 (1999). S. Peru et al., Eur. Phys. J. A9,35(2000). J. Mrkzek et al., Nucl. Phys. A734, E65 (2004) Zs. Dombradi et al., Nucl. Phys. A727, 195 (2003) D. Sohler et al., Phys. Rev. C66,054302 (2002). E. Caurier et al., Nucf.Phys. A742, 14(2004). S. GrCvy et al., Phys. Lett. B594,252(2004).
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567
A PLUTONIUM CERAMIC TARGET FOR MASHA P.A. WILK, D.A. SHAUGHNESSY, K.J. MOODY, J.M. KENNEALLY, J.F. WILD, M.A. STOYER, J.B. PATW, R.W. LOUGHEED, B.B. EBBINGHAUS, AND R.L. LANDINGHAM Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA YU.TS. OGANESSIAN, A.V. YEREMIN, S.N. DMITRIEV Flerov Laboratory of Nudear Reactions, Joint Institute for Nuclear Research, Dubna, Russia
We are currently developing a plutonium ceramic target for the MASHA mass separator. The MASHA separator will use a thick plutonium ceramic target capable of tolerating temperatures up to 2000 "C. Promising candidates for the target include oxides and carbides, although more research into their thermodynamic properties will be required. Reaction products will diffuse out of the target into an ion source, where they will then be transported through the separator to a position-sensitive focal-plane detector array. Experiments on MASHA will allow us to make measurements that will cement our identification of element 114 and provide for future experiments where the chemical properties of the heaviest elements are studied.
1.
Introduction
1.1. The MASHA Separator
The MASHA (Mass Analyzer of zuper Heavy Atoms) on-line mass separator is currently under development at the Flerov Laboratory of Nuclear Reactions at JINR. This separator is expected to have a number of improvements over existing recoil separators, and will provide at least a ten-fold increase in the production and detection rate for element 114. It will allow unambiguous mass identification of super heavy nuclei with a mass resolution better than 1 amu [ 13. One proposal for the MASHA target is a thick, ceramic target (uranium or plutonium) heated to a temperature of approximately 2000°C. The advantage of this type of target over thin targets is that the ceramic will see a large range of beam energies so more of the excitation function will be sampled. The effective width of the excitation hnction is about 25 MeV with a thick target compared to only 3 MeV with a traditional thin target.
* Corresponding author email [email protected].
568
The target will be a combination actinide target and ion source of the CERN-ISOLDE type [2,3]. Reaction products will diffuse out of the heated, porous target and drift to an ion source where they will be ionized and injected into the separator. After traveling through the separator, the products will impinge on a position-sensitive focal-plane detector array for mass measurement. Initial tests will use uranium ceramics, but ultimately, element 1 14 experiments will be performed using ceramics made from 244Pu(about 15% of the world's supply) bombarded with a 48Cabeam. 1.2. Target Requirements and Preparation
The plutonium target for MASHA must meet several requirements. It must be physically stable and able to withstand a temperature of 2000 "C without melting or thermally expanding. The target must be stable over a large temperature range without undergoing phase transitions that would result in an unstable compound or a compound with a much lower melting point. The plutonium vapor pressure cannot be so high as to contaminate the ion source or separator, and finally, the diffusion rate of reaction products from the target must be fast enough to allow measurement of short-lived products. The kind of material that would best meet these requirements appears to be a binary compound of uranium or plutonium, namely a carbide or oxide. In order to produce these compounds, they must be heated to temperatures on the order of 1800 "C. In addition, the use of large quantities of plutonium requires the use of a glove box for safety considerations. A custom made furnace (Micropyretics Heaters International) has been designed for this project that reaches these temperatures without emitting too much heat externally, which could pose a safety hazard inside of a glove box. The furnace is also small enough to fit inside the standard-sized glove boxes we currently have at LLNL.
2.
Relevant Plutonium Compounds
Of the known binary plutonium compounds, there are several that meet the first requirement of a melting point greater than 2000 "C. These include plutonium carbides, nitrides, sulfides, and oxides. In order to decide which compound is best suited for MASHA, the physical and thermodynamic properties, as well as the ease of preparation, of each compound must be considered. Plutonium mononitride may be a promising candidate, but it is highly reactive with a variety of common reagents and is readily oxidized in air. It also decomposes before reaching its reported melting point of 2830 "C [4]. The other disadvantage of PUN is its large vaporization pressure [5]. Likewise, plutonium monosulfide is reported to have a melting point of 2350 "C [6], but it
569
too has a large volatility and exhibits a high coefficient of thermal expansion [7]. More research on the plutonium-sulfur phase diagram would need to be done before ruling out plutonium sulfides completely. The two most promising systems are plutonium carbides and oxides. The one disadvantage with the plutonium-carbon system is that the phase diagram shows a phase transition at 1600 "C where Pu2C3 transforms into PuC2, which is unstable at lower temperatures [ 5 ] . If the target is to have repeated use, this makes the carbide unattractive, since heating it to 2000 "C would create a compound that would be unstable once the temperature was lowered. Although plutonium carbides should not be ruled out at this time, more work needs to be done to investigate their thermodynamic properties. Weight Percent Oxygen 3000
0 I..
10
. . . . . .. .................................................. !.-...,........!
Pu
20
Atomic Percent Oxygen Figure 1. Plutonium-oxygen phase diagram (Ref. [ 5 ] . )
More information is known about the plutonium oxides than any of the other binary compounds. PuOz satisfies most of the target requirements and is coincidentally the easiest compound to prepare. It has the lowest vapor pressure at elevated temperatures ( 6 ~ 1 0 atm - ~ at 2000 "C [S]) in addition to a high melting point (2425 "C [5]). One potential drawback is that PuOz reacts with tantalum to form Pu203(tantalum is currently proposed as the support material.) Pu203 has a comparable vapor pressure and melting point (2080 "C [ 5 ] ) ; the
570
effect this transformation would have on the structural integrity of the target needs to be investigated further. Figure 1 shows the plutonium-oxygen phase diagram from Ref. [5]. 3.
Conclusions
Candidates for the MASHA target are currently being prepared and characterized. On-line tests with MASHA will begin with uranium carbides, but subsequent irradiations with 242Puand ultimately 244Puwill be performed. The plutonium oxides appear to be the best candidates for the ceramic target, although the carbides should not be ruled out. Once the target is prepared and tested, experiments designed to measure the mass of element 114 will begin.
Acknowledgments This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. UCRL-PROC-205 183
References 1. Yu. Ts. Oganessian et al., Nucl. Instrum. Meth. Phys. Res. B204, 606
2. 3. 4. 5. 6. 7.
8.
(2003). L. C. Carraz et al., Nucl. Instrum. Meth. 148,217 (1978). H. L. Ravn, Phys. Reports 54,201 (1979). J. M. Cleveland, The Chemistry of Plutonium (American Nuclear Society, LaGrange Park, IL, 1979), 653. M. E. Kassner and D. E. Peterson, Editors, Phase Diagrams of Binary Actinide Alloys (ASM International, Materials Park, OH, 1995), 355. 0. L. Kruger and J. B. Moser, J. Inorg. Nucl. Chem. 28, 825 (1966). J. H. Handwerk, 0. L. Kruger, and J. B. Moser in Plutonium 1965, A. E. Kay and M. B. Waldron, Editors (Chapman and Hall, London, 1965), 739750. B. B. Ebbinghaus, personal communication (2003).
571
THE DETECTION SYSTEM OF THE DUBNA GAS-FILLED RECOIL SEPARATOR V.G. SUBBOTIN, YU.S. TSYGANOV, A.M. SUKHOV, S.N. ILIEV, A.N. POLYAKOV, AND A.A. VOINOV Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation
The Dubna Gas-filled Recoil Separator (DGFRS), operated at the U400 cyclotron at the Flerov Laboratory of Nuclear Reactions (FLNR), is one of the most efficient existing separator systems used to separate heavy products of the complete fusion nuclear reactions. The system of detecting of the compound nuclei, a-decay sequences and spontaneous fission events, data processing, readout and accumulation is described. The present system was successfully applied in our experiments aimed at the synthesis of superheavy elements (SHES) with Z=112-116 and Z=118.
1.
Introduction
Synthesis and study of properties of SHE’S is one of the most interesting and most expensive field of nuclear physics. That is why, if you have a modern charged particle’s accelerator, corresponding target material and high efficiency of separator facility, you have to get the most efficient detection system providing reliable identification of nuclei in the case of single events. So, such a system has to operate under low background conditions, to have high detection efficiency and small dead time. Data accumulating and readout system have to be as fast as possible to work in real-time mode. 2.
Detector and background conditions
To measure energies of evaporation residues (EVRs), sequential a-decays and spontaneous fission events, their time coincidences and positions we use timeof-flight (TOF) module and position-sensitive semiconductor detector (PSD) array (fig.1) located in the focal plane of the separator. Similar silicon detector without position sensitivity behind focal one is used in “veto” mode in order to eliminate signals from low-ionizing light particles, which could pass through the focal-plane detector without being detected in the TOF system. Using side detector array without position sensitivity allows us to increase detection efficiency for a-decays of implanted nuclei up to 87% of 4n.More detailed description of operation of both TOF module and PSD array could be found in our previous works [l-31. The energy resolution for a-particles absorbed in the focal-plane detector was about 40 keV before irradiation. For measuring the sum signals of a-
572
particles escaping the focal-plane detector at different angles and registered by side detectors the energy resolution was -190 keV. The FWHM position resolutions of the signals of correlated decays of nuclei implanted in the detectors were 0.8 mm for EVR-a and 0.5 mm for EVR-SF signals.
Fig. 1. Schematic view of the detecting module DGFRS provides deep suppression of 48Caions beam (by a factor more and target-like products (>lo5). To imthan 10l6), scattered particles prove background conditions for detecting long-time decay sequences the beam was switched off after a recoil signal was detected with parameters of implantation energy and TOF expected for corresponding Z of EVRs, followed by a-like signal with the expected energy in the same coordinate position [4]. A schematic view of the measurement equipment is shown in Fig. 2. To determine the event time the 16-bit counters KC-01 1 are used [ 5 ] . All the data are digitized and organized in events by CAMAC electronics. Specially designed crate controllers [6,7] are employed to transfer the data from the CAMAC crate to the buffer memory module [8] and to the memory of the PC ATl586. 3.
Development of measurements system
The present system was used in experiments aimed to synthesize isotopes of 114 and 1 16 nuclei in complete fusion reactions 48Ca+244Pu,248Cm,correspondently [ 12,131. In 2002, preparing next experiments aimed at synthesis of 118 and 1 15 isotopes we upgraded the measurement system to reduce its dead time (-85 ps). The second measuring CAMAC crate with the same multiplexers and ADC was added to the system. We made new logic of registration (block 9 in fig.2), so the second measuring crate operated only during main crate dead time.
573
fl fl
Enposition
E. a Exco,~position E. a Emenergy
El
Logic of registration
cl 7
v
u u u
I - posmon-sensitive strips of Si detector 2 - charge-sensitive preamplifier micro C a s a 3 - sum-mverl-amplifier SU-206K 4 - ampldierSU-4K 191. 5 -amplifier ORTEC575 6 -analog multiplexer AM-206K 7 - ADC PA-24K Ill]
[lo]
j
Errenergy
Measunng CAMAC Crate
8 - analog discrimmator of energies all A D C ~ ) 9 - loglcai block KL-202(to 10 - charge-sensitwe preamplifier PA-201 11 -timing filter amplifier Polon I 5 0 I 12 -nanosecond delay Polon 1506 13 - constant fraction discrimmator 14 - TIA converier Polon 1701A
Fig. 2. Block diagram of the measuring electronics (only one detector circuit is shown completely). 4.
Results
As a result we have got possibility to register first a-particle 7 ps later after recoil signal was recorded. We have used the upgraded system to study decay properties of isotopes of new SHE’S with Z=118 and Z=115; no events were registered by additional crate electronics because the decay times in the observed chains are longer than 85 ps [14,15]. Therefore they were recorded by main measuring crate. We plan to develop our registration system in oreder to reduce further the dead time.
Acknowledgments This work has been performed with support the of the Russian Foundation for Basic Research under grant No. 04-02-171 86.
574
References 1 . Yu.A. Lazarev et ul., JINR Report P13-97-238, Dubna, 1997. 2. Yu.S. Tsyganov et a/., Nucl. Instr. and Meth.A 392, 197 (1997); Yu.S. Tsyganov et al., Nucl. Instr. and Meth.A 525,213 (2004). 3. V.G. Subbotin et al., Acta Phys. Polonica B 34 (4), 2159 (2003). 4. Yu.S. Tsyganov ei a/., Nucl. Instr. and Meth.A 513,416 (2003). 5. N.I. Zhuravlev et al., JINR Report 10-8754, Dubna, 1975. 6. V.G. Subbotin, A.N. Kuznetsov, JINR Report 13-12953, Dubna, 1979; I.N. Churin, JINR Communication P10-90-589, Dubna, 1990; N.I. Zhuravlev et ul., JINR Communication P10-88-937, Dubna, 1988. 7. A.Yu. Bonyushkina et a/., JINR Report P10-95-284, Dubna, 1995. 8. N.I. Zhuravlev et al., JINR Report P10-88-937, Dubna, 1988. 9. V.G. Subbotin, A.N. Kuznetsov, JINR Report 13-12953, Dubna, 1979. 10. A.N. Kuznetsov, JINR Report P13-87-188, Dubna, 1987. 11. V.G. Subbotin, A.N. Kuznetsov, JINR Report 13-12953, Dubna, 1979. 12. Yu.Ts. Oganessian et a/., Phys. Rev. C 62,041604 (2000). 13. Yu.Ts. Oganessian et a/., Phys. Rev. C 63,011301 (2001). 14. Yu.Ts. Oganessian et al., JINR Report D7-2002-287, Dubna, 2002. 15. Yu.Ts. Oganessian et al., Phys. Rev. C 69, 021601 (2004).
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NEUTRON DETECTOR AT THE FOCAL PLANE OF THE SET UP VASSILISSA. *
A.I. SVIRIKHIN, A.V. BELOZEROV, M.L. CHELNOKOV, V.I. CHEPIGIN, V.A. GORSHKOV, A.P. KABACHENKO, O.N. MALYSHEV, A.G. POPEKO, R.N. SAGAIDAK, A.V. SHUTOV, E.A. SOKOL, A.V. YEREMIN F L N R JINR, 141980 Dubna, Russia. E-mail: sashaQsunvas.jinr.ru
For experiments aimed a t the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. The neutron detector consisting of 72 3He filled counters has been mounted around the focal plane detector chamber. In the first experiment the multiplicity of prompt neutrons emitted in spontaneous fission of z5zNo was measured.
1. Introduction
A big number of even - even isotopes of transfermium elements decay solely by spontaneous fission. Even in the case of some odd - mass heavy nuclei such as, for example, 259,261Lrand 2611263Db the spontaneous fission probability is comparable with that of a - decay. Presently available experimental information on spontaneous fission of transfermium elements mainly concerns partial half lives. For Fm and No isotopes and for a few Md, Lr and Rf isotopes the total kinetic energy (TKE) and mass distributions of fission fragments from spontaneous fission were also accurately measured '. A multiplicity distribution of prompt neutrons emitted in spontaneous fission was measured for elements not heavier than fermium 2 , and the only one measurement was performed for the isotope 2 5 2 N 3~. These experiments were mainly performed using different mechanical systems which accomplished transportation of evaporation residues (ERs), formed in complete fusion reactions with accelerated ions, from the target *This work was performed partially under the financial support of the Russian Foundation for Basic Research, contract N 02-02-16116 and JINR - BMBF (Germany), JINR Polish, JINR - Slovak Cooperation Programmes.
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to the detector area. Typically, ERs were implanted into the catcher foils or were stopped in gas after which they were transported to the detectors. These experimental set ups had critical limitations in the transportation speed, i.e. half life measurements, as well as in background conditions, due to rather low suppression factors of unwanted reaction products and necessity to place detectors close to the target position.
2. Experiment
From the early 80-es, the recoil in - flight separators were widely used for the synthesis and study of decay properties of transfermium nuclei '. A high level of suppression of beam particles and unwanted reaction products, having high production rates in the region of charge and mass of target nuclei, has been achieved. Slow heavy ERs which are studied in complete fusion reactions with heavy ions after passing through such experimental set-ups and time-of-flight detectors are implanted in the focal plane semiconductor detectors. The transportation time amounts to a few microseconds allowing the investigation of very short-lived isotopes. The focal plane detector assemblies can have a well structure 5, providing a possibility to measure energy of both fission fragments (TKE) from spontaneous fission of implanted ERs, when one of the fission fragments is registered by the focal plane detector and the second one - by the side detector. One of such experimental set ups used for the synthesis and study of decay properties of transfermium nuclei is a recoil separator VASSILISSA Extremely low background conditions at the focal plane of the separator, situated behind a 2 meter concrete wall, allow one to build sophisticated detector systems around the focal plane implantation detector. For the purpose of the study of spontaneous fission of transfermium nuclei in more detail a neutron detector consisting of 72 3He filled counters was mounted around the focal plane detector chamber. The detector system consisting of two (start and stop) time-of-flight detectors and an array of silicon detectors have been developed and installed in the separator focal plane. Thin plastic foils (30-70 pg/cm2 in thickness), emitting secondary electrons and microchannel plates for detecting these electrons are used in both time-of-flight detectors. Having passed the time-of-flight detectors, the recoil nuclei are implanted into the silicon detectors. In order to improve the sensitivity of the experimental set-up, a new detector array has been manufactured and installed at the focal plane. The detector array consists of five identical 16 - strip silicon wafers. 617.
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The active area of a single silicon strip detector is 60x60 mm2. As for the stop detector, its every strip is position sensitive in the vertical direction with a resolution of 0.3 - 0.5 mm between a decays of the a decay chain. The average energy resolution is 20 keV for a's of the 241Am source. Four wafers are mounted in the backward hemisphere facing the stop detector. They measure escaping a's or fission fragments, and the total geometrical efficiency is 85 % of 4 T . As for the backward detectors, the strips do not have any position resolution and each four neighboring strips are connected galvanically so that 16 energy sensitive segments are formed 5. In the case of backward detectors, we obtained an energy resolution of about 150 keV. The reason for that is a broader range of energy losses for escaping a-particles hitting the backward detectors over a wide range of angles. The focal plane detector assembly was lodged in a cylinder vacuum chamber 210 mm in diameter. Neutron counters were placed around this cylinder chamber and thus three layers were arranged. From the outside, neutron counters were covered by plexiglass, 5 cm in thickness, and boron polyethylene, 5 cm in thickness too, to slow down background neutrons from the outside of the neutron counter. It allowed us to reduce the neutron background by one order of magnitude. When the 48Cabeam intensity was about 0.5 ppA on the Faraday cup of experimental set up, the counting rate of background neutrons was equal to 50 Hz. The spectroscopy amplifiers used in the electronic system of the recoil separator were designed and manufactured at the Flerov Laboratory and have two outputs with different amplifications: the first one of up to 200 MeV to measure fission energies and the other one of up to 20 MeV for ER's and a particles 5 . The electronic system developed for the prompt neutrons multiplicity measurements was started by pulses from SF outputs of amplifiers. Signals from 3He counters passed through pulse shapers, discriminators and started univibrators, which produced 150 nsec pulses. In one module 16 channels were collected using ',OR7logics. A special module collected all signals from 5 such modules. SF - like signal started the timing counter having the length of 128 psec and all events from neutron counters during that time period were stored in a FIFO buffer. The accuracy of time measurements was 1 psec. The efficiency of detection of one neutron measured using a 248Cm source, placed in the position of focal plane semiconductor detector, was 25 %. A multiplicity distribution of prompt neutrons emitted in spontaneous fission of 2 5 2 N ~formed , in the reaction 48Ca(206Pb,2n),was measured in
578
'
T
Figure 1. A multiplicity distribution of prompt neutrons emitted in spontaneous fission in dependence of Z and A of transuranium nuclei. Our result is indicated as *.
test experiments and was equal to 4.43 f 0.45. This value is in good agreement with that from literature (4.15 f 0.3) '. In Fig. the obtained result is presented with the data, taken from literature. In the nearest future it is planned to carry out the experiments delivered to the measurements of a multiplicity distribution of prompt neutrons isotopes recently identified in emitted in spontaneous fission of 249,250N~ the experiments using the VASSILISSA separator 8 , it is also planned to study spontaneous fission of Rf isotopes.
References 1. 2. 3. 4.
5. 6. 7. 8.
E.K. Hulet, Sou. Journ. of Nucl. Phys., 57 (1994) 1165 - 1173. D.C. Hoffman, Nucl. Phys., A502 (1989) 21c - 40c. Yu. A. Lazarev et. al., Phys. Lett., B52 (1974) 321 - 324. A.V. Yeremin and A.G. Popeko, Physics of Element. P a r t . and At. Nuclei, 35 N4 (2004) 895 - 927. O.N. Malyshev et. al., Nucl. Znstr. Meth., A440 (2000) 86 - 94. A.G. Popeko et. al., Nucl. Instr. Meth., A510 (2003) 371 - 376. O.N. Malyshev et. al., Nucl. Instr. Meth., A516 (2004) 529 - 538. A.V. Belozerov et. al. Eur. Phys. J., A16 (2003) 447 - 456.
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MULTICHANNEL ELECTRONIC MODULE FOR COMBAS SEPARATOR A.G. ARTUKH~,S.A. KLYGIN’, YU.M. S E R E D A ~YU.G. ~ ~ , TETEREV], A.N. VORONTSOViP2,G. KAMINSKI’23@,A. BUDZANOWSK13, J. SZMIDER3, N.I. ZAMIATIN4, D.A. SMOLIN4,N.V. GORBUNOV4, A.A. POVTOREIK04 AND P.G. LITOVCHENKO~ ’Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980, Dubna, Russia ’NC “Institutefor Nuclear Research”, NAS of Ukraine, 252650, Kyiv-22, DSP, prospect Nauki 47, Ukraine ’The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Science Radzikowskiego str. 152, 31-342, Cracow, Poland ‘Laboratory of Particle Physics, Joint Institute for Nuclear Research, 141980, Dubna, Russia A multi-channel electronics system has been commissioned for the COMBAS focal plane detector. It is designed to record multiparameter events with many potential channels, but where only a little fraction of these are active in any registered event. Custom-built 32 channel preamplifiers and 32 channel shaping amplifiers process the amplitude signal from each strip of 32 strip detectors and electronics system records both amplitude and arrival time for each signal within the event. Additional custom-built analog CAMAC module - 32 channel multiplexer, allows to use only one ADC channel for one strip detector and also generates the numbers of acted strips. Thus the processing load reduces considerably. The last module in the spectrometric line is ADC (analog to digital converter), which using the multiplexer information converts only useful information. An additional module is custom-build Multiplicity Logic, it organizes a fast trigger to determine a potentially interesting multi-parameter event. The electronic performance is illustrated with results from heave ion inelastic scattering recorded with position sensitive strip detectors.
1.
Introduction
To increase the coincident detector efficiency for breakup reactions, a large area, position sensitive silicon detectors with 32 individual strips each 2 mm by 65 mm were used as focal plane detectors of COMBAS separator (Fig.1). This
@
E-mail address: Grzegorz.Kaminski@if .edu.pl
580
design increases the solid angle, diminishes the dead space between detectors, and improves the angular resolution. A major advantage of multi-strip detectors is that events with high multiplicity may be detected since each strip acts as an independent detector. The multi channel electronics system and the appropriate data acquisition system has been designed and built to satisfy the demand of multi-particle spectroscopy measurements [ 11. - ---_-.
_/
/'
Fiml selecbon
by
BP
S s m d seledlon by BP
/ /'
, '
\.__ '\
I "B (33 A MeV)
\
Figure 1. In-flight fragment separator COMBAS (FLNR JINR, Dubna) with focal plane detector system for correlation experiment.
2.
Electronics system
There are three main stages of processing in this system (Fig.2): Preamplification: detector signals are preamplified in the 32 channel preamplifier connected directly to the detector and mounted inside the vacuum chamber. From here preamplified signals go through individual coaxial cables to the shaping amplifier. Amplification and analog multiplexing: a 32 channel shaping amplifier mainly works as shaper with shaping time equal to 0,5 ps and also it marks all acted strips. It is closely linked to a 32 channel analog multiplexer which sends all 32 signals into one sequence, generates the numbers of acted strips and sends this numbers in parallel with main sequence. Digital conversion and readout: ADC obtains analog sequences from multiplexers and converts only significant information. So it is possible to save a lot of registration time on reading through CAMAC only useful information.
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An electronics system has been designed and tested with detector array in COMBAS separator focal plane, in “B* breakup experiments “B (33 A MeV) + 12C(0.28 mg/cm2) + (’Li + 4He) + I2C* (Fig.3).
A.
I
Figure 2 Multichannel electronics line for one 32 strip detector
Figure 3. The 2-dimensional plot E(AE), where AE - one of the strips D1.2 and E - D1.3
Acknowledgments This work was supported in part by grant RFBR-01-02-16427
References 1. A.G. Artukh, G.F. Gridnev et al, Proc. On International Symposium on Exotic Nuclei “Exon-2001”,Bakal lake, 24-28 July 2001, World Scientific Pages 682-689
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NEW LINES OF RESEARCH WITH THE MAGNEX LARGEACCEPTANCE SPECTROMETER F.CAPPUZZELLO', A.CUNSOL01s2,A.FOT12*3,A.LAZZARO'32,S.E.A.0RRIG0'92, J.S.WINFIELD', C.NOCIFOR04, H. LENSKE' 1. INFN - Laboratori Nazionali del Sud, Catania, Italy 2. Dipartimento di Fisica e Astronomia, Universita di Catania, Catania, Italy 3. INFN - Sezione di Catania, Catania, Italy 4. GSI, Darmstadt, Germany 5. Institutf i r Theoretische Physik, Universitat Giessen, Giessen, Germany A new design of magnetic spectrometer, MAGNEX, is under construction for the INFNLNS, Catania. The unique features of MAGNEX are its solid angle of acceptance (5 1 msr), momentum acceptance (+ 1O%), overall momentum resolution of 1/2000 and mass resolution of 1/200, together with a focal plane detector having a low detection threshold (0.5 MeV/A). The spectrometer is based on a 55" bend angle dipole magnet with mean radius of 1.6 m. It is designed for a maximum rigidity of 1.8 Tm. Despite the large acceptance, a good momentum resolution is achieved by a combination of careful ionoptical design and software ray-reconstruction. The latter depends on three things: the availability of detailed field maps, the precise measurement of position and angle by the detection system, and the solution to high order of the equation of motion based on, in our case, the program COSY INFINITY. The MAGNEX spectrometer, connected with the broad choice of both stable and radioactive beams at the LNS, will provide new opportunities for, e.g., spectroscopy of weakly-bound nuclei by direct reactions, reaction mechanisms with large isospin and nuclear astrophysics.
1. Introduction
The concept and layout of the MAGNEX spectrometer has been described in refs. [l-31. In brief, it is a large-acceptance device (50 msr) based on a vertically-focussing quadrupole and 55' bend-angle dipole. The angles and profiles of the dipole entrance and exit pole faces are used to correct partly the aberrations in the ion-optics. Further corrections are performed in software by a ray-reconstruction technique, resulting in an expected average momentum resolution of about 1/2000. The ray-reconstruction is based on a solution to high orders of the equation of motion using, in our case, the program COSY INFINITY of Berz [4].Detailed measured field maps in five vertical planes are used. A position-sensitive timing detector (PSD) between the target and quadrupole gives both the angle of the scattered particles and a start signal for the time-of-flight. The focal plane detector (FPD) measures positions and angles and provides particle identification [5]. The spectrometer will be commissioned at the beginning of 2005. In this paper we focus on some of the experimental opportunities opened by the advent
583
of MAGNEX at the LNS, including its use with radioactive ion beams (RIB) from the ISOL-type EXCYT facility [6]. In particular a deeper analysis of the MAGNEX capabilities is shown in reference to a specific RIB experiment simulated with our Cosymag routine libraries [7]. 2.
Direct Reactions with Tandem Stable Beams
Recent studies have shown how the (7Li:Be) reaction at Tandem energies is a powerful tool to explore the excited states of light neutron rich nuclei [8]. In ref. [9] an energy resolution of 50 keV has been obtained in the excitation energy spectrum of "Be by detecting the 7Be ejectiles with the Split-Pole magnetic spectrometer at IPN-Orsay. The observation of narrow resonances embedded in the continuum (BSEC), well beyond the "Be neutron emission threshold, has raised the question whether this phenomenon is connected to the exotic properties of the "Be structure. It has stimulated similar studies for different ions such as I2B, I4B, ''C and 7He. A clear indication of the BSEC has been obtained [10,1 I ] for the "C nucleus, which presents many similarities with "Be. It is worthwhile to note that both "Be and I5Chave a structure of three neutrons coupled to an integer number of a particles. Such resuits have determined the development of sophisticated microscopic theories based on QRPA. The microscopic model of Dynamical Core Polarisation (DCP) of refs. [ 12,131, which accounts for the correlation of core phonons with single particle excitations of the external neutron, predicts the existence of narrow resonances at low excitation energy for light neutron rich odd nuclei. The calculated response functions have shown how these narrow resonances cannot be explained by two quasi-particle (2QP) excitations and need a broader phase space including at least 4QP configurations. This is due to the presence of a weakly bound, and thus easily polarizable core, e.g., ''Be for the "Be nucleus and 14C for the "C, that effectively exchange energy with the unpaired neutron. Consequently the energy produced in a collision, proceeding through a direct mechanism, can be directly transferred to the core, the valence neutron being weakly influenced. In the extreme case of BSEC the valence nucleon remains bound to the core even when the energy transferred to the whole nucleus by the direct process would be enough to extract it. A systematic study of the Na + 3n nuclei up to the Iron-Nickel region via the (7Li,7Be)reaction at Tandem energies would allow us to follow the evolution of this phenomena as a function of the mass, charge and binding energy. This would greatly help to understand the microscopic origin of the BSEC and to clarify whether this phenomenon can be described within the general framework of mean field theory or many-body correlations are unavoidable. This study has been hindered up to now by the reduction of the cross sections for heavier systems at the low incident energy necessary for achieving the high resolving power in the energy spectra. The use of the MAGNEX spectrometer with its
584
large solid angle (more than 25 times the Split-Pole) will open new opportunities in this field. The energy resolution achievable will be of the order of 50 keV, thus allowing a clean separation of most of the excited states.
3.
Direct Reactions with EXCYT RIB'S
The (7Li,7Be)studies described above may be extended to the use of RIB'S from the EXCYT facility. Inverse kinematic reactions and the large acceptance of MAGNEX would be used to overcome the low secondary beam intensity. One of the first beams from EXCYT will be 'Li [14]. We can use this beam to study the neutron-rich nucleus 'He. The proposed experiment is 7Li(8Li,7Be)8He,i.e., the 8Li beam bombards a 7Li target and 7Bereaction products are detected in the focal plane of the spectrometer. Besides the possibility of exciting bound states in the continuum, in a similar manner as for "Be, the low-lying level structure of 'He could be more firmly established. In the most recent evaluation of mass 8 nuclei, three excited states in 'He (&,, = 2.14 MeV) are listed at 3.1, 4.36 and 7.16 MeV [15] with evidence for another at 6.03 MeV [16]. In fact, it is not clear whether the 2+ first-excited state at 3.1 MeV is a single state or two states: some groups have reported a level near 2.7 MeV [17,18], others give the excitation energy as about 3.6 MeV [19-211 (in addition, Belozerov et al. [17] report a level at 1.3 MeV). With a high resolution spectrum from a generally unselective reaction as (7Li,7Be),one might expect either to excite both levels (if there are two) or determine a more precise energy of a single level. For the above 'He experiment a complete simulation accounting for the ray reconstruction technique [7] is presented in the following. In the simulation a beam of 57 MeV incident energy and of 3 x lo5 pps intensity is assumed, according to the preliminary predictions for the EXCYT facility [14]. From the systematics for light nuclei of the ('Li?Be) reaction at 57 MeV, the cross section for the Gamow-Teller transition from the 'Lig,(2+) ground to the 8He(2') excited state is estimated to be about 100 + 200 pb/sr at forward angles. The weak intensity of the beam and the low value of the cross section put severe constraints on the solid angle and target thickness, in order to perform the experiment in a reasonable time. For this purpose the large solid angle (50 msr) of MAGNEX, connected to the precise reconstruction of the scattering angle and the effective compensation of the kinematic effect, are key elements for the feasibility of this experiment. The kinematic broadening of the peaks in the energy spectra due to the large scattering angle interval (from 1O to 15') is more than 2 MeV when the spectrometer aperture is fully open. Under these conditions the counting rate per each micron of target thickness is about 0.8 counts / 1 pm x hour, while the effect of target on the energy resolution is about 28 keV I 1 pm. To limit this contribution to the energy resolution to within 200 keV a counting rate of about 6 counts I hour is achievable. This leads to the necessity to fix the spectrometer in the same conditions for at least 100 hours to
-
..
585
get enough counts in the GT transition peak. In Fig. 1 the initial conditions of the simulations are shown for a sample of 20000 particles distributed over 12 simulated levels, which corresponds to about 300 hours of measurement. The ground and the known excited states at 2.7, 3.6, 6.03, 7.16 MeV are visible. A broad resonance known at 4.5 MeV is also included. In the simulation the 2.7 and 3.6 MeV states are both considered to exist, and the experimental goal is to resolve them. For each 'He excitation two lines are present in the spectra, arising from either the population of the ground or the 0.429 MeV bound first excited state of the 'Be ejectiles. The strong kinematic effect is evident in the plot, appearing as a noticeable curvature of the kinetic energy lines as a function of the scattering angle. In the right panel of Fig. 1 the projected kinetic energy spectrum is shown, emphasizing the need to measure the scattering angle with good precision. It is important to bear in mind that any possible angular segmentation of the data is hindered by the very low counting rate of this experiment. The distribution of particles along the focal plane after tracking through the spectrometer is shown in Figure 2. In the simulations all the active and dead layers are included realistically. To reduce the kinematic effect, the detector has been shifted to the predicted location of the focal plane (as allowed by MAGNEX), and the quadrupolar and sextupolar surface correction coils are used. The scatter plot and the one dimensional spectrum give an idea of the difficulties with the large acceptance condition if trajectory reconstruction is not employed, even with an optically-refined spectrometer such as MAGNEX [3]. The position resolution is obviously not enough to distinguish the peaks at 2.7 and 3.6 MeV.
M
4
c
4.
0
m
Kinetic energy (MeV)
Kinetic energy (MeV)
Figure 1. Left panel; initial conditions (after the target) for the simulation of the 'Li('Li,?Be)'He reaction at 57 MeV. Right panel; initial energy spectrum.
586
In Figure 3 the result of the application of the ray reconstruction method is shown. The order of reconstruction has been set to the ll*. The reconstructed kinematic scatter plot clearly indicates the power of this technique in compensating both the kinematic effect and the effects of residual aberrations that were observed in Figure 2. The excitation energy spectrum shows the clear separation of the peaks in the region of interest around 3 MeV. The broadening of the peaks is almost entirely due to the effect of target thickness which is unavoidable and not dependant on the instrument itself.
8
-E
v
W
M
Figure 2. Left panel; scatter plot at the focal plane for the 'Li('Li?Be)'He panel; focal plane position spectrum.
'He excitation energy
reaction at 57 MeV. Right
'He Excitation energy (MeV)
Figure 3. Reconstructed scatter plot (left) and energy spectrum (right) at the target for the 7 , 8 .7 Li( Li, Be)'He reaction at 57 MeV. The 1I* order algorithm of Cosymag has been used.
587 References 1.
2. 3. 4. 5. 6. 7.
8.
9. 10.
1 1.
12. 13. 14. 15. 16. 17. 18. 19. 20. 2 1.
A.Cunsolo et al., Proc. Workshop Giornata EXCYT, Catania (1996) pp. 143-161, Proc. Workshop I1 Giornata EXCYT, Catania (1997) pp. 71 -80. A. Cunsolo et al., Proc. gth Intl. Conf. on Nucl. Reaction Mechanisms, Varenna, 2000, p. 66 1. A. Cunsolo et al., Nucl. Instr. and Meth. A 481 (2002) 48; 484 (2002) 56. M. Berz, Nucl. Instr. and Meth. A 298 (1990) 473. A. Cunsolo et al., Nucl. Instr. and Meth. A 495 (2002) 216. G. Ciavola et al., Nucl. Phys. A616 (1997) 69c. A. Lazzaro, PhD thesis, University of Catania, (2003), A. Lazzaro et al., Proc. 71h Int. Computational Accelerator Physics Conf., East Lansing, Michigan, Oct. 2002 (M. Berz, ed., IOP Publishing, in press). F. Cappuzzello et al., Proc. Int. Conf. on Nuclear Physics at Border Lines, Lipari, May 2001 (G. Fazzio, G . Giardina, F. Hanappe, G. Immb, N. Rowley, eds., World Scientific, Singapore, 2002) p. 64. F. Cappuzzello et al., Phys. Lett. B 516 (2001) 21. C. Nociforo, PhD. thesis, Univ. of Catania (2002) and Proc. XXXVII Zakopane School of Physics, 2002, Acta Physica Polonica B 34 (2003) 2387. S. Orrigo et al., these proceedings, and F. Cappuzzello et al., EuroPhys.Lett. 65 (2004) 766. G. Baur, H. Lenske, Nucl. Phys. A 282 (1977) 201. H. Fuchs et al., Nucl. Phys. A 343 (1980) 133. G. Cuttone, Workshop on forthcoming facilities at LNS, Catania, 2003. J.H. Kelley et al., Energy levels of light nuclei A = 8, preliminary version #1, TUNL Nuclear Data Evaluation Group, February 2002. H.G. Bohlen et al., Prog. Part. Nucl. Phys. 42 (1999) 17. A.L. Belozerov et al., Izv. Akad. Nauk. SSSR Ser. Fiz. 52 (1988) 100. K. Markenroth et al., Nucl. Phys. A 679 (2001) 462. A.A. Korsheninnikov et al., Nucl. Phys. A 588 (1995) 123c. W. von Oertzen et al., Nucl. Phys. A 588 (1995) 129c. Y. Iwaka et al., Phys. Rev. C 62 (2000) 0643 1 1.
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THE MODIFIED MINI-FOBOS SETUP D.V. KAMANIN, A.A. ALEXANDROV, I.A. ALEXANDROVA, S.V. DENISOV, E.A. KUZNETSOVA, S.V. MITROFANOV, V.G. TISHCHENKO, A.N. TYUKAVKIN, I.P. TSURIN, YU.E. PENIONZHKEVICH, E.A. SOKOL Joint Institute for Nuclear Research, 141 980 Dubna. Russia YU.V. PYATKOV Moscow Engineering Physics Institute, I I5409 Moscow, Russia S.V. KHLEBNIKOV, T.E. KUZMINA Khlopin-Radium-institute.194021 St. Petersburg, Russia
YU.V. RYABOV Institute for Nuclear Research RAN, I I7312 Moscow, Russia S.R. YAMALETDINOV Department of Physics of University of Jpaskyla. FIN-40014 Jpaskyla,
The TOF-E-Z spectrometer mini-FOBOS being the modified mobile subset of the wellknown spectrometer of charged particles FOBOS is turned into operation. It is desired for the study of multi-cluster decays of nuclei in the wide range of masses and excitation energies. The first experiments have been performed this year.
1. Introduction
The present paper is devoted to the new mobile double-arm TOF-E-Z spectrometer of the charged fragments named the Modified Mini-FOBOS setup (MMF) turned into operation this year. It inherits the standard detector modules of the 47t spectrometer of charged particles FOBOS [l]. The main feature of the FOBOS detector modules is the independent identification of the mass and charge for each fragment without any kinematical assumptions on the reaction mechanism. This makes possible precise study of the reactions in the most general case - non-binary processes. These excellent capabilities in registering charged products of nuclear reactions have been confirmed in different experiments on the study of the multi-body decays both with HI beams [2] and with the spontaneously fissile sources since the new type of decay, namely, collinear cluster tripartition (CCT) has been intensively studied [3].
589
2. Experimental Setup
The general idea of the MMF consists in using a small reaction chamber, which might be unique for each experiment and the basic universal system maintaining the detectors. Depending on demands of the particular experiments the additional detectors can easily be installed (gamma-detectors, neutron-detectors, forward-angle arrays, etc.). In particular, the high-efficiency neutron counter has been coupled to the spectrometer during the winter experiment this year dedicated to the search for the collinear cluster tripartition (CCT) (Fig. 2, left). The FOBOS-modules are fit to the universal reaction chamber of 44 cm in the diameter by means of the adapting cones at the angles of 65", 90" and 135' available with respect to the beam axis for both detector modules (fig. 1). Such a configuration of the spectrometer is well suited for study of heavy-ion induced reactions and for spontaneous fission as well. The particle flight-path of 50 cm in this configuration is the same as at the FOBOS setup, therefore, we keep the resolution parameters specific for FOBOS. Thus, all the advantages of the FOBOS spectrometer (except of the declared geometrical efiicigilcy closed to 4n) have been adopted entirely by the MMF. earn axis (both direction)
Alternative target nodes.
Mini
uB 0 S
F 0
Figure 1 . The layout of the modified mini-FOBOS spectrometer. Dimensions age given in cm.
This mechanical construction has been initially built as the stand for testing the FOBOS detector modules [4],however, a number of modifications have
590
been performed. The arm-angle of 90" is facilitated by the specially designed wedge flanges and by the alternative target (source) position (fig. 2, right). The completely new independent gas-supplying system which can support several pairs of detector modules has been developed. Besides this, modifications have concerned the start detectors, the electronics and the data acquisition system, which can also work now on the Windows platform. The comprehensive numerical model of the neutron registration channel has been developed [ 5 ] . In order to improve the mass calibration in the framework of the TOF-E method the procedure aimed to restore the fragment energy developed for the FOBOS data has been significantly refined [ 6 ] .This is a difficult task for slow CCT fragments since they loose up to 70% of their initial energy in entrance foils. Although the mass resolution achieved in the frame of the TOF-E method is less good than the corresponding value obtained by the TOF-TOF analysis but in the case of incomplete kinematics the TOF-TOF method becomes unusable. We also introduced independent estimate of the nuclear charge of the fragments by means of the drift-time [7]. The latter is especially critical for the heavy and slow fragments since the conventional Bragg-spectroscopy cannot be used.
Figure 2 The view of the spectrometer during the experiments performed this year. The special short central chamber is used to fit into the neutron barrel (left). The detector arms are set opposite to each other by means of the wedge flanges, gas supplying system is seen behind the spectrometer (right).
591
The most intriguing task among the modifications of the mini-FOBOS consists in the resolution of the multiple hits in the detectors since it could be, in particular, a direct hint to the multiple collinear decays and deliver additional information. The signals from the avalanche counters are specially processed in order to reveal particles, which hit the detectors shortly after each other. This method is already under development at the MMF spectrometer. The detector system of the MMF is quite similar to that of the Berlin Reaction Spectrometer (BRS) [S] also the present object of study (multi-cluster decays, collinear pre-scission configurations) and the experimental methods (missing mass, kinematical coincidenses) of the MMF are similar to those of BRS [9]. However, the longer TOF base of the MMF allows more accurate mass analysis and the transformable geometry of the MMF permit to study both the symmetric and far asymmetric projectile-target configurations. 3. Conclusions
The MMF has been already successfilly tested in experiments and it is provided with all the necessary equipment, software and also with the men power. We plan to exploit the advantages of the MMF setup in forthcoming experiments dedicated to the decay study of super- and hyper-deformed heavy nuclei focused, in particular, on collinear cluster tripartion in the reactions with light particle beams and on the molecular states in N=Z nuclei in HI induced reactions. References 1. 2. 3. 4.
5. 6. 7. 8. 9.
H.-G. Ortlepp et al. NIM A 403 (1998) 65-97. V.G. Tishchenko et al., Nucl. Phys. A 712 (2002) 207-246. Yu.V. Pyatkov et al., Contribution to this Proceedings M. Andrassy et al, FLNR Sci. Rep. 1989/90, JINR E7-91-75, Dubna 1991. p. 175 D.V. Kamanin et al., Physics ofAtomic Nuclei, v. 66 (2003) 1655 Yu.V. Pyatkov e f al., Preprint JINR E15-2004-65 Yu. V. Pyatkov et al., Physics ofAtomic Nuclei, v. 66 (2003) 1631 S. Thummerer et al. Nuovo Cimento 111 A (1998) 1077 W. von Oertzen, Contribution to this proceedings
592
GAS FEEDING SYSTEM SUPPLYING THE U-400M CYCLOTRON ION SOURCE WITH HYDROGEN ISOTOPES A.A. YUKHIMCHUK, V.V. ANTILOPOV, V.A. APASOV, YU.1. VINOGRADOV, A.N. GOLUBKOV, YE.V. GORNOSTAEV, S.K. GRISHECHKIN, A.M. DEMIN, S.V. ZLATOUSTOVSKI, V.G. KLEVTSOV, A.V. KURYAKIN, I.N. MALKOV, R.K. MUSYAEV, V.I. PUSTOVOI Russian Federal Nuclear Centre - All-Russia Scienti3c Research Institute of Experimental Physics (RFNC-VNIIEF), 607188 Sarov, Nizhny Novgorod Reg., Russia V.V. BEKHTEREV, S.L. BOGOMOLOV, G.G. GULBEKIAN, A.A. YEFREMOV, A. ZELENAK, M. LEPORIS, V.N. LOGINOV, YU.TS. OGANESSIAN, S.V. PASHCHENKO, A.M. RODIN, YU.1. SMIRNOV, G.M. TER-AKOPIAN, N. YU. YAZVITSKI Joint Institute for Nuclear Research, 141980, Dubna, Moscow Reg., Russia
Automated system feeding into ion source hydrogen isotopes as molecules with preset ratio of the fluxes is described. The control system automatically maintained the working parameters and provided graphic and digital representation of the controlled processes. The radiofrequency (RF) ion source installed at the axial injection line of the cyclotron produced ion beams of H D , H T , D T , D2W, etc. At a several months DT' beam acceleration the tritium consumption was less than 108 Bqhr. The intensity of a 58.2 MeV triton beam (T'ions) extracted from the cyclotron chamber was about 10 nA.
1.
Introduction
Study of exotic light nuclei and nuclear systems at the boundary of neutron stability is currently one of the central trends in the nuclear structure research. Transfer reactions with radioactive nuclear beams offer a good possibility for such investigations. Creation of the radioactive beam accelerating system (DRIBs project) called for obtaining molecular ion beams of hydrogen isotopes (HI), including tritium. This enabled us to proceed to experiments on studying resonance states of 4H and 5H nuclei, produced in reactions t+t-+5H+p, t + t j 4 H + d and t+d+4H+p [ 1,9,lo]. A 58.2 MeV triton beam was produced at the U-400M cyclotron of Laboratory of Nuclear Reactions, JINR. The cyclotron accelerated single
593
charged ions TD' to energy of 19.4 MeV/nucleon. The molecular ion beam dissociated on a thin graphite foil and, as a result, the T' ions were extracted from the internal cyclotron orbit into the external beam channel. To deliver the triton beam to the target the separator ACCULLINA was employed [2], where the ions were selected with maximum energy and trajectory angle spreads not greater than +0,25% and *7 mrad, respectively. Assuming that the required tritium ion beam intensity at the target should be about 10' ion/s and taking into account the beam losses at the transportation with the selection in energy and angular divergence, we supposed that maximum tritium current of the accelerated triton beam should constitute 10 nA (6.10'' ionh). Both the tritium feeding system and the RF ion source itself must be able to produce as much tritium ion beam current as possible at condition that a minimum consumption is achieved for the tritium radioactive gas. The purity of the source vacuum chamber and cyclotron environment must be assured for a long-term operation of the entire system. This paper presents a detailed description of the system of HI feeding into the cyclotron ion source and HI mixture disposal upon the work completion. A brief description of the RF ion source is presented along with the results of its testing and the acceleration of a TD' beam at the cyclotron. 2.
HI Feeding System and Its Operation
2.1. General Considerations
The amount of tritium involved into the experiment had to be restricted to the value of minimal significant activity (MSA), lo9 Bq or 0.01cm3 at normal conditions, according to the radiation safety standard NRB-99. It was because the lack of free space at the cyclotron did not permit the installation of any exhaust hoods, while the radiation level obtained at the time of the cyclotron operation excluded a possibility for the use of ionization chambers to monitor possible tritium leaks. At the same time, one had to adjust HI fluxes going to the ion source at a rate of up to several cm3/hour. Therefore, at the ion source adjustment the HI feeding system (HIFS) must supply the protium and deuterium fluxes up to several cm3/hour and, in the working mode, the deuterium and tritium fluxes should be regulated within a limit of 5.10-3cm3/hour. The layout of the hydrogen isotope feeding system is presented in Fig. 1. HI feeding unit (HIFU) provides for the feeding of hydrogen isotope molecules of preset composition to the ion source of U-400M cyclotron along
594
with the tine adjustment of the isotope composition. A recovery system (RS) is designed to perform technological operations with HIFS gas lines and purifL the cryogenic pumps of the U-400M cyclotron ion source.
Figure 1. HIFS basic scheme. HIFU - HI feeding unit; RS -recovery system; BSI-BS3 - HI sources; BS4-BS5 - traps; HI, H2 - leaks; D1 - vacuum gauge (PRK261); D2 - pressure gauge (TRK261); D3 - pressure gauge (CMR261); NJ - roughing pump (BOC EDWARDS GVSP30); VP - bellow vacuum valves; T - thermocouples.
The automated monitoring and control system (MCS) fixes the status of the HIFS elements, maintains the working parameters, accomplishes the graphic and digital representation of controlled processes, the experiment logging, and the control of the system elements made from the cyclotron control panel. 2.2, HI Sources and Traps
The feeding system includes three thermo-desorption sources - BS1, BS2 and BS3. Hydrogen isotopes in the sources are stored in chemically bound state on 238U.All the sources have similar design: they are manufactured as cylindrical ampoules with an external electric heater, contain 3 g o ~ each ~ and ~ are ~ capable of generating up to 380 cm3 of gas.
U
595
The deuterium source BS3 is used at the ion source adjustment and in the working mode. The protium-deuterium source BS2 is used to tune the cyclotron and the ACCULLINA separator. The tritium source BS1 is used in the working mode of the cyclotron when tritium molecular ions are being accelerating. BS2 and BS3 sources are positioned outside the HIFU hermetical volume (Fig.l), whereas the B S l source is placed inside the hermetical volume of the feeding system. Peculiarity of the tritium source BSI is its body made as an auto-fastened vessel with an evacuated hydrogen barrier [4,5]. The vacuum barrier is pumped with an autonomous titanium getter. The equilibrium pressure of uranium tritide at room temperature does not exceed 10-3 Pa, and its decomposition temperature, at which the equilibrium pressure exceeds the standard atmosphere, is greater than 650 K. The design of the BS1 source and hermetic volume HV of the feeding system FS provides for three protection frontiers. Taking into account the properties of uranium tritide, one could regard the BSl source as a closed source of ionizing radiation. Traps of two types, one employing Ti and another filled with the intermetalloid compound Zr(Vo8Cro2)2, are used in the gas recovery system. Titanium traps have a high specific capacity for hydrogen and show low equilibrium pressures of hydrogen gas above the hydride. They are effective in the case of pure HI disposing. But the presence of such admixtures as 02,N2, CO, C02,CH, etc. in the gas decreases the titanium sorption capacity and its hydrogen absorption rate. Therefore, in the recovery system the intermetallide Zr(Vo*Cr02) traps are employed in series with the titanium traps. Zr(Vo8Cro2)2possesses less sorption capacity, but is by far less sensitive to admixtures [7]. The trap BS4 contains 300g of Zr(Vo8Cro2)2alloy, and the trap BS5 carries 250g of titanium. The working temperature of the traps is 20O+25O0C. Such temperature results in a high hydrogen absorption rate, while the equilibrium pressures of HI above the corresponding hydrides are rather low yet. Thus, practically complete absorption of HI is attained. In the range of the working temperatures (200+250°C) the titanium specific capacity makes about 350 cm3/g at an equilibrium pressure of 1.10-2Pa. The specific capacity of Zr(Vo8Cro2)2 is 50+70 cm3/g at an equilibrium pressure of 1.10’’ Pa. Thus, the titanium trap absorbs more than 100 liters of gas. This amount is 100 times as much than the HI sources could ever produce.
596
2.3. Leaks
To control the gas flux leaks made of made with the use of a nickel capillary (0.45 mm diameter and 0.1 mm wall) is used (see Fig.2). The leak operates in a mode of the HI diffusion spilling over from the leak ampoule volume (1) through the capillary wall (2) into channel (A), directly connected with the ion source through appropriate gas lines. The leak flow capacity is determined both by the material of the diffusion element and by geometry, and their specific operating conditions: the HI pressure in the ampoule and the temperature of the nickel capillary. The lack of the hydride embitterment of nickel in a rather wide temperature range allows one to obtain a wide range for flux control. Also, relatively low specific hydrogen permeability of nickel, as compared to palladium alloys, makes possible the fine tuning of the HI fluxes. In the temperature range of 400t5000°C the specific permeability of nickel varies between 1.90.10-5 and 5.71,10-5 and between respectively, for protium and 1.15.10-5 and 3.45.10-5 cm3~mmlcm2~c~atmi’2, deuterium [3].
B
4h Figure 2 Leak diagram 1 - leak body, 2 - nickel capillary, 3 - electrlc connector
597
The capillary is heated directly by electric current passing through it. For that reason, one end of the capillary is properly isolated from the unit body. The low nickel permeability for hydrogen at room temperature makes it possible to employ, in the regular operation mode, the leak filled with the initial HI. Switching into the working mode implies only heating the capillary to the working temperatures. This takes a few minutes only. The peculiarity of this leak is that the HI flux at its output is almost independent on the HI pressure, but it is a function of the electric current value, applied to the capillary. 2.4. Feeding HI to Ion Source and Their Disposal
The system operates in the following way. The required source (e.g. BS2) is connected to the leak volume pumped in advance and is heated up to the temperature providing the desired pressure of the chosen HI. The temperature dependences of the HI equilibrium pressure above uranium hydride have the following forms: lgP(Pa)=-4590/T+l1.59; lgP(Pa)=4500/T+l1.56 and lgP(Pa) = -4471/T+11.73 for protium, deuterium and tritium, respectively [6]. To fill the volume with gas under a pressure of about 0.1 M Pa a source temperature of (680t705)K is sufficient. In process of the storage at room temperature the tritium pressure makes about 7.10-4Pa. Upon attaining the preset pressure in the volume the source valve is closed and its heating is turned off. Heating of leak H2 (see in Fig. 1) turns on, and its temperature is regulated to set a needed deuterium flux of 0.1-5 cm3/hour. Then a minor amount of DT molecules (5~10-3...0,1cm3/hr)is added to the deuterium flux from the BSI source, according to the scheme outlined above. The flux value is set to provide the necessary beam intensity for the DT' (HT', HD') ions. The needed intensity of the ion beam could be adjusted by tuning the temperatures of both leaks, H1 and H2. When the work with the beam is finished, the valves of appropriate sources are opened, and the gas from the buffer volumes is absorbed by these sources. Thus, it enables one to employ the multiple uses of the gas, and, what is even more important, to reduce to a minimum the amount of tritium coming into the recovery system. When pressure in the volumes reaches -lo-* torr, the valves of the sources are closed and the volumes are pumped out and the leaks are degassed through the recovery system RS, the gas lines and the buffer. Upon attaining a vacuum of about torr in the gas lines of HIFU the leak heating is turned off and all the valves are closed. HI disposal is mainly implemented by means of reverse placing of the unspent isotope to the source, from which it was
598
evolved. However in some cases (while "washing" and pumping the gas lines contacted with tritium) the disposal is implemented by the HI absorption in the BS4 and BS5 traps.
2.5. Monitoring and Control System This is a distributed system consisting of a computer and a set of autonomous network modules, interconnected via RS-232 and RS-485 interface units. The control computer, located in the room of the cyclotron control panel, accomplishes the data acquisition and active control of the hardware. To acquire data from the sensors and to control the regulating units of remote analog and discrete input/output are used of 1-7000 series (produced by ICP DAS) with RS485 interface; vacuum sensors of Balzers Instruments with TPG-256 6-channel controller (RS-232 interface). The HI feeding system is mounted on an electrically isolated platform placed directly on the top of the cyclotron magnet. A 15 kV potential is applied to the platform. The electronics of the monitoring and control (MCS) system, providing for HIFS operation, is located in an isolated rack with the same potential applied. To provide the subsystem galvanic decoupling by data exchange channel a fiber-optic communication line is used. HIFU is powered through a transformer with isolation for 30 kV. The electronics of the tritium recovery system is located in a grounded rack. The HIFU subsystem provided temperature measuring and controlling of HI sources BSI-BS3 and absorbers BS4, BS5. Absolute error of temperature measurement in a range from 20°C to 600°C is approximately h5"C. The stabilization precision is 2-3°C. Vacuum in the gas lines of the preparation system and the hermetic volume is measured with the gauges PKR261 (Dl-D3) of Balzers Instruments having the measurement range 5.10-4-1O3 mbar. The nickel capillaries of the leaks are connected to the power amplifier (PA) giving the output current in a range from 0 to 6 A. Control voltage to the amplifier input is fed from the output of the analog module 1-7024 (14-bit DAC). The MCS was controlled by a computer program, which provided for a real time presentation, in a form suitable for the user, i.e. in a symbolic circuit, the status of the gas system units (valves), the graphic and digital presentation of monitored processes and remote control of the system elements. The program made experiment logging with the data storage to the hard disk.
3.
Tritium Ion Beam Production
To produce molecular ions a RF source was employed mounted at the axial injection line of the U-400M cyclotron [S]. The schematic view of the ion source with its electrostatic optics is shown in Fig. 3. The extracted ion beam was focused by an electrostatic lens in the object plane of the magnet, which bent the beam by 90" into the vertical injection channel. The ion mass spectrum was measured by varying the bending magnet field. The operation modes of the ion source were optimized at a test bench intended for the H2+ ion production. To test and adjust the entire system hydrogen and deuterium were fed through the feeding system into the source. Produced (D2H)' ions were injected onto the central orbit of the cyclotron, and the ion beam was accelerated to the cyclotron final radius. The ion spectra obtained during the feeding of the mixture of deuterium and hydrogen is shown in Fig. 4.
L 1
Figure 3 . The schematic view of the ion source
After these tests, hydrogen was substituted by a mixture of gases that contained 99% of deuterium and 1% of tritium, and the TD' ion acceleration was started. Typical operating parameters of the ion source were as follows: RF requency of about 50 MHz, a power of 40+50 W, a sweeping electrode voltage of 0.5 kV, a focusing electrostatic lens potential of 10 kV, a beam injection energy of 15.9 keV. The ion spectra obtained during the adjustment of the injecting and accelerating systems are shown in Fig. 5.
600
H+ I
-l.-ll-
4 6 8 10 BP (arbitrary units)
1!
Figure 4.Ion spectrum obtained with the deuterium-hydrogen mixture
40 n
330a
v
G
-
20-
u5 10--
“0.6 c:
u 0.2
t
6 8 10 12 Bp (arbitrary units) Figure 5. Spectra of deuterium-tritium ions: top - full scale; bottom - magnified scale.
60 1
During the many months acceleration of the DT' beam both the gas feeding system and the ion source operated reliably and stably. The consumption of the basic gas, deuterium, was 1.4 cm3/hr. The consumption of the deuterium-tritium mixture was about 0.06-0.1 cm3/hr resulting in a tritium consumption less than 10' Bq/hr. The value of the triton beam current (T') extracted from the cyclotron was about 10 nA. The 58.2 MeV triton beam was delivered to the object plane of the ion-optical system of the ACCULINNA separator without a visible loss in the beam intensity. The system of HI feeding operated successfully in experiments on studying the resonance states of the 4Hand 'H nuclei produced in transfer reactions of the triton beam colliding with the liquid tritium and deuterium targets [ 1,9,10]. Acknowledgments
Partial support of the work by INTAS, grant No. 03-05 1-4496 is acknowledged. References
1. 2. 3. 4. 5. 6. 7. 8.
.9. 10.
M.S. Golovkov, et al., Phys. Lett., B556, 70 (2003). A.M. Rodin, et al., Nucl. Znstr. Meth., B204, 114 (2003). V.A.Goltsov. Vopr. Atomnoy nauki I techniki. Ser.: Atomno vodorodnaya energetika. Issue l(2). Moscow, (1977), p. 65. N.S.Ganchuk et al., J. Moscow Phys. SOC., 9, No. 4,289 (1999). A.M. A.A. Yukhimchuk et al. RF patent Nc2136064, Bul.N? 24,27.08.99. N.S. K.Mackey. Chemical compounds of metals. Moscow: Mir (1968). A.N.Perevezentsev, B.M.Andreev, 1.L.Selivanenko. Purity materials, 1, 122 (1990). G.G. Gulbekian, eta]., Proc.l4* Int. Conf. on Cyclotrons and their Applications, Cape Town, South Africa, 1995, Ed. J.C.Cornel1, World Scientific, Singapore, 1995, p. 95. M.S. Golovkov, et al., Phys. Rev. Lett., 93,26501 (2004). S.I. Sidorchuk, et a]., Phys. Lett., B594, 54 (2004).
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605
STATUS OF THE SPIRAL2 PROJECT AT GANIL REMY ANNE GANIL BP 55027, 14076 Caen Cedex 5, France FOR THE SPIRAL2 GROUP
The spiral 2 project is now in the Detailed Design Study phase. This paper presents the progress of the studies concerning the main components of the future facility, except the safety aspects. This second phase, based on the “LINAG Phase 1” report [I], in which were defined the main orientations, started in January 2003 and will be achieved by the end of 2004.
1. Specifications of the project and summary of the physics objectives [2].
The SPIRAL 2 project aims at delivering high intensities of rare isotope beams by adopting the best production method for each radioactive beam. The unstable beams will be produced by the ISOL “Isotope Separation On-Line” method via a converter, or by direct irradiation and by in-flight techniques. The combination of all these techniques (i.e. via fission induced by fast neutrons in a uranium target or by direct bombardment of the fissile material, or via fusion-evaporation with unstable beams or heavy ion beams), will allow to cover broad areas of the nuclear chart. In addition to fundamental research in nuclear physics, the SPIRAL2 facility will also offer a high performance multidisciplinary tool, especially in fields of science requiring high fluxes of neutrons, such as material sciences, atomic, plasma and surface physics [3]. The specifications resulting from the physics needs are summarized below: 1.l. Driver and primary beams
0 0 0
the driver must deliver deuterons up to 40 MeV with a beam current up to 5 mA and also heavy ions with beam currents up to 1 mA and 14 MeVh. it will also be able to accelerate ions of mass-to-charge ratio A/q=6. the beam energy will be adjustable. a fast chopper is required for some physics experiments to select one bunch out of a few hundred to a few thousand.
606
1.2. Production hall
the production rate of the radioactive beams will be based on a maximum rate of l O I 4 fissionsh of the UC, target, which defines the concrete shielding. the neutron converter has to withstand a maximum beam power of 200 kW. without the use of a converter, the primary beams will consist of deuterons or other species (such as '*4He,12,13C)and the maximum power will be limited to about 6 kW for a UC, target). different thick targets will replace the uranium target for fusion evaporation reactions with stable ion beams. different types of ion sources will be studied in order to get the best efficiency for the selected ion specie. a mass separator must deliver at least two independent beams, with a mass resolution of 250. an identification station will check the desired specie output. the isotopes will be bred to higher charge states by means of an ECR charge breeder prior to post-acceleration. 1.3. Experimental area
without post-acceleration, the secondary beams will be transported to a lowenergy experimental hall. after post-acceleration in the existing CIME cyclotron (SPIRALl), the secondary beams will be transported to the existing experimental areas . new direct beam transfer lines will directly deliver the CIME beam to the VAMOS spectrometer and the gamma detector EXOGAM caves. for the study of hsion evaporation reactions with the in-flight method, the beams from the linac will be transported to a new experimental hall. 1.4. Use of neutrons for other applications
the possibility of material irradiation studies, using the large converter neutron flux, especially for the study of the behaviour of materials considered for future fusion machines (ITER, DEMO), is being investigated. room must be left for possible installation of a pulsed neutron beam facility including an experimental hall and a -10 m long neutron line to be used for neutron-TOF like experiments. 2.
A few key-dates of the project, (Detailed Design Study Phase).
1 November 02 26-27 June 03 December 03
I
Official start of the Detailed Design Study First Technical Advisory Committee (TAC) Intermediate Report (retained technical solutions, first cost estimation of the project, first construction time schedule)
1
607
3.
Baseline of the project and additional options
The schematic layout is shown in Fig. 1. The facility can be divided into 4 main areas: accelerator driver, production station (including converter, target and ion source), secondary beam transfer lines and high energy RIB beam lines.
TfS Stafian
Irradiation Neutrons 14 MeV
Figure I . Schematic layout of the accelerator driver, converter-target-ion source station, secondaly beam transfer lines, and high energy RIB beam lines.
Section 1 listed the specifications. We can distinguish a baseline project, which contains the basic parameters (e.g. primary beam power, fission rate, etc) and additional options which can be installed from day one or at a later stage, see Table 1. The baseline project comprises 0 the driver able to deliver a full-power deuteron beam and a heavy ion beam (q/A=1/3) at intermediate intensity. 0 the production station and two plugs for RIB production, equipped with converter, low density UC, target, and ECR source. 0 A new experimental hall for high intensity stable heavy ion beams. 0 the secondary beam lines including the separator, the charge breeder and transport to the CIME cyclotron.
608 Table 1: technical upgrades that will be purchased in the course of the operation of the facility, additional options that can be installed from day one or at a later stage, and the possible future extensions
. =
. . .
Technical upgrades a high-performance heavy ion source (Q/A=1/3) Other targets (high density UCx target, targets w/o converter, heavy ion targets) Other ion sources (thermo-ionisation, febiad, laser) additional production Plugs spare cryomodules
. = *
Additional Options new experimental hall for stable ion beams RIB transfer to low energy experimental hall (LIRAT) Direct lines to GUG2 caves 2”d heavier ion line (1/6)
9
.
. =
Future extensions Energy extension (up to 100 MeV/u) Experimental area for nTOF like experiments (including low- power converter and 10 m long neutron line)
Fast chopper in the MEBT Neutron plug for material irradiation
3.1. Accelerator driver
The driver must accelerate beams of high power (200 kW deuteron beam power), different ion species and mass-to-charge ratios (deuterons as well as heavier ions with mass-to-charge ratio A/q=3), from 40 MeV deuteron energy down to energies as low as a few MeV for heavy ions. The concept of “Independently Phased Superconducting Linac” has been chosen. See schematic view in Fig.2. 1. One injector will deliver both kinds of ion beams at the energy of 0.75 MeV/u. The deuteron source (5 mA) will be either a version of SILHI-type the source or a Micro-Phoenix one. To get rid of other ion species (i.e. D2+,D3+), deuteron beam will be magnetically analysed before injection in the RFQ. For the A/q=3 ions, high confinement fields (Br 2-3 T) and high frequency (f> 28 GHz) will be required to reach high ion beam currents (1 mA Ar 12 +), which is the goal of A-Phoenix source. For the production of metallic ions (Ni, Cr, etc), R&D will be carried out on existing sources at GANIL. The RFQ cavity must bunch and accelerate the beam with a high transmission to allow for hands-on maintenance. The four-vane structure at 88 Mhz was finally chosen because the RF power consumption is the lowest. A second injector for (A/q=6) ions, including a second RFQ cavity, is planned in a second step to feed ions into the MEBT (Medium Energy Beam Transport) A fast chopper has to be inserted in the MEBT line to select one bunch from N = 10’ to lo5 bunches. It needs significant R&D effort owing to the small rise time required ( 8 ns) and can be installed later, as a short-term option.
-
609
2. Superconducting linac: The choice of short cavities, exhibiting very wide velocity acceptance, allows the optimisation of the output energy by re-adjusting the individual RF phases. Quarter-Wave Resonators have been studied and 88 MHz for the whole linac was the retained solution, as well as the use of 2 families of QWR resonators (p=0.07 and fk0.12) for the following reasons: 0
0 0
0
the total number of cavities is low. there is no frequency jump which would require longitudinal matching. the cavity aperture is potentially large. the frequency is identical for all RF sources. the cost is slightly reduced. aa Mnz
aa
Mnz
Eacc = 6-7 MV/m
Figure 2: schematic view of the accelerator
In addition, the focusing by means of room-temperature quadrupoles, instead of superconducting solenoids, resulting in one cryostat per focusing lattice, has been chosen, it offers many advantages: the cryostats are much simpler, the cavity and magnet alignment is much easier, the space available for diagnostics is larger and the linac tuning is simplified. A conservative accelerating field of 67MV/m was chosen because the resulting maximum peak fields (Epk < 40 MV/m, Bpk < 80 mT) can be achieved, without too much effort. 3. High energy beam transport: From the linac exit, the beam will be transported either straight to a beam dump (10-20% of full beam power) or to a new experimental hall for stable ions, or down to the RIB production station. 3.2. RIB production station
In order to provide against radiation and contamination, the “plug” technology, developed at TRIUMF (Canada) was chosen and adapted for SPIRAL2 [4].
610 1. The production plug, see Fig.3, comprises essentially: 0 0 0
containment tanks for the converter, the target and the ion source. shielding for biological protection against radiation. a service cap for the ancillary equipment (e.g. Pumping, motors for converter, valves, etc)
After target irradiation and enough cooling time, the plug will be isolated by valves and disconnected from all external supplies. It can be then remotely transported to a shielded bunker. After a few months of storage, the plug will be transported into a hot cell for maintenance (disassembly and replacement of components). A minimum of two production plugs, one in place and one in preparation, will be needed to ensure an acceptable RIB production time. 2. Converter: The converter is a 1 m rotating carbon wheel, it has to withstand a maximum incident beam power of 200 kW.
Figure 3: exploded view of the SPIRAL2 converter target ion-source plug.
The beam impinges horizontally onto the rim of the wheel, made of individual graphite tiles. The maximum temperature has been fixed at 1750OC in order to limit the evaporation. 3. Targets [ 5 ] : The neutron-rich isotopes are produced by fission of a depleted uranium carbide target. A low-density target, based on the technology used at PARRNE and ISOLDE, has been designed to reach at least lOI3 fissionds. A
611
high-density target reaching 1OI4 fissionsh is under study, in collaboration with the Gatchina and Legnaro laboratories. A tantalum oven has been designed to stabilize the target temperature around 220OOC to allow efficient diffusion.
4. Ion sources: Different types of ion sources have to be coupled to the target in the same plug, to cover the largest range of radioactive isotope beams: 0
0 0
0
ECR ion sources for the production of gaseous elements such as noble gases FEBIAD-type ion sources for less volatile elements. thermo-ionisation sources for alkalis, alkali-earth and some rare earth elements. laser sources for a large variety of non-volatile elements such as metallic ions. First studies were concentrated on an optimised ECR ion-source.
3.3. Secondary beam transport lines
The radioactive isotopes extracted from the ion source are collected and mass selected through a separator. The beams of different ion species are split up, some to the charge breeder [ 6 ] , prior to post-acceleration in the existing CIME cyclotron, and some to the low-energy experimental hall, see Figs. 1 and 4. The section between ion source and separator is short and included in plugs. Two solutions have been considered for the separator: 1. a BRAMA, “Broad-Range Acceptance Mass Analyser” type solution [7], based on sliding electrostatic deflectors: fully studied. 2. a solution based on a WIEN filter 18-91, minimising the space charge effects: under study. The final choice will be done in the next months. 4. infrastructure and conventional facilities The infrastructure and conventional facilities were designed around the baseline project in such a way that all the additional options and future extensions can be implemented, such as a 100 MeVh linac. The site layout is shown in Fig. 4. The planned production building is underground, (the accelerator is at the zero level), will contain all high-radiation components and is designed to comply with all safety and radiation protection rules. The cycle of a production plug has been carefully studied, from assembly, testing, and irradiation to temporary storage, refurbishment in a hot cell before a new irradiation and final dismantling. The storage area can house a total of ten storage pits. A nuclear ventilation system will ensure the pumping of the different areas of containment.
612
5. Conclusion The detailed studies of SPIRAL2 are in progress in all domains of the project, some prototypes are under construction, like the two types of resonators and the RFQ which will be tested by the end of this year. The development of a high density uranium production target is going on by means of tests in Gatchina, the results will be evaluated in December. Concerning the separator, BRAMA or Wien filter, a choice will be done in 2005. The cost of the whole project is 117 millions of euros, 72 Meuros of which are dedicated to the capital cost. A “green light” from the research ministry is expected by the end of 2004, which could permit to start the construction phase in 2005 and expect the first beam by the end of 2009.
SPiRAL 2 layout
to I
Figure 4: Site layout.
References 1.
2.
W.Mittig and al., “LINAG Phase I”, GANIL internal report, June 2002, http://www,ganil .fr/research splreports. A.Mosnier, joint project review, DSM/CEA-INZP3/CNRS, Orsay, April
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3. 4.
5. 6. 7. 8. 9.
2004, internal report. M.Lewitowicz, “Physics at SPIRAL2”, this conference. P.Bricault and al., “ISAC radioactive ion beam facility at TFUUMF”, LINAC96 proceedings, August 1996,Geneva Switzerland. 0.Bajeat and al., “UCx target design for the SPIRAL2 and ALTO projects”, this conference. T.Lamy and al., “Charge breeding method results with the Phoenix booster ECR ion source”, Rev. Sci. Instrum. Vol73, no.2, pp 717-719 (2002). J.M. Nitschke, “BRAMA, a Broad Range Atomic Mass Analyser for the ISL”, particle Accelerators, 1994, Vol47, pp.153-158. R.Cee, “beam optical calculations for SPIRALZ”, GANIL Report R0401. B.Jacquot, R.Cee, and M.Duva1, “beam lines for radioactive nuclei: Design notes for the Wien filter option”, internal report.
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NUSTAR: NUCLEAR STRUCTURE RESEARCH WITHIN FAIR OBJECTIVES AND ORGANISATION.
G. MUNZENBERG Gesellschaft f u r Schwerionenforschung, GSI mbH Planckstr. 1, 64291 Darmstadt, Germany and Johannes Gutenberg- Universitat Mainz, Germany E-mail: [email protected]
FAIR, the new international facility for antiproton and ion research at GSI will cover a broad spectrum of nuclear and atomic physics research, and application, using intense heavy ion beams of intermediate energy and secondary beams of radioactive nuclei and antiprotons. Nuclear structure research within NUSTAR will be extended to regions far-off stability. New probes for reaction studies with unstable nuclei become available. Experimental developments include: a largeacceptance fragment separator of high resolution which delivers radioactive beams to a low energy branch with detection systems for precision spectroscopy such as an ion trap and gamma arrays with tracking capability, a high energy branch for reaction studies in reversed kinematics, and a storage ring complex for experiments at high precision including direct mass measurements and reaction studies. In this contribution the challenges and opportunities for structure research with the new facility will be outlined. The organization of the FAIR project will be outlined. Nuclear structure research at GSI on an international basis, organized within NUSTAR will be addressed.
1. Introduction
FAIR the new Facility for Antiproton and Ion Research at GSI will cover five areas of research including: - Structure and dynamics of nuclei, based on radioactive beams with high energy to investigate nucleonic matter, fundamental symmetries] and for astrophysics. - Hadron structure and quark-gluon dynamics, based on antiproton beams to investigate non-perturbative QCD, quark-gluon degrees of freedom] confinement and chiral symmetry. - Nuclear matter and the quark-gluon plasma, based on relativistic
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heavy-ion beams to investigate the nuclear phase diagram, compressed nuclear and strange matter, and de-confinement and chiral symmetry. - Physics of dense plasmas and bulk matter based on bunch compression to create short intense pulses of energetic ion beams to investigate the properties of plasmas at high density, phase transitions and the equation of state, and laser - ion interactions with plasmas. - Ultra high electromagnetic fields and their applications based on the combination of intense ion beams and a petawatt laser to investigate QED and critical fields, ion-laser interactions, and the ion - matter interaction.
Rapidly cycling superconducting magnets Cooled beams Figure 1. The Layout of FAIR
To meet the various beam conditions required by these research areas FAIR is a flexible system of heavy-ion accelerators and storage-rings. It provides beams of all chemical elements of the periodic table up to uranium, including rare isotopes with low abundances in naturelas well as secondary beams of radioactive isotopes and antiprotons. Parallel operation of the ring systems and dedicated beam-lines permit to run different experiments simultaneously. This guarantees sufficient beam-time for all
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research programmes. Fig. 1 displays the present and and future accelerator systems. The existing system consists of the the UNIversal Linear Accelerator UNILAC, serving as accelerator for low energy beams and as injector, the booster synchroton SIS 18, and the Experimental Storage Ring ESR. The new facility comprises the double-ring synchrotrons SIS 100/300 and a system of storage-cooler rings: The Collector Ring CR, the New Experimental Storage Ring NESR, and the High Energy Storage Ring HESR. The UNILAC provides beams for experiments near Coulomb-barrier, primarily the superheavy-element program’ and simultaneously serves as injector for the synchrotron complex. Two RFQ-injectors, one equipped with an ECR ion source, the other one with two high-intensity sources allow for flexibility in the choice of the primary ion beams. For the existing UNILAC, SIS18, ESR facility parallel operation of different experiments is already routine now. The parallel operation of different experiments in the ring system is sketched in Fig. 2 on the example of one supercycle serving radioactive beams (RIBS), nucleus-nucleus collisions, antiprotons, and plasma physics. The fast cycling SIS 100 accelerates ions up to uranium to energies of 1 AGeV at high intensity of the order of 1OI2/s. This mode is used for radioactive beam production. The SIS 300 can then be used as a stretcher ring for DC operation. The radioactive nuclei, separated in-flight by the Super-FRS are optionally injected into the storage-ring system consisting of the Collector Ring CR and the New Experimental Storage-Ring NESR for experiments with stored and cooled beams. Antiprotons are produced with intense proton beams of 30 GeV at a secondary target placed in front to the CR-NESR. They are collected and cooled and then injected into the SISlOO for acceleration up to 30 GeV. The accelerated antiproton beams are transferred into the high energy storage and cooler ring HESR for fixed-target in-ring experiments. The program to investigate nuclear matter and the quark-gluon plasma, requires highest energies at moderate intensities using the SIS100/SISI300 accelerator combination. Energies up to 35 AGeV are available for uranium. Intense, bunched beams at moderate energies around 35 AGeV are prepared for the plasma physics programme.
617 UNlLAC
300 Tm Ring
r
-
* .. active
a
Nuclear
Collision
Figure 2. Use of the different rings for parallel beam operation for the four principal experimental programmes. A supercycle for multibeam operation is sketched.
2. NUSTAR, the radioactive beams program
Central topics of NUSTAR, the FAIR research program for Nuclear Structure, Astrophysics, and Reactions are: the exploration of the limits of nuclear stability in isospin and charge including exotic decay modes and the evolution of shell structure. The aim is to understand correlations and pairing, in-medium modification of the nucleon-nucleon interaction, and the behavior of nuclear matter with extreme neutron-to-proton ratios. The storage and cooler ring system for the nuclear structure investigations are a unique feature of fair. Electron-cooled beams are the ideal tool for precision experiments. Direct mass measurements in storage rings allow systematic studies e.g. mapping the nuclear mass surface to get a first overview on the evaluation of nuclear structure towards the limits of nuclear stability to explore isospin effects. Irregularities in the nuclear mass surface are first indications for structure change^^>^. Atomic nuclei can be stored as bare, hydrogen- or helium-like systems. Decay studies in storage rings explore nuclear lifetime under interstellar conditions5. The success of the first generation experiments with unstable nuclear beams motivated a number of second generation projects with improved accelerators and new experimental equipment. The major ones are SPIRAL11 (France) the new RIKEN accelerator facility RARF (Japan), the MSU upgrade (USA), and the planned facilities EURISOL (a European project)
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and RIA (USA). The new GSI facility will play a premier role among these new facilities. It will provide high beam intensities in combination with the highest energies available for unstable nuclear beams. I is the only facility which uses a storage-ring system for structure research. It includes a small collider, the heavy-ion storage ring combined with small intersecting electron storage ring which will provide electrons as a new probe for the investigations of unstable nuclei6. SUDerconductingBagrnent Separator > High-Energy Reaction Setup > Multi-Storage Rings (CR, NESR,
I Ring branch I Figure 3. The GSI NUSTAR facility with the SuperFRS and the three: branches: low energy, reactions and the storage -ring complex with the small electron-heavy ion collider
Fig. 3 displays the NUSTAR facility4. Its principal instrument component is the superconducting FRagment Separator, a two-staged high resolving energyloss-type spectrometer for in-flight separation of radioactive nuclides, created in the production target by fragmentation of fission of heavy ions. SuperFRS can optionally be operated in the spectrometer mode for reaction studies of high resolution. It feeds three branches: - A low energy branch for spectroscopy or reaction studies at low and intermediate energies e.g. below 100 AMeV. The special feature is a combination of a dispersive magnet and a shaped energy degrader acting as energy buncher to create in-flight separated isotopic beam of low momentum spread. The low-energy branch will be equipped with a broad spectrum of instrumentation including an ion catcher-trap system, silicon arrays for
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decay studies with implanted radioactive nuclei, a laser setup, and an advanced germanium ball, AGATA, for in-beam gamma spectroscopy. - A high energy branch for reaction studies with unstable nuclei. This setup will be an upgrade of the existing LAND-Aladin setup with a superconducting dipole of 4 T m bending power, an upgraded large area neutron detector, and improved particle detectors for heavy fragments and charged particles. The setup will also contain a target calorimetcr and proton detectors to investigate proton scattering in reversed kinematics’. - A ring branch, equipped with the CR to collect and cool the in-flight separated fragment beams. This ring will be equipped with stochastic cooling to achieve fast cooling with cooling times of the order of seconds. Experiments will be performed in the NESR which has an electron cooler and an internal gas jet and cluster target. The internal gas target will allow reaction studies with highest precision and scattering a t low momentum transfer. Cooled beams are well defined in momentum and angle. The use of thin targets avoids atomic interactions such as energy loss or angular scattering. The effective target thickness is enhanced as the beam passes the target on each revolution with a frequency of 106/s. A new and challenging development is a small electron ring, operated in colliding mode with the NESR t o perform low-energy electron scattering for structure studies. With this setup it will not only be possible to investigate electron scattering on radioactive nuclei but also to measure electron scattering in complete kinematics with the detection of all participants in the exit channel, not possible with the presently used stable-beam electron scattering facilities.
3. The organization of FAIR and NUSTAR
The GSI FAIR project is a multi-national project, steered by the International Steering Committee ISC-FAIR. The total cost according to the technical design report is 675 M . The German government will pay 65% the state of Hesse 10%. The final decision on the construction of FAIR will be made after the commitment of partner states to contribute 25% of construction cost. The committee for Administrative and Funding Issues, AFI-FAIR, is responsible for international funding issues which will be organized on the administrative government level( Fig. 4). To organize funding and contraction, phase one of the project will be run of the basis of Memoranda of understanding (MOUs). The Scientific and Technical Issues STI-FAIR committee will combine the three scientific Program Advisory Committees PAC QCD, PAC NUSTAR (Nuclear Structure, Astrophysics,
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AFI-FAIR
STI-FAIR Scientific + Technical Issues Sidney Gales
-
Administrative + Funding Issues H.F.Wagner
PACQCD
-PAC NUSTAR
-
PACAPPA
Figure 4. Organisation of the multi-national FAIR project
+ Project Management
International nominate representatives for the FAIR
Figure 5.
FAIR Project
Members of FAIR and their representation
and Reactions), and PAC APPA (Atomic Physics, Plasma Physics, and Applications), as well as the Technical Advisory Committee. The international member states of FAIR will nominate representatives for the Council FAIR ahead of the project management (Fig. 5). Within this this project nuclear structure research will be organized by NUSTAR (Fig. 6). Each of the member institutes of NUSTAR nominates
62 1
International Nuclear Structure and Astrophysics Community
LOIS submitted 12 members 465 a9 institutes
Figure 6.
NUSTAR organisation
a representative. These representatives elect the board of representatives which will on the one hand interact with the differen collaborations and initiatives and represent the interest of NUSTAR in the GSI future Project. For NUSTAR twelve letters of Intent (LOIS) have been submitted. All of them have been accepted by the PAC. NUSTAR has presently about 470 members in about 90 institutes, the collaboration is open for participants, they can register under the NUSTAR WEB-page: www.gsi.de/nustar.
References www.gsi.de S. Hofmann, contribution to this conferece T. Radon, H. Geissel, G. Munzenberg et al., Nucl. Phys. A 677, 75 (2000) H. Geissel, contribution to this conference F. Bosch et al. Phys. Rev. Lett. 77, 5190 (1996) Conceptual design report for "An International Accelerator Facility for Ions and Antiprotons", GSI, Darmstadt 2002 7. H. Emling, contribution to this conference J . Phys. G 20, 1681 (1994)
1. 2. 3. 4. 5. 6.
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PERSPECTIVE OF RI-BEAM BASED RESEARCH AT RIKEN
T . MOTOBAYASHI R I K E N , 2-1 Hirosawa, W a k o , Saitama 351-0198, Japan E-mail: [email protected] Since 1990, the RIKEN Accelerator Research Facility has provided variety of fast radio-isotope (RI) beams. Their intensities are highest in the world for many light unstable nuclei. Various studies have been made and are in progress using these exotic beams. In order to greatly extend the region of study to more exotic and heavier nuclei, the RIKEN RI Beam Factory is now being build. It is expected to come into operation during the year 2006 and first experiments will start in 2007.
1. RI-beam Based Research at RIKEN
1.1. R I K E N Accelerator Complex The current RIKEN Accelerator Research Facility (RARF) has a main ring cyclotron (RRC) with K=540 MeV with two injector machines, an AVF (azimuthally varying field) cyclotron for ions up to Kr and a linear accelerator (RILAC) for heavier ions. The schematic view of the accelerators is shown in Fig. 1 with the RIBF facilities discussed later. The RARF started its routine operation in 1987. It provides various kinds of beams of light heavy-ions with energies up to 135 MeV/nucleon, including 270 MeV polarized deuterons. In the middle of 199Os, an 18-GHz ECR ion source (ECRIS-18)l and a frequency variable RFQ (radio frequency quadrupole) linac2 have been developed and installed in the RARF for efficient acceleration of intense heavy-ion beams. Recently, for better matching between the RILAC and RRC, a booster linac system called CSM (Charge State Multiplier) has been installed in collaboration with CNS (Center for Nuclear Study, the University of Tokyo). Due to these developments, performance of the facility has been much improved, and various experiments requiring high beam intensities become possible. For example, intense lower-energy ions at several MeV/nucleon energies are also available in the RILAC experimental hall. They are used for super-heavy element search by heavy-
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ion fusion reactions3. The most recently, an experiment for production of the new element Z=113 are being performed using a 5 MeVfnucleon 70Zn beam". Another consequence of the developments is increase of the energy and intensity of the beams provided by the RILAC-RRC acceleration scheme.
Figure 1. Schematic view of the RIKEN accelerator complex. The part with gray background is the RIBF facility being built.
1.2. Intermediate-Energy R I Beams
RI Beams of light unstable nuclei have been produced by the projectile fragmentation scheme since 1990. The primary beams are mainly light heavy ions with energies at around 100 MeVfnucleon accelerated by the RRC with the AVF injector. Due t o the large angular and momentum acceptance and high bending power of the fragment separator RIPS5, intense RI beams are available. These intensities are highest in the world for many light unstable nuclei. Due to the upgrade of the liniac injector system mentioned before, available RI beams have been extended to the A x 7 0 region, which could not be realized by the AVF-RRC acceleration scheme, where the energy and intensity are too low for efficient production of RI beams. "This attempt was successful by observing one event for the production of the isotope 278113 by the 209Bi(70Zn,n)reaction in July 2004*.
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Besides the intermediate-energy RI beams, beams of light unstable nuclei at lower energies, typically 5 MeV/nucleon, are also available by the CRIB facility6 constructed by CNS. Several experiments of astrophysical reactions and spectroscopy have been performed7.
1.3. Spectroscopy of Unstable Nuclei
Direct reactions are useful for nuclear spectroscopy. However, new methods were necessary for studies with the fast RI beams because of difficulties due to the reversed kinematics, poor beam-energy resolution, and low beam intensity. We have developed the following two methods of nuclear spectroscopy to overcome the difficulties. The invariant mass is measured if the excited state of interest is particle unbound. For bound states, deexcitation y rays are measured t o identify the excited levels. In the invariant mass method, particles decaying in flight is measured. The excitation energy of their parent nucleus is obtained from the invariant mass measured by their momentum vectors. The energy resolution is almost free from the energy spread of the incident secondary beams. We have been studying, by this method, astrophysical (p,y) processes with the Coulomb dissociation method. The processes with p-shell nuclei, such as sB-+7Be+p, gC+sB+p, and 140+13N+p have been measured with P b targets for studying astrophysical hydrogen burning and solar neutrino production processess. Recently we extended the Coulomb dissociation study to a few rp-process nuclei as 22Mg+23A1. Another application of the Coulomb dissociation is to measure the E l strength function of very neutron-rich nuclei. Coulomb dissociation experiments with 11Li9and ''Belo beams are such examples. Coulomb excitation to particle-bound states has also been studied for various unstable nuclei. Comparing the yield of deexcitation y rays and theoretical prediction of the Coulomb excitation cross section, the transition probability of the relevant state can be extracted. We built arrays of NaI(T1) scintillators called DALI and DALI-2, and measured the probabilities of the 2+-0+ transitions in 32Mg, 34Mg, 56Ni, etc, and the El transitions in llBe, 12Beand 150.The same method was extended recently to (p,p') and (d,d') reactions. Efficient measurements were realized by using a liquid hydrogen target. The location of the first 2+ state in 30Ne11, the most proton deficient N=20 even-even nucleus, has been determined for the first time by the (p,p') experiment. Recently, we have performed another (p,p') experiment, and found two bound excited states in 27F12,
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which are not expected in any of recent theoretical calculations. One of the recent highlights is the 2+-0+ transition in “ C . A lifetime measurement and a I6C+Pb inelastic scattering experiment point to a surprising nature of the transition: very much hindered E2 and enhanced neutron excitations13. This indicates almost complete decoupling of protonand neutron-motions in this nucleus. 2. RI Beam Factory Project 2.1. Overview
Encouraged by these achievements together with those in other RI beam facilities with the fragmentation scheme, RIKEN has planned to extend the research with RI beams by building a new experimental facility called “RI Beam Factory (RIBF)”. The goal of the project is to provide a wide range of experimental opportunities by increasing the variety of RI with a good beam quality.
Neutrons + (Isotopes) Figure 2. Nuclear chart covered by the RIBF project. The thick solid curves indicate the limit of RI productions of 1 particle per day.
The RIBF facility is illustrated in Fig. 1.
It includes three
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ring cyclotrons in cascade to accelerate heavy ions up t o uranium to 350 MeV/nucleon with high intensities up to 1 ppA. RI beams will be produced via the projectile fragmentation or in-flight fission of uranium ions. The limit of production in the rate of one particle per day is indicated by the shick solid curves in Fig. 2. As shown in the figure, most of the expected path of the r-process nculeosynthesis is covered. In view of its outstanding performance on the variety and intensity of RI beams, the RIBF is one of the next-generation facilities competitive to the ones planned in the United State (RIA) and Europe (GSI FAIR), and is their first realization in the in-flight fragmentation scheme. In order to fully exploit this scientific opportunity, we set three major categories of research that should be pursued in the RIBF: i) establishment of a comprehensive picture of atomic nucleus by studying new dynamics in asymmetric nuclei, ii) understanding of element genesis by studying properties and reactions of unstable nuclei, and iii) new applications in multi-disciplinary fields such as medicine, environmental researches, and material science, hoping that some of them will create new industrial domains. We plant t o build the facility of its phase I by the end of 2006.
2.2. Phase 1
In the phase I, a magnetic analyzer called Zero-degree spectrometer and a few beam lines will be constructed together with the BigRIPS1*. Experiments, which will be starged in the year 2007 after the RIBF comes into operation, should be in this condition. Experimental possibilities without the Zero-degree spectrometer in the first few years are: interaction cross-section measurement, p-decay study in the vicinity of the ‘?-process path”, P-y and/or isomer spectroscopy, proton elastic scattering in inverse kinematics, spectroscopy with degraded RI beams, studies with stopped RI’s, unbound state spectroscopy by correlation measurements, and spectroscopy of pionic atoms. With the Zero-degree spectrometer, search for new isotopes using the long TOF line realized by coupling the Zero-degree spectrometer and BigRIPS, in-beam y spectroscopy with direct reactions (inelastic scattering including Coulomb excitation, charge exchange and fragmentation reactions), Missing mass spectroscopy, proton elastic scattering / giant resonance studies, and so on will be possible.
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2.3. Phase 11
The phase I1 of RIBF is in the planning state at present. It contains constructions of the following devices. The budget request will be made in 2005, and the construction will be in five years.
2.3.1. A large-acceptance superconducting spectrometer ( S A M U R A I ) Its large angular and momentum acceptance allows for multi-particle correlation measurements. Experiments currently planned are electromagnetic dissociation for nuclear structure and nuclear astrophysics studies, proton elastic / inelastic scattering and knockout reactions to study density distribution and single-particle orbit of nucleus, polarized-deuteron induced reactions to explore 2-3N forces and short-range correlation, and multiparticle measurement for the EOS study.
2.3.2. Gas catcher and rf ion guide system ( S L O W R I )
RI beams from the BigRIPS are stopped in a gas and efficiently extracted by the rf ion guide scheme to conduct various experiments15 such as mass measurements with the multi reflection time-fo-flight (MR-TOF) spectrometer and determination of charge radii of unstable nuclei by the collinear laser spectroscopy. It is also expected as a possible ion-source of the SCRIT system.
2.3.3. Polarized RI beams Polarized RI beam with high-intensity and low to medium-energy will be developed as a new probe for material science as well as for nuclear structure study. They are generated with the existing RIPS separator which is newly connected to a return beam line delivering heavy-ion beams from the IRC.
2.3.4. High resolution RI beam spectrometer ( S H A R A Q )
A high-resolution spectrometer coupled with a newly designed beam line with dispersion matching is proposed. Exploiting the advantage of the high momentum resolution, experiments with RI beams as reaction probes rather than the object are planned such as study on double Gamow-Teller states and production of neutron nuggets by RI-induced double charge-exchange reactions. Choice of spin- and isospin-transfers and Q-value of the reaction can control the kinematical conditions.
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Triggeredinjection
of faint RI beams
Figure 3.
Scheme of the rare RI ring.
2.3.5. Rang for rare RI ions Conceptual design of an isochronous ring with individual injection of rarely produced RI ions is proposed (Fig. 3). It is mostly dedicated for new precision mass measurement scheme for short-lived nuclei very far from the valley of stability. Time-of-flight measurements with a long flight path with an accurate isochronous field enable one t o determine the ion mass with a great precision. Almost 100% injection efficiency is realized with cyclotronbased (rare) RI beams with the individual injection method. The ring is equipped with a kicker for ion extraction as well as the one for injection for the mass measurement.
e-linac
7 ISOL (e-t-m 2 :
gas catc3er
Figure 4.
’
;C
Scheme of elestron scattering experiments using the Self Confining RI Target.
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2.3.6. Self confining RI target Conceptual design of new electron-scattering experiment scheme for RI ions are proposed (Fig. 4). RI ions produced by an ISOL apparatus are delivered to the area of strong electron beams running. These ions are trapped transversely with the help of the attractive force caused by the electrons. With longitudinal confinement by appropriate potentials, the RI ions form a target in the electron beam, and electron-RI scattering measurements can be performed. 3. Summary
The RIKEN RI beam factory (RIBF) will provide various possibilities for experimental studies using radioactive beams. Construction of the three ring cyclotrons (fRC, IRC and SRC) will be completed in 2006. Then the first experiments will start with 200-300 A MeV beams of various unstable nuclei from 2006. To extend further the research with RI beams, the “phase 11” of RIBF is being considered. References T. Nakagawa et al., Jpn. J. Appl. Phys. 33,378 (1996). 0. Kamigaito et al., Jpn. J . Appl. Phys. 33,L537 (1996). K. Morita, this symposium. K. Morita e t al., J . Phys. SOC.Jpn. 73,2593 (2004). T. Kubo e t al., Nucl. Instr. Meth. B70 309 (1992). Y. Yanagisawa et al., Nucl. Instr. Meth. B in print. T. Teranishi, this symposium. T. Motobayashi, Nucl. Phys. A693 258 (2001), and references therein. S . Shimoura et al., Phys. Lett. B348 29 (1995). N . F’ukuda et al., Phys. Rev. C70 054606 (2004). Y. Yanagisawa et al., Phys. Lett. B566 84 (2003). Z. Elekes et al., Phys. Lett. B599 17 (2004). N. Imai e t al., Phys. Rev. Lett. 92 062501 (2004); Z.Elekes et al., Phys. Lett. B586 34 (2004). 14. T. Kubo, this symposium. 15. M. Wada, this symposium. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
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UCX TARGET DESIGN FOR THE SPIRAL 2 PROJECT AND THE ALTO PROJECT 0. BAJEAT', F. AZAIEZ, c . BOURGEOIS, M. CHEIKH MAHMED, H. CROIZET, M.
DUCOURTIEUX, S. ESSABAA, R. FIFI, S. FRANCHOO, F. IBRAHIM, C. LAU, F. LEBLANC, H. LEFORT, M. MIREA, C. PHAN VIET, JC. POTIER, B. ROUSSIERE, J. SAUVAGE, D. VERNEY, F. POUGHEON Institut de Physique Nucldaire. 91406 Orsay, France G. GAUBERT, Y. HUGUET, N. LECESNE, P. LECOMTE, R. LEROY, F. PELLEMOINE, M.G. SAINT-LAURENT GANIL, Bd H. Becquerel14076 Caen cedex 5, France F. NIZERY, D. RIDIKAS DAPNIA. CEA Saclay, 91 191 Gifsur Yvette, France R.V. RIBAS Instituto de Fisica. Universidade de Sao Paolo, SP ,CP. 66318, 05315-970, Brazil
Two ways of production of radioactive beams using uranium carbide targets are taken into consideration: fission induced by fast neutrons and by bremsstrahlung radiation. For the SPIRAL 2 project, the fission of 238Uin uranium carbide target will be induced by a neutron flow created by bombarding a carbon converter with a 40 MeV high intensity deuteron beam. Calculations and design of the target in order to reach I O l 3 fissionsk with good release time have been done. The second way is the photofission using an electron beam. In 2005 the ALTO project (Accelerateur Lineaire Aupres du Tandem d'Orsay) will give a 50 MeV/lOpA electron beam. This facility will allowmore than 10" fissionds. In this case, the electron beam hits the target without converter. Calculations realised in order to estimate the production are used to choose the best target shape. For the two cases some R & D on targets to improve release is described.
1. The UCx targets For both projects: Spiral 2 with fast neutrons and Alto with electrons same kinds of targets will be used. Targets are based on the Isolde method [l]. Such a type of thick target is an assembly of disks (thickness about 1 mm) composed of a mixing of uranium carbide and graphite. These pellets are obtained by compressing a mix of uranium oxide and graphite powders. The carbonation is made by heating the pellets up to 2000 "C under vacuum to make the reaction:
*
Corresponding author: [email protected] .fr
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U02+6C-+UC2+2C+2COt or UO2+6C-UC+3C+2COT. The graphite allows limiting the carbide grain size for minimizing diffusion paths. The figure 1 shows the structure of a target. During irradiation, targets are heated up to 2200°C.
Fig. I: structure of an UCx target by Scanning Electron Microscopy. The graphite appears in black, the uranium carbide UC and UC2 in white. The uranium carbide grain size is about 20 to 30 pm.
2.
The Spiral 2 target
For the SPIRAL 2 project the specification is to reach 1013fissionsls in the case of UCx target using a 40 MeV/5 mA deuteron beam with a rotating carbon converter [ 2 ] . The production of a target irradiated by fast neutrons is estimated using the FICNER code [3]. Thanks to this code one can see the effect of geometrical parameters onto the production. The figure 2 illustrates the importance to put the target as close as possible to the converter. Effect of the distance converter-target on the production target 80 mm diam. / 80 mm length
10
30
50
70
90
dist, converter I target in m m
Fig. 2. Effect of the converter-target distance on the production for 2 beam sizes (diameter 30 mm and 60 mm).
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With this calculation code it has also been demonstrated that a conical target would not be better for the production than a cylindrical one with the same volume. For the Spiral 2 project we plan to make a target diameter 80-mm, length 8O-mm, at about 40 mm from the entrance of the converter (figure 3).
660 g U
Figure 3. The SPIRAL 2 target: 19 series of about 60 pellets diameter 15 mm, thickness 1 mm, spacing about 0.3 mm between each pellet.
The target has to work at a temperature hgher than 2000 "C in order to allow an efficient release of the produced radioactive elements. The power deposited inside the target by the fission reactions is about 500 W for 1.6 1013f/s.Then, an extra heating must be added in order to reach convenient temperatures. A tantalum oven prototype is under construction. The pellets will be in a graphite container surrounded by a tantalum foil to avoid reaction between carbon and the oven. The main difficulty will be to obtain a homogeneous temperature of the target for a long duration (3 months). Moreover the transfer tube between the target and the ion source has to be heated at 2000°C too for the ionization of non volatile nuclei. 3.
The Alto target
For this project the goal is to reach 10" fissionsh using a 50 MeV/10 pA electron beam to produce radioactive beams in the PARRNE separator already existing at IPN Orsay [4]. The production of the target is estimated using the FICEL code [ 5 ] . Due to the absorption of photons the production does not increase proportionally with the target density. The case of a conical shape has been also studied. The table 1 shows that a conical target 3 times larger would produce only 30 % more fission events than the cylindrical one.
633 Is' diameter 2"ddiameter mm
Mm 14 14
14 34
Length mm
Nb fissionh
Volume cm'
for 10 pA 1.0 10" 1.3 10"
15 41
100 100
Table 1. Comparison of acylindrical and a conical target for photofission (target density = 2.4 g/cm3)
For ALTO project we'll use an assembly of about 90 pellets of 14 mm diameter, 1 mm thickness and within a small spacing between each pellet. For such a target, 350 W of the 500 W incident beam will be absorbed in the target, 150 W being re-emitted out of the target as photon radiation.
3.1. The case of a converter The production in the target using a tungsten converter has been studied. In this case a part of the fission events is due to the photons emitted from the converter. Another part is due to brernsstrahlung radiations produced in the target by the electrons which hit the target if the converter thickness is lower than the electron range in the tungsten. tar get p r o d u c t i o n w ith c o n v e r t e r UCx target 1 4 m m d i a m . , 1 0 0 mm length, density 3.6 g/cm3, tungsten converter irradiated b y 50 M e V electron b e a m
1.2E+10 I.OE+IO 8,OE+09 6,OE+09 4,OE+09 2.OE+09 O.OE+OO without cow
Imm
2mm
4mm
5mm
7mm
8mm
converter thickness
Figure 3. Fission produced in the target with various converter thicknesses. In black: fission induced by photons produced in the target. In gray: fission induced by photons produced in the converter. The converter is in direct contact with the target.
These results proved the production is better without converter. Nevertheless, a converter could be useful to reduce the energy deposited in the target. In fact if the converter thickness is equal to the range of electrons, no electrons will hit the target and only the energy due to photon absorption will be deposited in the target. The results given in the table 2 show that the converter
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would reduce the production by a factor 5 while the energy deposited would be reduced by a factor 10.
Nb of fission per
pC Without converter Converter W 10 mm
1.3 10" 0.26 10"'
Energy deposition in MeV 35 3.4
3.2. Gamma dose rate
Knowing the angular and energy distribution of photons N(E,8) emitted out of the target, the dose rate is given by :
F(E) being the photon conversion factor. The figure 4 confirms that dose rate is very high in forwarded direction, but even in other directions some important shielding becomes necessary.
I
Gamma dose rates in Gylh at 1 rn
y
: E i
(3
1000
100
10
0
10
20
30
40
50
60
70
80
angle in degres
Figure 4: gamma dose rate by an UCx target d i m . 14 mm, length 150 mm, density 3.6 g/cm3 irradiated by a 50 MeV / 10 pA electron beam.
4.
The release times (Spiral 2 and Alto)
Some effusion calculations using the Monte-Carlo method have demonstrated that spacing between the pellets can decrease the mean number of collisions of radioactive atoms to get the entrance of the ion source. But in the same time the
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production will be lower due to the lower effective density of the target. A new kind of target made by an assembly of disks with 3 bumps thickness 0.3 mm has been realized as presented on the figure 4. This new target has been tested on line using the PARRNE set up (25 MeV11 pA deuteron beam on a carbon converter).
Figure 4: view of a piece of the target with 0.3 mm bumps. The target is constituted of 123 pellets diameter 15 mm. Each pellets thickness is about 1 mm (0.7 mm + 0.3 mm)
Release time measurements on I3’Sn and 139Xehave been performed using this new UC, target. Assuming that the predominant release mechanism is effusion in case of tin and diffusion in case of xenon, the measurement and the analysis have been carried out as indicated in refs. [6,7]. Preliminary results indicate TR=: 40 s for tin and TR =: 20 s for xenon. Previous measurements performed with the same target+ion-source temperature conditions (TtXget= 2080°C and Tlhe = 2 1OOOC) but using a target without bumps had led to TR = 55 s for tin and TR = 21 s for xenon. Consequently, this seems to confirm that with the new target the release time remains the same for diffusion but decreases for effusion.
References 1. H.L. Ravn et al. Nucl. Instr. And Method B 26 (1987) 183. 2. R. Anne. Technical status of Spiral 2. These proceedings. 3. M. Mirea et al. Modeling a neutron rich nuclei source. European Physical Journal A 11, 2001, pp. 59-78. 4. F. Ibrahim. The ALTO project at IPN Orsay. These proceedings. 5 . M. Mirea et al. Exploratory analysis of a neutron-rich nuclei source based on photo-fission, NIM B 201, 2003, pp. 433-448. 6. B. Roussikre et al. Release properties of UCx and molten U targets. NIM B 194, 2002, pp. 151-163. 7. C. Lau et al., NIMB204 (2003)246.
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PRESENT STATUS OF THE KICK-JAERI JOINT RNB PROJECT H. MIYATAKE' Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Oho I - I p Tsukuba, lbaraki 305-0801, JAPAN E-mail: [email protected] A new RNB facility, TRIAC (Tokai Radioactive Ion Accelerator Complex) facility, based on the ISOL and post acceleration scheme, has been constructed under the collaboration of High Energy Accelerator Research Organization (KEK) and Japan Atomic Energy Research Institute (JAERI) since 2001. It will be open for the RNB science with 1.1 MeVh RNB from FY2005.The higher energetic (5 - 8 MeVh) RNB will be available in the near future. The present status of the R&D works of TRIAC together with some results of pilot experiments are presented.
1. TRIAC Facility 1.I Layout
The facility in a final goal [ 11 consists of the Isotope Separator On-Line (ISOL), a charge-breeding ECR (CB-ECR), split-coaxial RFQ (SCRFQ-) linac, interdigital-H type (IH-) linac, three Bunchers, and superconducting (SC-) linac (see Fig. 1). The output *"Primavbem energy is variable from 0.1 to 8 MeVIu. The characteristics of this facility are summarized in Table 1. The primary beam is Figure 1 The layout of the TRIAC. supplied from 20 MV Tandem accelerator. The CB-ECR, a key device for the acceleration of heavy radioactive nuclei, was newly designed to achieve the high efficiency for the charge-breeding of radioactive ions [2]. The SCRFQ- and IH-linac, constructed at the pilot RNB facility of KEK-Tanashi in 1997 [3], were re-installed by changing those rf-frequencies for matching one of the SC-linac (130 MHz). The higher energy RNBs up to 8 MeVlu will be realized by modifying cavities of SC-linac [4] as mentioned in the following sub-section. This paper is a group report made on behalf of about 40 scientists, engineers, technicians of KEK and JAERI.
*
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There are two experimental halls. One is for the low-energy experiments with 1.1 MeVlu, and the other is for the high-energy experiments with 5-8 MeVlu RNB. Some experimental devices are already placed as mentioned in Ref. 1. It is noted that another 14 GHz ECRIS for the stable nuclear beam is also important device not only for a tuning of the linac complex, but also for some research subjects with using intense heavy-ion beams independent from Tandem beams. Table I Some parameters of TRIAC 40 MeV, p. 31A, HI (20MV) UC2, etc M/AM, I/S 1200, FEBIAD, Surface, etc. ECRIS 18 GHz, 1 kW ECRIS 14 GHz, 200 W [ 5 ] injection energy 2.1 k e V h output energy 0.14-8.52 M e V h (variable) duty cycle 100% (A/q516), 30% (A/q=29) frequency, output energy 25.96 MHz, 178.4 keV/u ( N q 9 9 ) SCRFQ-linac frequency, output energy 51.92 MHz, 0.14-1.09 M e V h (A/qSlO) IH-linac frequency, output energy 129.8 MHz, < 5.25 M e V h (A/q57), SC-linac* < 8.52 M e V h ( A / q g ) *The structure of the SC-linac will be modified from the original one (40 cavities ) to the one with 8 low-p cavities + 36 original cavities.
Primary Beam Production Target ISOL Charge Breeder I/S for stable HI linac complex
energy, intensity
1.2 RNBs
I ClfY
.
. ‘)?
,
ct3 \
,
‘1.4
)
PF
46
1:s
I 14.8
142
111)
A
m
.
a
\
A
Figure 2 Measured intensities of mass separated Rb, Cs, Sr, and Ba isotopes The solid circles indicate the expected production rates, and squares and triangles, showing measured intensities, correspond to the different graphite materials, fiber (1 1 Km+) and block (porosity 50%), respectwely, in which UC2 is deposited
Various kinds of neutron rich radioactive nuclei produced via the proton induced fission or the heavy-ion transfer or the fusion reaction have been ionized so far in R&D works at Tokai site [ 6 ] . The RNB intensities of fission fragments at the exit of the ISOL have been measured by utilizing the surface ionization type ion-source and FEBIAD type ion-source. In this measurement, parameters of the proton beam energy, intensity, and the thickness of UC-target were 20 MeV, 100 nA, and 0.33 g/cm2,respectively.
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As can be seen in Fig. 2, the extraction efficiency decreases from several 10 th % to the order of 1 % due to the short lifetime of each extracted fragment. The UC-target in the graphite fiber (11 pm$) indicates rather high efficiency than one in the graphite block (porosity 50 %). Based on these data, the intensity of '43Cs-RNBfor the realistic conditions of the 30 MeV and 3 pA proton beam and 2.6 g/cm2UC-target is, for example, expected to be 5x10' pps. RNBs of 'Li, "F, 2oF,and '"In elements were also produced via the heavy-ion transfer and fusion reactions and were mass-separated with those intensities of 1O', 106,3x 1 05,and 3x 1O5 pps, respectively. 1.3 The CB-ECR
The 18 GHz CB-ECR converts singly charged radioactive ions, axially injected from the ISOL, to higher charged ones in its ECR plasma in-flight. It aims at realizing the small mass to charge ratio (A/q) required from the acceleration by the post linac complex. The maximum acceptable Nq-values for linacs are 29 for SCRFQ, 10 for IH, and 7 for SC-linac, respectively. So far, the charge breeding efficiencies for the externally injected 1'-ions were measured at KEK-test bench before the installation [7]. The highest efficiencies reached to 13.5% for Ar9', 10.4% for Kr"', and 6.8% for XeZDC, respectively. It is noted that these high charge states fulfill the overall acceleration condition ( N q < 7) for the linac complex and is comparable to the results of the PHOENIX booster [8]. A charge-breeding time was also measured with Xe-ions [7]. This quantity is characterized as a delay time of the extracted q+-ion beam Figure 3 The time spectrum of extracted Xe-ions from CB-ECR Solid circles, open from the beginning of the injection of circles, and crosses indicate extracted ions the l'-ion beam and a time constant of with q = 15+, 18'. and 2 I+, respectively, while a growth curve of the extracted beam. the solid line is the 1 'injection beam. The singly charged Xe-ions were injected to the CB-ECR for 300 ms with repetition frequency of 1 Hz. Fig. 3 shows the time structures of the extracted charge-bred ions with q = 15+, 18+, and 21'. From this figure, the charge-breeding time was as short as 60 ms including the delay time of 20 ms for the conversion from q = 1' to 21'. This value is enough short with respect to the half-lives of almost all of heavy neutron-rich fission fragments available in TFUAC.
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1.4 Connecting the IH-Linac to the SC-Linac
i q y d He channel
RNBs from IH-linac will be more accelerated by the SC-linac, which was constructed in 1993 as a booster of the Tandem accelerator [4]. This linac comprises 10 cryostats and inter-cryostat quadrupole doublets. Each cryostat has 4 cavities. This linac has a large energy acceptance, since there are only 2 accelerating gaps in a cavity and the rf-phase of each cavity can be independently tuned. The optimum incident velocity is designed to be p = 0.1, which is, however, still high for the output energy of IH-linac (1.1 MeVh). Therefore, we will modify the upstream 8 cavities to accept the low+ beams by replacing new ones, each of which will be optimized for the beam velocity of p = 0.06. Moreover, one cryostat including the original 4 cavities will be added to the last r n i n c t a t tn arhipve I . , U"I.I"." "'JVU'"'
Niobi
c
RF input
Figure 4 Cut-view of a low-p cavity as a superconducting twin-quarter wave resonators (fo = 129.8 MHz, P o = 0.06, La-= 0.15 m).
the hiohpr nntmit eneruv "'O""* vuy... b J . l l l "
-1.v.
Figure 4 shows a newly designed low-p cavity having 3-gaps. The expected output energy of the modified SC-linac, consisting of 8 low-p cavities and 36 original cavities, is 5.25 MeVIu for Alq=7 and 8.52 MeV for Mq=4 [9]. The test of the modified cryostat with low-p cavities has been performed since the year of 2004. All of these modifications will be finished in 2 to 3 years. 2.
Some Experimental Subjects
At the early stage of the facility, the kinetic energy of the RNBs is limited to be 1.1 MeVlu. The interesting research fields with low-energy RNBs are solid-state physics, atomic physics and nuclear astrophysics, as well as nuclear physics by means of spectroscopic work on exotic nuclei. Some topics being discussed and preparing are (1) direct measurements of astrophysical reaction rates, (2) mechanism of the thermal diffusion in materials, (3) studies of electromagnetic structures of materials by means of the PAC-spectroscopy or
3 1000 E
800 UJ
g
Boo
ij 400
'rn 0
E,fM@Jf Figure 5 The reaction cross section of the 'Ll(a, n) "B. The open and solid triangles show previous inclusive measurements [12, 131, while the open squares are the previous exclusive ones
640
P-NMR technique and (4)spectroscopic studies of exotic nuclei by means of the nuclear spin polarization technique or the Coulomb excitation technique. Some pilot experiments with low-energy RNBs have already been performed. The RNBs are obtained by utilizing the inverse transfer reactions and the recoil mass separator in Tandem facility [lo]. 2. I Direct Exclusive Measurements of Astrophysical 'Li (a,n) Reaction
One of these experiments is for a direct measurement of the 8Li(a, n) reaction cross section in an energy region from E,, = 0.3 to 2 MeV corresponding to the Gamow peak at Tg=1 to 3, which is an interesting region for the heavy element synthesis in going across the stability gap of A=8 under the explosive stellar condition such as an inhomogeneous big-bang and supernova explosions. The 'Li-RNB was produced via the transfer reaction of 'Be('Li, 8Li). Its purity and typical beam intensity are 99 % and 2x105 pps. In order to overcome such relatively weak beam intensity, we newly constructed a detector system consisting of a large solid angle neutron wall and a 3-dimensional tracking gas chamber [ 111. The typical detection efficiency for the 8Li(a, n) reaction events at E,, = 2 MeV is about 15 YO. The obtained first result from E,, = 0.7 to 2.5 MeV has revealed ten times better statistics compared to the previous exclusive measurement as shown in the Fig. 5 [ 151. There is a large discrepancy between our exclusive measurement and the previous inclusive ones [ 12,131. More precise measurement in the energy region around 0.5 MeV has already been performed and its analysis is now in progress. 2.2 A Novel Method f o r Measurements of the Thermal Diffusion of Li-Ions
Another experiment is for a non-destructive measurement of the thermal diffusion constant of Li-ions in LiAl intermetallic compound. A new technique to measure the diffusion constants in solids has been developed [ 161. In the test experiment, the energetic (- 0.5 MeV/u) 'Li has been implanted with the depth of 12 pm into the LiAl compound. The implanted 8Li decays into two a-particles, whose average range in LiAl is 8 pin. Then a charged particle detector, located close to the sample
nme (mst Figure 6 Normalized time spectra of a-yields at the temperature as indicated in the legend. Solid, dashed, and dotted lines are fitted results with the most probable diffusion constants at 2OoC, 150"C, and 300"C, respectively.
641
surface, could efficiently detect a particles from 'Li diffusing toward the sample surface. Therefore the time-dependent yields of a-particle are supposed to be a good measure of the Li diffusion in the sample. Figure 6 shows the time spectra of a-particle yields normalized by the time-dependent amount of 'Li-nucleus in LiAl, where a pulsed 'Li beam was used. The experimental spectra measured at different temperatures (20' C, 150°C, and 300°C) show a clear diffusion effect. The preliminary comparison is also presented with diffusion constants assumed in the one-dimensional simulation based on the Fick' s 2nd raw, well demonstrating that the present method could be applied for the direct measurement of diffusion constants in a non-destructive way for other methods such as an indirect NMR technique [17]. The method will be extended to measure the Li-ion difhsion constants in the super ionic conductors such as electrode materials in Li-batteries in order to study the mechanism of the ionic conductivity. 3. Summary KEK-IPNS (Institute of Particle and Nuclear Studies) and JAERI-Tokai have been collaborating to construct the radioactive-nuclear-beam facility based on the ISOL and post-acceleration scheme. From FY2005, the low-energy RNB having its energy up to 1.1 MeVh will be available at the LE-experimental hall for various scientific subjects of nuclear astrophysics, nuclear physics, material science, and related research fields. Along with the installation of the components of TRIAC, following developments have been performed. (1) Development of UC-target for producing RNB by proton-induced fission. (2) Measurement of the charge-breeding efficiency and its characteristic time in CB-ECR. (3) Performance test of the newly designed low+ superconducting cavity. In parallel with the developments for TRIAC, several pilot RNB experiments have been also performed such as: (1) Direct measurement of 'Li(a, n) reaction rates relevant to the heavy-element synthesis in the astrophysical explosive environments, (2) Measurement of the diffusion constants of Li in Li super ionic conductors. These subjects will be hIly performed at LE-experimental hall in TRIAC facility. References
1. H. Miyatake, et al., Nucl. Instrum. Meth. B204(2003)746. 2. S.C. Jeong, et al., Nucl. Instrum. Meth. B204(2003)420. 3. S. Arai et al., Nucl. Instrum. Meth. A390(1997)9. 4. T. Ishii et al., Nucl. Instrum. Meth. A328(1993)231. 5. NANOGAN: P. Sortais et al, Proc. 12th Int. Work. on ECR ion sources, RIKEN, (1995)44.
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6. S. Ichikawa et al., Nucl. Instrum. Meth. B204(2003)372. 7. S.C. Jeong et al., Rev. Sci. Instrum. 75(2004)1631. 8. T. Lamy et al., Proc. 8th Int. Conf. EPAC02, Paris, France, 3 - 7 June (2002) 1724. 9. S. Takeuchi, private communication. 10. H. Ishiyama et al., Proc. Tours Symp., AIP 704(2004)453. 1 1. T. Hashimoto et al., to be published in Nucl. Instrum.Meth. 12. R.N. Boyd et al., Phys. Rev. Lett. 68(1992)1283. 13. X. Gu et al., Phys. Lett. B343(1995)31. 14. Y. Mizoi et al., Phys. Rev. C62(2000)065801. 15. Preliminary results were summarized in H. Miyatake et al., Nucl. Phys. A738C(2004) 40 1. 16. S.C. Jeong et al., Jpn. J. Appl. Phys. 42(2003)4567. 17. J.C. Tarczon et al., Mat. Sci. Eng. A101(1988)99.
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DUBNA CYCLOTRONS - STATUS AND PLANS G.G.GULBEKYAN, B.N.GIKAL, S.L.BOGOMOLOV, S.N.DMITRIEV, M.G.ITKIS, V.V.KALAGIN, YU.TS.OGANESSIAN, V.A.SOKOLOV Joint Institutefor Nuclear Research, Dubna. Moscow region, Russia In Laboratory of nuclear reactions there are 4 accelerators of heavy ions. Cyclotrons U - 400, U - 400M, U - 200 and DC-40 accelerate ions from P up to Bi with energy from 3 up to 100 MeV/nucleon with high intensity. The large program of scientific and applied researches is carried out on the beams of heavy ions. In Laboratory the project DRIBs allowing obtaining beams of the accelerated radioactive ions is being realized. The first experiments on 6 He and 8 He beams are carried out.
1.
Introduction
The FLNR scientific program on heavy ion physics included experiments on the synthesis of heavy and exotic nuclei using ion beams of stable and radioactive isotopes and studies of nuclear reactions, acceleration technology and applied research. Presently Flerov Laboratory of Nuclear Reactions of Joint Institute for Nuclear Research has four cyclotrons of heavy ions, U-400, U-400M, U-200, DC-40, which provide performance of the basic and applied researches. Total operating time of cyclotrons is about 8000 hours/ year. The intensive beams of 48Ca ions on the cyclotron U-400 have provided performance of the program on synthesis of a number of new isotopes of the superheavy elements. The Tritium beam with the energy of 19 MeVIn and intensity of lo9 pps was accelerated on the cyclotrons U-400. The beams of He6 (28 MeVIn) and He8 (25 MeV/n with intensity 3.10' pps and 3.104 pps respectively were received in flight method using a thin Beryllium production target in the separation channel. The realization of the project DRIBs (Dubna Radioactive Ion Beams) based on ISOL scheme is completed at the Laboratory. It will allow increasing the intensity of the He6 and He8 beams up to 10" pps and lo8pps respectively. The first physical experiment is being planned to carry out by the end of this year. Last year the modernization of DC-40 cyclotron was carried out. The task of modernization is acceleration of an intensive beam of Kr with energy about 1.2 MeVh that will be used for irradiation of deferent polymer materials. FLNR works on creation of the cyclotron DC-72 for Slovak cyclotron center in Bratislava are being conducted. The accelerator is developed for acceleration
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of protons with energy up to 72 MeV and heavy ions with energy from 3.5 up to 18 MeVIn. The first beam is being planned to obtain in 2005. Flerov Laboratory of Nuclear Reactions in collaboration with Nuclear physics Institute (Almaty) the cyclotron DC-60 for applied researches has been developed for the Research Center at L.N.Gumilev Euroasia State University in Astana (Kazakhstan). The cyclotron is capable to accelerate ions from Carbon to Xenon with energies 0.35 + 1.67 MeVln. 2.
The U400 Cyclotron
The U400 has 12 experimental channels, the main experimental setups are [i]: GFRS- gas filled recoil separator, VASILISSA- the electrostatic separator, CORSETDEMON- the setup for study of fusion-fission reactions, U600- the setup for production the track membranes, MSP144- the magnetic separator. The diagram of U-400 operation in 1997-2004 and using the beams is shown Figure 1. In 1998-2004, the U400 was mainly used for experiments with 48Ca5+ ions for the purpose of synthesis the new super heavy elements. The isochronous U400 cyclotron has beeq in operation since 1978 [ii]. Until 1996, the PIG- ion source has been used for ion production. Since 1996, the ECR-4M ion source (made in GANIL, France) has been installed at the U400. The axial injection system was created to inject ions from the ECR4M to the U400 center [iii]. To increase the capture into acceleration the sine and linear bunchers were installed into the axial injection canal [iv]. The essential modernization of the U400 axial injection in 2002 included sharp shortening of the horizontal part of the injection canal [v]. To increase the capture in acceleration efficiency, the combination of line and sine bunchers are used [vi]. The linear buncher is situated at 4.4m and the sine one is placed at 0.8 m above the median plane. The modernization gave us the possibility to increase the 48Ca+5current into the injection line from 40a60 to 80a100 pA at the similar capture in acceleration efficiency. Correspondingly, the average output 48Ca+18 ion current was increased from 15 to 25 pA. The average intensity of 48Ca+5ions at the U400 extraction radius is about 4.3 ppA (21SPA). The typical 48Ca+5 ion energy is 250a270 MeV. Since 2003, the TOF method [vii] with two capacitive pickup electrodes has been used at the U400 to measure the extracted ion energy and to adjust the ion acceleration regime.
645 The U-400 operation in 1997-2004
Hours 7000 6000 5000
4000
3000 2000
1000
0 1997
1998
1999
2000
2001
2002
2003
2004upto December
Year
Figure 1: The diagram of U-400 operation in 1997-2004
The required energy of extracted ions received by means of changing the charge of accelerated particle (rough method) and by means of changing the stripping foil position, or changing the RF frequency and the magnetic field level (fluent method). To realize the 48Ca+5ion extraction with energies more than 260 MeV with keeping the beam intensity, the special magnetic channel has been constructed and situated at the hill outer edge. The aim of the channel is additional focusing of the extracted ion beam at the second turn after the stripping foil, when the foil is moved to the big radius. In experiments on synthesis the new super heavy elements, the average intensity of the 48Cat'8before the experimental target is about 1.4 ppA (25pA). The main line of the ion spectrum after the stripping foil is mainly used for the physical experiments. The results of 48Caacceleration in 2003 presented in [8]. In the regimes, the average consumption of solid 48Ca is about 0.8 mghour.
9
Ir = 8,2 . 1014pps Matter consumption -0,8 mghour Utilization - 0,16 mghour (20%) 48Ca enrichment (60% in matter) dN/dt (48Ca) = 12 . 10'4pps (0,4 mghour) Efficiency &O-n= 65% Efficiency &0-5+= 10%
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3.
Modernizaion of the U400 Cyclotronat the FLNR JINR
The modernization of the U400 has been suggested to improve the cyclotron parameters. The aims of the modernization are: 1. Decreasing the magnetic field level at the cyclotron center from the region of 1.93t2.1 T to 0.8t1.8 T, that allows us to decrease the electrical power of the U400R main coil power supply in four times. 2. Providing the fluent ion energy variation at factor 5 for every mass to charge ratio A/Z at accuracy of bE/E=5.10"; 3. Increasing the intensity of accelerated ions of rare stable isotopes at factor 3. The beginning modernization of the U400 axial injection included sharp shortening the injection canal horizontal part. As the result, the distance from the ECR to the AM90 bending magnet became equal to 730 mm. The changes allows us to increase the 48Ca"' ion intensity at the U400 output from 0.9 to 1.4 ppA. Further modernization intends decreasing ion losses by means of increasing the SL solenoid inner diameter from 68 to 100 mm and the AM90 bending magnet horizontal aperture from 70 to 94 mm. In the future, we are planning to search possibility of increasing the injection voltage from the range of 13t20 kV to 40t50 kV. As we estimated, the changes can give us increasing the U400R accelerating efficiency in 1.5+2 times, it is particularly important for &a ions. To extract ions out of the U400R we suppose to use two ways: electrostatic deflector and stripping foil method. Both the methods allow us to extract ions in the directions of the existing ion transport channels. The RF system of U400R will consists of two RF generators that will excite two separated RF dee resonators. The RF resonators will be made from iron with copper coating to decrease the outgassing rate from the vacuum surface. The modernization of vacuum system will include changing five diffusion pumps VA-8-7 with N2 pumping rate of Q=4250 l/s each to five cryopumps with Q=3000 l/s each and two turbopumps with 1900 l/s each. In addition the materials of the cyclotron vacuum chamber and RF resonators will be changed to decrease their outgassing rate. The given changes allow us to improve vacuum in the cyclotron chamber from (1S t 2 ) x 10-7Torr to 1O-' Torr.
4.
The U400M Cyclotron
The axial injection system of the U-400M [9] was put into operation in 1995. The design of the axial injection system of the U-400M cyclotron is similar to that of the U-400 cyclotron, but on cyclotron two sources of ions are installed:
647
ECR - for production of heavy ions and high-frequency source of ions, which in our case was used for generation of Tritium ion beam. The DECRIS-2 (Dubna ECR Ion Source) installed at the cyclotron is created at the FLNR [lo]. The beam is focused by a lenses and three solenoids placed in the axial channel. The channel is pumped out by two turbomolecular and three cryogenic pumps, which provide vacuum of 2.5* lo" Torr. Due to good vacuum in the cyclotron chamber (better than 1* 10-7 Tom) and high acceleration rate, the beam loss during the process of acceleration up to the final radius is less than 10%. The diagram of U-400M operation in 1997-2004 and using the beams is shown in Figure 3. 5.
Beam Extraction from U-400m Cyclotron
The beam is extracted from the cyclotron by a stripping on a thin foil. The beam extraction system allows the beam to be extracted with a stripping ratio Z,/Z, = 1.4s1.7 (Z,, - the charge of ions of the internal beam, Z ,, - the charge of ions of the extracted beam). The modernized this year extraction system provides a beam extraction efficiency of 70-80%. The beam is extracted from the cyclotron by a stripping foil. The beam extraction system allows the beam to be extracted with a stripping ratio Z,,/Z,, = 1.4sl.7 (Z,,, - the charge of ions of the internal beam, Z,,, - the charge of ions of the extracted beam). Main ion energy range of extracted ions is 30 + 50 MeVIn. The beam extraction efficiency constitutes of 70-80%. At present a number of new set-ups have been mounted, including the ACCULINNA [ 111 channel, intended for the production of radioactive ion beams. To carry out these experiments, the ECR source has been specially adjusted, which has enabled the production of high intensity beams of light ions both of gaseous and solid materials. Table 1: The efficiency of the "B" beam transportation from the ECR source to the physical target.
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The intensity of beams of light ions in the range from Li to Ne with an energy of 30t50 MeVInucl was 3+5*1013 pps. This was achieved with using a bunching system, which increases the intensity of the beam by a factor of 3 ~ 5 . Table 1 shows the efficiency of the beam transportation from the ECR source to the physical target obtained for "B3+.
6. Tritium Acceleration The tritium ion beam was required for study of 4H and 'H resonance states in neutron transfer reactions t+t+'H+p and t+tj4H+d. Experiments were performed at the separator ACCULINNA 1111. At the U400M cyclotron the tritium ions should be accelerated as molecular ions (DT)+ from the point of view beam extraction by stripping. The required beam intensity on the liquid tritium target was about 10' pps. Taking into account the beam losses on transport and monochromatisation the intensity of the accelerated beam should be about 10 nA (6 10" pps).
..
The main requirements to the ion source were: minimal consumption of radioactive tritium; high output of molecular ions; long lifetime. For production of molecular ions the RF ion source was chosen. During the operation at the test bench the ion source was optimized for production of H: ions. The schematic view of the RF ion source with electrostatic optics is shown in Figure2.
.
Figure 2: The schematic view of the RF ion source.
For feeding of the tritium atoms into the ion source the special gas feed system was developed in RFNC - VNIIEPh (Sarov, Russia) that provides fine regulation of gas flow and safety handling with tritium. The system has two channels for the gas feed - one was used for feeding of deuterium-tritium mixture with the tritium content of 1%, and the second - for the main gas deuterium.
649
& I
12
5
497
495
42 Yo
I
94 Ya 96 Yo
A beam of 58-MeV tritons was obtained from the U-400M cyclotron and delivered to the tritium target. The ACCULINNA separator ion optics was used to select the beam having an energy spread smaller than 0.5%, angular dispersion of A0<0.5' and a 4-mm beam spot in the final focus plane. The average intensity of the delivered beam was around 2x 10' s-' [ 121. Table 3. Radioactive ion beams produced by ACCULINNA facility at the Be target (primary beam intensity - 6.25 . 10'' pps).
Table 2 shows the efficiency of the tritium beam transportation from the ion source to the physical target. I
The U-400M operation in 1998-2004
Hours
3500 3000
2500 2000 1500 1000
500
1998
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2001 Year
2002
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Figure 3: The diagram of U-400M operation in 1997-2004 In 200012002 the first stage of the DRIBs project has been realized at the U400-U400M accelerator complex.
All together, the beam quality, target parameters and performance of detector telescopes, allow one to have an experimental resolution of -500 keV for the widths of 'H resonance states which could result from the t+t reaction.
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A series of experiments on the production of radioactive ion beams from Li to 0 with energy of 3 0 ~ 5 0MeV/nucl was carried out at the ACCULINNA facility [ l 11. On the focal plane of the facility spots of 6He, 8He, “Li, ”Be beams were about 10 mm in diameter, the ion energy spread - AE/E = 5%. The obtained results are presented in Table 3. This year a series of experiments on 6He ion beam will be started.
References 1. “JINR FLNR Scientific report 1995-1996”, Dubna (1 997), “JINR FLNR Scientific report 1997-1998”, Dubna (2000). 2. G. Gulbekian and CYCLOTRON Group, “Status of the FLNR JINR Heavy Ion Cyclotrons” in Proc. Of 14‘hInt. Conf. On Cyclotrons and Their Applications, Cape Town, South Africa, 1995, p. 95. 3. Yu.Ts. Oganessian, G.G. Gulbekian, B.N. Gikal, M. El-Shazly et al., ”Axial injection system for the U-400 cyclotron with the ECR-4M ion source”, in JINR FLNR sci. rep. 1995-1996, Heavy Ion Physics, Dubna 1997, p. 270. 4. 0. Borisov, B. Gikal, G. Gulbekyan, I. Ivanenko, I. Kalagin, “Optimization of the axial injection system for U-400 cyclotron (linear buncher)”, in Proc. of EPAC2000, Vienna, Austria, 2000, p. 1468. 5 . Yu. Ts. Oganessian, GG. Gulbekyan, B.N. Gikal, I.V. Kalagin et al. “Status of the U400 cyclotron at the FLNR JINR”, in Proc. of APAC-2004 Conf., Gyeongju, Korea, 2004. 6. I.Kalagin, I. Ivanenko, G.Gulbekian “The experimental investigation of the beam transportation efficiency through the axial injection system of the U400 cyclotron”, in Proc. of the 2001 Particle Accelerator Conf., Chicago, 2001, p. 1568. 7. Wolf B., “Handbook of ion sources”, CRC Press, 1995. 8. Yu.Ts.Oganessian, http://flerovlab.iinr.ru/flnr. 9. G.G.Gulbekian, I.V.Kolesov, V.V.Bekhterev et al. “Axial injection system for the U-400M cyclotron with an ECR ion source” in JINR FLNR Sci. Rep. 1993-1994, Heavy Ion Physics, Dubna 1995, p. 227. 10. A. Efremov, V. Behterev, S.L.Bogomolov, V.B.Kutner, A.N.Lebedev, V.N.Loginov, N.Yu.Yazvitsky “Performance of the ion source DECRIS14-2”. Rev. Sci. Instrum. 69(2) (1998) 662. 11. Rodin A.M. et al., NIM B126 (1997) 236. 12. Yu.Ts. Oganessian, G.G.Gulbekian, S.L.Bogomolov et al. “Production and acceleration of tritium ion beam at the U-400M cyclotron”, Proc. of the 16‘h Int. Conf. On Cyclotrons and Their Applications, East Lansing, USA, ( 2001), p. 466.
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RADIOACTIVE ION BEAMS IN BRAZIL (RIBRAS)*
R. LICHTENTHALER, A. LEPINE-SZILY, v. GUIMARAES, c. PEREGO, v. PLACCO, 0. CAMARGO JR., R. DENKE, P.N. DE FARIA, E.A. BENJAMIM, R.Y.R. KURAMOTO, N. ADDED, G.F. LIMA, M.S. HUSSEIN Departamento de Fisica Nuclear Instituto d e Fisica da Universidade d e S i o Paulo CP 66318, 05315-970 ,960Paulo S P J. KOLATA University of Notre Dame - USA A. ARAZI Tandar - Argentina
A double superconducting solenoid system is being installed at the Pelletron Laboratory of the University 0s SLo Paulo. This system allows the production of secondary beams of light exotic nuclei like 8 L i , 6 H e and others. The first results using this facility are presented.
1. The RIBRAS project
The Pelletron Laboratory of the University of ,550 Paulo installed the first South America Radioactive Ion beam device (RIBRAS) l Y 2 .This facility extends the capabilities of the original 8MV Pelletron accelerator by producing secondary beams of unstable nuclei. The most important components in this system are the two new superconducting solenoids. The solenoids have 6.5 T maximum central field (5 T.m axial field integral) and a 30 cm clear warm bore, which corresponds to an angular acceptance in the range of 2 5 0 _< 15deg in the laboratory system. The solenoids were conceived by Cryomagnetics INC and were designed to operate in conection with the Linac post-accelerator, presently under construction. With the LINAC, the energy of the primary beam will be about 2 - 3 times larger * Auxilio pesquisa fapesp no.97/9956-5, projeto temAtico fapesp no.2001/06676-9
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than the maximum energy of the present Pelletron Tandem of 8 MV terminal voltage (3 - 5MeV.A). The presence of two magnets is very important to produce pure secondary beams. The first solenoid makes an in-flight selection of the reaction products emerging from the primary target in the forward angle region. As the first magnet transmits all ions with the same magnetic rigidity m E / Q 2 the radioactive secondary beam can be rather contaminated. With two solenoids, it is possible to use differential energy loss in an energy degrader foil, located at the crossover point between the magnets (crossing mode of operation). This degrader foil will allow the second solenoid to select the ion of interest by moving the contaminant ions out of its bandpass. An additional future possibility of the two solenoid system is the production of tertiary beams using a secondary target in the middle scattering chamber. The second solenoid can be tuned to select a different magnetic rigidity producing low intensity (1-100/s) tertiary beams like 9Li,8He.3t4This is in principle possible with secondary beams of 107/s and assuming a typical conversion efficiency of for the production reaction.
2. Recent developments
The RIBRAS beam line is presently mounted in the experimental room of the Pelletron accelerator Laboratory. Figure 1 The production system consists of a gas cell, mounted in a I S 0 chamber followed by a tungsten Faraday cup which suppress the primary beam and measures its current. The gas cell was mounted with a 2.2pm Havar entrance window and a 9Be vacuum tight exit window 12p thick which plays the role of the primary target and the window of the gas cell at the same time. The gas inside the cell has the double purpose of cooling the Berilium foil heated by the primary beam and as production target. In case we want t o use a gas target to produce secondary beams, the Berilium foil can be replaced by another Havar foil and the pressure inside the cell can be increased up t o several Bars. In table 1 we present some typical production rates and reactions used at Notre Dame and at RIBRAS, SBo Paulo. The first radioactive beams produced by this system were delivered during the XI11 J.A. Swieca Summer School on Experimental Nuclear Physics on February/2004 using only the first solenoid. The 8Li and 6 H e particles produced by the reaction of the 7Li primary beam on the ' B e primary target were focused by the first solenoid in the the scattering chamber located at the crossover point between the two solenoids. The secondary beam
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Figure 1. RIBRAS Facility installed in the 45B Pelletron beam line
Production reaction
secondary beam (part/s/pAmp) lo6 105 105 105 103
Note: (*)Production reactions measured at RIBRAS
profile (x-y) was measured by a Paralell Plate Avalanche Counter (PPAC) placed in the crossover point. A triple E(150pm) -E(15Opm) - AE(20pm) silicon telescope placed at zero degrees and 5cm after the PPAC allowed the identification of the atomic number; mass and the energy of the secondary beam particles. The secondary beam spot measured at the PPAC position was of about 7mm in diameter which is consistent with a primary beam spot size of 4 - 5mm multiplied by a magnifying factor of 1.5 of the first solenoid. Figure 2 and Figure 3 show the telescope spectra with the solenoid 1 tuned to select 'Li and 6 H e ions, respectively. The production rates measured at RIBRAS for these two exotic ions were of about 1 0 5 p / s and 106p/s, respectively per microampere of primary 7Li beam.
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Figure 2.
E - A E spectrum for the gBe(7Lz,6H e ) reaction
637 334 175
92
Figure 3.
E - A E spectrum for the 9Be(7L2,8Lz) reaction
The second solenoid is mounted and in place waiting for the secondary scattering chamber to accomplish the system.
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3. Conclusions In conclusion, a double superconducting 6.5T facility is installed at the Pelletron Laboratory of the University of SBo Paulo to produce secondary beams of radioactive nuclei. The two solenoids are mounted and tested on the 45B beam line of the Pelletron experimental area. The system begun its operation with only the first solenoid and using the ’Li primary beam delivered by the 8MV Pelletron Tandem. Secondary beams of Li and H e were produced. Experiments using these secondary beams are in progress.
’
References 1. Progress in RIBRAS Radiactive Ion Beams in Brasil Project.
R. Lichtenthaler, A. LCpine-Szily, V. Guimarfies, G. F. Lima, M. S. Hussein. Nuclear. Inst. and Methods. A505 (2003) 612-615c. 2. Radiactive Ion Beams in Brasil (RIBRAS) R. Lichtenthaler, A. LCpine-Szily, V. Guimarks, G . F. Lima, M. S. Hussein. Brazilian Journal of Physics 33,110.2 (2003)294
3. “A Radioactive Beam Facility using a Large Superconducting Solenoid”, J. J. Kolata, F. D. Becchetti, W. Z. Liu, D. A. Roberts and J. W. Janecke, Nucl. Instrum. Meth. B40/41 (1989) 503. 4. F.D. Bechetti, M.Y.Lee, T.W. O’Donnell, D.A. Roberts, J.A. Zimmerman,
J.J. Kolata, V. Guimariies, D. Peterson, P. Santi Nucl. Instrum. and Methods in Phys. Res. A422 (1999)505
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THE S L O W R I - B E A M FACILITY AT R I K E N RIBF
M. WADA Atomic Physics Laboratory, RIKEN, 2-1 Himsawa, Wako, Saitama, 351-0198 JAPAN E-mail: mwOriken.go.jp The slow RI-beam facility (SLOWRI) at RIKEN aims at providing universal slow or trapped unstable nuclei of high purity by combining a projectile fragment separator BiglUPS and a deceleration and cooling device, rf ion-guide. This new facility will provide a unique opportunity to perform precision atomic spectroscopy for a wide variety of unstable nuclei.
1. Introduction
Using modern atomic spectroscopy techniques extremely high precision measurements have become possible of various fundamental quantity of atomic nuclei. For instance, atomic masses and the hyperfine structure of ground state ions have been measured with a precision of using ion trapping and laser spectroscopy techniques. Such techniques should play important roles also in studies of radioactive nuclear ions (RI). So far low energy RI beams have been provided mainly by ISOL (Isotope Separator On-line) facilities. However the available nuclides at ISOL facilities are limited, since chemical processes in production targets and in ion-sources are element dependent. The slow RI-beam facility (SLOWRI) at RIKEN aims at providing universal slow or trapped RI of high purity by combining a projectile fragment separator BigRIPS' and a deceleration and cooling device, rf ion-guide2. This will allow a unique opportunity to perform precision atomic spectroscopy for a wide variety of RI, not available in so far existing facilities. 2. Facility
SLOWRI will be located at one of the branches of the projectile fragment separator BigRIPS (Fig. 1). The energetic RI-beams from BigRIPS are then passed through an energy degrader and thermalized in a catcher gas cell
657 filled with He of -133 mbar. Because of the high ionization potential of He most radioactive ions end up as singly charged ions that can be manipulated by applied electric fields. A combination of dc fields and inhomogeneous rf electric fields in the gas drive the ions to a small exit nozzle (rf ion guide). There the ions are extracted from the gas cell and entered into a rf six-pole beam guide (SPIG)3 that separates the ions from the He gas, forms a lowenergy bunched beam and delivers this beam to high vacuum where various trap experiments can be performed. This beam can also be entered into an electro-magnetic mass separator to provide -10 keV pure isotope beams. Stopping 5 Cnoltng of >I00 MeV/u R l beom
Figure 1. Schematic overview of the SLOWRI facility.
Beam intensities at SLOWRI as expected in 2007 are shown in Fig. 2. These values are based on the evaluated beam intensities of energetic RIbeams at BigRIPS4 and the present performance of the rf ion-guide5. In online tests using a -100A MeV beam of *Li ions from the RIKEN fragment separator RIPS an overall efficiency of -5% was obtained when the beam intensity was lo4 ions per second. A major loss in the test setup was in the degrader which is placed in front of the catcher gas cell which for sLi ions allows only 15% of the nuclei to be stopped in the gas cell. In case of heavier ions larger percentages are expected. For higher beam intensities the overall efficiency decreases, however, because of space-charge effects in the gas6. For nuclei close to stability the intensities would be limited by this effect, which is not included in the figure. For nuclei far from stability the obtainable intensities are mainly limited by the lifetime of the nuclides due to the finite extraction time from the gas cell. In the evaluation therefore for nuclides of < 1 s. the efficiency was assumed to be E = 0.01 x In total we expected that in the year 2007 more than 2800 nuclides will be available with intensities higher than 0.01 ions per second. Characteristics of the SLOWRI facility are summarized to be:
658 0 0
0 0
0
A wide range of nuclides High purity, no isobar or isotone contamination Small emittance (lr mm mrad) and short bunched beams Variable energy range, in traps (-0 eV) or accelerated (1-10 keV, 1 MeV f u : future option with additional accelerators) Human accesibility during on-line experiments
Estimated Yield of Slow RI-Beams
Figure 2.
Expected yield of slow RI at SLOWRI.
3. Planned experiments
Various static properties of nuclei can be determined through precision atomic spectroscopy of trapped unstable ions and beams of low energy unstable nuclei. Typical quantities to be measured at SLOWRI are: 0
0
0 0 0
Atomic masses Charge radii Valence neutron radii Nuclear moments Abundances of protons and neutrons at the surface of nuclei
In addition, the obtained pure and small emittance beams of low energy unstable nuclei can be injected into other accelerators for further acceleration and into other experimental facilities. One such possibility of the latter
could be the SCRIT facility, where electron scattering experiment could be performed to determine nuclear charge form factors.
3.1. Mass measurements The atomic mass is one of the most important quantities of a nucleus and have been studied in various methods since the early days of physics. Among many methods we chose a multi-reflection time-of-flight (MR-TOF) mass spectrometer. Slow RI beams extracted from the rf ion-guide are bunch injected into the spectrometer with a repetition rate of -500 Hz. The spectrometer consists of two electrostatic mirrors between which the ions travel back and forth repeatedly. These mirrors are designed such that energy-isochrononicity in the flight time is guaranteed during the multiple reflcctions while thc flight time varys with the masses of ions. A massresolving power of >60000 has been obtained with -500 reflections in a 30 cm length spectrometer7 which should allow to determine ion masses This accuracy is lower than that obtained with with an accuracy of a Penning trap mass spectrometer, however, it is sufficient to study many r-process nuclides, for instance. The advantages of the MR-TOF spectrometer are: 1) short measurement periods, typically 2 ms, which allows all neutron rich nuclei to be investigated, 2) the device is compact and its operation is simple, especially, it is independent from the upstream devices, such as accelerators and fragment separators, 3) isobars can be measured simultaneously, so that mass reference can easily be established in the mass spectra. In total, the number of measurable nuclides within a limited beam time would be larger than that can be achieved by other methods. It should be noted that this method can be used even during a low-duty parasite beam time. 3.2. Collinear laser spectroscopy
The root-mean-square charge radii of unstable nuclei have been determined exclusively with isotope shift measurements of the optical transitions of singly-charged ions or neutral atoms by laser spectroscopy. Many isotopes of alkaline, alkaline-earth and noble-gases which all have good optical transitions and are available at conventional ISOL facilities have been measured by collinear laser spectroscopy. However, isotopes of other elements have not been investigated so far. In SLOWRI, isotopes of all atomic elements will be provided as well collimated mono-energetic beams. It should expand the range of applicable
660 nuclides of laser spectroscopy. In the first years of the RIBF project, Ni and its neighboring elements, such as Ni, Co, Fe, Mn, Cr, Cu, Ga, Ge are planned to be investigated. They all have possible optical transitions in the ground states of neutral atoms with presently available laser systems. Some of them have so called recycle transitions which enhance the detection probability noticeably. Also the multistep resonance ionization (RIS) method can be applied to the isotopes of Ni as well as of some other elements. The required minimum intensity for this method can be as low as 10 atoms per second.
3.3. Hyperfine s t r u c t u r e spectroscopy The proton distributions in a nucleus can be studied by determining the charge radius of the nucleus, however, the neutron distributions are hard to be determined, since neutrons have no net electric charges but only have magnetization. High precision measurements of the ground state hyperfine splittings for a series of isotopes enable us to study the isotope shift in M1 term which is the so called differential hyperfine anomaly. The main part of this anomaly is due to the Bohr-Weisskopf effect which stems from the finite distribution of the magnetization in the inhomogeneous hyperfine field at the nucleus8. This effect would be particularly useful to investigate the root-mean-square radius of a valence neutron in case of neutron-odd nuclei. For Be isotopes such experiments are already in progress at the present facility as phase-0 experiments of the SLOWRI projectg>l0."Be is known as a neutron halo nucleus so that a large hyperfine anomaly is expected due to a large root-mean-square radius of the valence neutron of 'lBe1'. The precision hyperfine structure spectroscopy would provide the first confirma tion of the halo nuclei with a reliable probe of electro-magnetic interaction. Other elements such as Mg, Ca, Sr, Ba and Ra are planned to be investigated at SLOWRI.
3.4. Antiprotonic R I atoms The different abundances of protons and neutrons at the surface of a nucleus are important for nuclear structure studies. Antiprotonic atoms would be excellent probes for such different nucleon abundances at the surface of a nucleus, since annihilation of an antiproton occurs dominantly with a surface nucleon and since the vanished nucleon can be identified by the total charge of the emitted pions or the residual nucleus.
Antiprotonic RI atoms12 are planed to be produced in a nested Penning trap, in which a cloud of p can be trapped and into which slow RI ions are repeatedly bunch-injected. A typical value of the antiproton capture cm2 for llLi+ ions when the relative energy cross sections is 1.3 x is 0.1 atomic units13 which corresponds to a "Li+-beam energy of 33 eV in the p rest frame. Assuming the number of trapped p to be 5 x lo6 and that they are confined to 1 mm2, the target density is Np = 5 x 10' cm-2. Slow RI ions are then bunch-injected and pass through the p cloud for 5 x lo5 s-l if the ions are 33 eV llLi+ and the trap length is 4 cm. Since we are mainly interested in very short-lived nuclei, we assume a short measurement cycle of 10 ms, in which only 10 RI-ions are involved when the RI-beam intensity is lo3 s-'. The production rate per cycle is then Y = 1.3. x 5.10' x 10 x 5 . lo3 = 3 . Thus, in total 3 p-11Li2+ ions can be expected per second. Since the RIKEN accelerator facility does not provide p, a major effort should be devoted to the development of a portable trap for p in order to bring them to RIKEN from the CERN antiproton decelerator facility. 4. Summary
The combination of BigRIPS and the rf ion guide technique enables us to obtain universal slow RI-beams which will be used for various nuclear structure studies. Although there are several similar devices being build at other facilities worldwide, none of them are yet in operation for high energy RI-beams. The SLOWRI facility will be a unique and single facility, at least, until RIA in the US or FAIR in Germany will operate. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
T. Kubo, Nucl. Instrm. and Meth. B204, 97 (2003). M. Wada et al., Nucl. Instrm. and Meth. B204, 570 (2003). S. Fujitaka et al., Nucl. Instrm. and Meth. B126, 386 (1997). T. Suda, private communication. M. Wada, Nucl. Instrm. and Meth. A532, 40 (2004). A. Takamine, et al., to be submitted. Y. Ishida et al., Nucl. Instrm. and Meth. B219-220, 468 (2004). A. Bohr and V. F. Weisskopf, Phys. Rev. 77,94 (1950). K. Okada et al., J. Phys. SOC. Jpn. 67, 3073 (1998). T. Nakamura et al., Opt. Commun. 205, 329 (2002). T. Fbjita et al., Phys. Rev. C 59, 210 (1999). M. Wada and Y. Yamazaki, Nucl. Instr. and Meth. B214, 196 (2004). J. S. Cohen, Phys. Rev. A69, 022501 (2004).
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FIRST RADIOACTIVE BEAMS AT THE EXCYT FACILITY M. MENNA, G. CUTTONE, M. R E , F. CHINES, G. MESSINA, A. AMATO, L. CALABRE'ITA , F. CAPPUZZELLO , L. CELONA , L. COSENTINO , P. FINOCCHIARO, S. GAMMINO, D. GARUFI, S. PASSARELLO, G. RAIA, D. RIFUGGIATO , A. ROVELLI , G. SCHILLARI , V. SCUDERI Laboratori Nazionali del Sud, Istituto Nazionale di Fisica Nucleare, Via S. Sofia, 44 Catania, 95123, Italy The EXCYT facility (Exotics with CYclotron and Tandem) is based on a K-800 Superconducting Cyclotron injecting stable heavy-ion beams (up to 80 MeV/u, 1 epA) into a target-ion source assembly (TIS) to produce the required nuclear species, and on a 15 MV Tandem for post-accelerating the radioactive beams. The TIS was successfully tested at the SIRa test-bench of SPIRAL, GANIL, by shooting a 13C primary beam (60 MeV/u) on a '*C target under the same operational conditions that will be initially used at EXCYT. The measured yields and production efficiencies for 9Li were compatible with the ones obtained at SIRa. The commissioning of the facility and the start of the first experiment with 'Li is foreseen by the end of 2004 ST
1.
Introduction
At the EXCYT facility the primary beam coming from the SERSE ECR ion source goes into a K = 800 cyclotron (up to 80 MeV/u, 1 epA) [l], and then to the target-ion source assembly. Here the species of interest are produced mainly by projectile fragmentation in a target thicker than the projectile range. The products diffuse through the heated target, are desorbed from its surface and via a transfer line go to the ion source where they are ionised. Then, when positive, they pass through the vapours of a charge exchange cell to become negative. They are mass-separated in two stages, the first with a resolving power of about 2000 and the second up to 20000. It is worth noting that besides the 15MVTandem post acceleration there is also an option to supply 300 keV beams for low energy experiments. The facility is described and shown in a number of publications and reports [2, 31. For the development of prospective radioactive ion beams or to check their feasibility, it is of paramount importance to carry on tests for determining the efficiencies of the production, diffusion, desorption, effusion and ionisation. The first radioactive beam will be 'Li taking into account the requests of the nuclear scientists.
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2.
Target-Ion source assembly
The main characteristics of an ideal target are: 1. High production rates. 2. Large diffusion coefficient of the products. 3. Low product hold-up time on surfaces. 4. Large values of dissipated power. 5. High melting point. 6. Low vapour pressure to preserve ion-source efficiency (c lo-' mol/s). 7. Low Z to reduce activation. 8. No sintering. 9. No reaction with the container. 10. No damage due to the radiations. By taking into account some of these characteristics (points 4-10), graphite has been chosen as a suitable material. To comply also with favourable production and release properties (points 1-3), it has been necessary to select among the many kinds of commercially available graphite the ones with higher open porosity, lower closed porosity, smaller grain size, hgher thermal conductivity, higher density. After an extensive market research, we chose the graphite type UTR146 manufactured by XYCARB with a few parts per million of impurities. Experiments with radiotracers in collaboration with CERNISOLDE clearly showed a better release from this kind of graphite compared to other materials [4]. As for the production rates, we were less restricted on the choice since at EXCYT the radioactive nuclides are obtained mainly by projectile fragmentation. Nevertheless, we obtained good yields of "F, 'Be and 22324Na by shooting "F (48 MeVh) on a XYCARB UTR146 graphite target [5]. The previous TIS assembly [6] proved to be unreliable and was therefore radically modified in its shape, dimensions and geometry. In the new TIS (shown in figure 1) the graphite target is standing in a tantalum container, whch in turn is inside a tantalum heater. In this configuration, the heater doesn't touch the container and the primary beam impinge on the target from the top. The target container is heated by irradiation from the heater; the target is heated by : irradiation, contact at its bottom, primary beam power. The effective target volume is mainly its upper part, the rest constituting a simple mechanical support. In fact, the thckness of the upper part is chosen according to the range of the primary beam and of the produced radioactive nuclides. In collaboration with PNPI-US, Gatchina, a new type of combined targetion source unit has been developed for the on-line production of radioactive singly charged ions [ 7 ] . In this configuration the target is able to withstand high temperatures and acts also as an ioniser (ionising target), thus making the
664
effusion delays shorter. Off and on-line experiments with the ionising target using different target materials have been carried out. The off-line ionisation efficiency measured for stable atoms of Li and Mg varied between 0.14 and 5.4 %, while the on-line yields of neutron-deficient isotopes of Pm, Sm, Eu, Gd, Tm, Yb and Lu, which depend on the ionisation and on the release efficiencies, were within the interval 0.1 - 7%. Rather unexpectedly, we observed for the first time an axial magnetic field influence on the ionising efficiency of atoms with low potential of ionisation such as Li and neutron-deficient isotopes of Pm, Sm, Eu, Gd, Tm, Yb, Lu produced in on-line experiments
ioactive Beam
the Ion Source Figure 1 . The new TIS. The real target area is mainly constituted by its upper part and is tilted to enhance both power dissipation and desorption.
Given its physical and chemical properties, the ionisation efficiency of lithium can be conveniently studied by implanting stable 6Li into the graphite substrate. The substrate is then positioned nearby the ion source and heated to allow for the escape of the implanted atoms to be ionised. Then the heating temperature is varied in order to keep the extracted beam in the range 1-10 nA (operational values for radioactive beams), the total extracted charge for the selected mass is recorded and converted into the number of extracted ions. The ratio of the latter to the implanted atoms gives the lower limit for the ionisation
665
efficiency. By using an ISOLDE-type Positive Ion Source with a hollow W cathode, we obtained an efficiency of 75% for 6Lif, very close both to its theoretical value and to on-line extrapolations [8]. 3.
The experiment of '9Liproduction
Since the TIS had been radically modified, it became necessary to verify off-line its behaviour with respect to the mechanical and thermal stresses at 2300 K, as well as to obtain information about the on-line production and the release processes. The target behaviour in terms of mechanical and thermal stresses was simulated by means of the ANSYS code under the simultaneous influence of the heating power and of a 400 W primary beam, while off-line tests at LNS showed that the assembly was mechanically and thermally stable up to 2300 K. For radioprotection purposes this was done long before conducting the real experiment. The results allowed to determine the target and the heater temperatures as functions of input heater current and primary beam power. In addition, we could extrapolate the best operational parameters and ascertain that under those conditions the target was not going to break. Details of the simulations can be found in reference [9]. Since these preliminary studies suggested the reliability of the new TIS assembly, it was decided to run an experiment at the SIRa test-bench of SPIRAL, GANIL, by shooting a 13C primary beam (60 MeVIu) on a '*C target under the same operational conditions that will be initially used at EXCYT [ 101 3.1. The on-line experiment In May 2003 the TIS was mounted and outgassed at SIRa. Because of radioprotection constraints the maximum primary beam power was limited to 370 W but there was no sign of leaks or breakdown, despite three abrupt thermal cycles from about 1700 K to room temperature due to failures of the power supplies and five additional on-line failures of the primary beam (i.e. unwanted thermal cycles). The graphs in figure 2 show the production efficiencies for *' 'Li: these are defined as the ratios of the extracted radioactive ions to the atoms produced by nuclear reactions in the target core, as estimated via the EPAX code. It is clear from the plots that the efficiencies increase by passing time. This is mainly due to three factors: loss of impurities from the target, increase of target temperature by the primary beam power and increase of the heater temperature by o h c resistive heating. Currently we are estimating the influence of the second factor
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on the third one as well as trying to unfold the three stages of the production process: diffusion, effusion and ionisation, thus getting the efficiencies for each stage. 0.5
1
4
0.4
8 0
g 0.3
i
:0.2 4
-a
iE
0.1 0.0 14W5833
Figure 2. Production efficiencies for 8. 9Li versus time. The production efficiency is the ratio of the extracted radioactive ions to ones theoretically produced into the target (estimation by the EPAX code)
The yields for ” 9Li (0.480% and 0.063% respectively) are compatible with the estimation made at the beginning of the project and will fulfil the EXCYT requirements providing 3.3 1O5 pps of post-accelerated ‘Li beam and 7.4 1O3 pps of post-accelerated 9Li beam (rescaled to 500 W primary beam). However, they can be improved by adding a Re liner inside the ioniser and by increasing the target and ion source operational temperatures. A comparison with SPIRAL graphite target shows that its efficiency (1.2% for ‘Li and 0.14% for 9Li) [l 11 is about twice compared to EXCYT. Some tentative explanations for this difference are: 1. A better performance of GANIL assembly with respect to EXCYT’s one. 2. We could not push the TIS to its real limits in terms of duration (schedule), target temperature (power supplies limitations) and primary beam intensity (radioprotection constraints). 3. The figures for the production efficiencies are affected by a large error (+ 100%) coming from the uncertainties in the EPAX code. We remind that the figures for SPIRAL are based on another nuclear reaction, namely 95 MeVIu 36Aron graphite.
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By taking into account the third point, the values are consistent and their overlap seems to indicate an order of magnitude for the efficiency of lithium produced from graphite matrices: around 1% for ‘Li and 0.1% for 9Li. 4.
Conclusions
This year signed two milestones for the EXCYT activity: the radioactive beams *’ ’Li were successfully obtained at GANIL with the EXCYT TIS and a 100 W I3C primary beam was extracted at LNS [3]. In 2004, EXCYT will be authorised to work with a primary beam power of 500 W. The commissioning of the facility and the start of the nuclear experiments with ‘Li is foreseen by the end of 2004. The experimental programme takes into account the availability of the MAGNEX detector [12, 131, the requests and the f i s t results obtained by the “Big Bang” collaboration [ 141 and the RSM experiment [ 151.
References 1. D. Rifuggiato et al., Status report of the LNS Superconducting Cyclotron, Nukleomka Vol. 48 Supplement 2,2003, pp. S131-S134 2. G. Cuttone et al., Status of the EXCYT facility at INFN-LNS, Proc. of the International Conference on the Labyrinth in Nuclear Structure, Crete, Greece 2003 (in publication on American Institute of Physics, 2004) 3. G. Cuttone et al., The EXCYT facility towards its commissioning,LNS Report 2003 (2004) 4. M. Menna et al., Release of implanted fluorine from C and S i c matrices, (in preparation) 5. M. Menna, NIM B184 (2001) 466 6. G. Ciavola et al., LNS Report 1996-99, (2000) 225 7. V. N. Panteelev et al., Off-line and on-line tests of ionizing targets, PNPI preprint 251 1 (2003) 1 8. Ravn et al., NIM B88 (1994) 441 9. M. Re et al., Thermal simulation for the EXCYT target assembly, LNS Report 2003 (2004) 10. M. Menna et al. , The experiment E435 at Ganil , LNS Report 2003 (2004) 11. S. Gibouin, Ph.D. Thesis No T 03 02,2003, University of Caen, France 12. A.Cunsolo et al., NIM A481 (2002) 48 13. A.Cunsolo et al., NIM A484 (2002) 56 14. S. Cherubini et. Al., Eur. Phys. Journ. A20 (2004) 355 15. A. Di Pietro et al., Using the resonance scattering method to study Li-He cluster states in boron exotic isotopes, LNS Report 2003 (2004)
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ISOTOPES PRODUCTION FROM UCX TARGET FOR THE SPES PROJECT A. ANDRIGHETTO Laboratori Nazionali di Legnaro, Viale dell 'Universita ' 2, 35020 Legnaro (Padova), ITALY - (e-mail:[email protected]) The study of an optimal solution for an Exotic Beam target system using a high intensity (up to 10 mA) proton primary beam impinging on an UCx Target is reported in the framework of the SPES Project of Legnaro INFN Laboratories (Italy). Two different topologies on the production Targets using a low energy primary beam (E<50 MeV) have been considered: The l-Steo configuration (primary beam direct on UCx Target device) and the 2-Stem one (beam impinging a neutron converter, followed by an UCx Target). In both configurations Isotopes Fragments Yields and Spectra Shape have been studied as a hnction of the primary beam energy.
1. Introduction Large part of the experiments using exotic beam, performed at the existing firstgeneration facilities are, at the present, limited by the poor beam intensity. Nevertheless, the development of high intensity, up to tens of mA, primary beam accelerator (Trasco Project) [l], opens a new field in RIB'S technologies. The present study investigates the solutions, in the framework of the R&D of the LNL SPES Project, to optimize the nuclei production by means of UCx target. In the report, only the On-Target isotopes production is considered. Technological problems related to the power dissipations or isotopes extraction from the target (e.g. diffusions, effusions, ionisations, etc), are not analyzed. 2. The SPES project The SPES project (Study for the Production of Exotic Species) [2] consists of an accelerator facility providing intense neutron-rich radioactive ion beams of highest quality, in the range of masses between 80 and 160. This facility is planned to be constructed at the Legnaro INFN Laboratory (Italy). The first design project 131 was based on a high intensity 100 MeV (100 kW) proton Linac driver. As a first step toward SPES and High Intensity accelerators, the socalled SPES-l phase has been funded by the Italian Institute for Nuclear Physics (INFN). In addition, this phase will be useful for interdisciplinary physics and medical applications. The total cost for SPES-1 is estimated to be 18.7M€ for the next 5 years. The Preliminary layout of the project is reported here:
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--
5 MeV 30 mA
TRIPS
RFQ
V 10 mA
M~BT
Superconducting Linac
BNCT n-source
Figure 1. Layout of the SPES-I
The SPES-1 Project includes:
1. The construction of a high intensity proton 5 MeV injector, based on protons source and on RFQ structure (with the TRIP source) developed inside TRASCO project. 2. The R&D on RFQ driven intense thermal neutrons source, aimed at interdisciplinary applications and for BNCT test (Boron Neutron Capture Therapy). 3. The R&D of a superconductive LINAC for protons, with a 20 MeV final energy and a 10 mA maximum intensity. 4. A feasibility design study of the SPES-2 phase, aimed at the generations of radioactive ion beams which is planned to be the further step of the project. At the present the RFQ is in the construction phase, the TRIP source is available at the Laboratori Nazionali del Sud (Catania, Italy), the target prototype for the BNCT facility is ready for preliminary beam power tests, and the LINAC is presently at the design stage.
3. Target configuration for the SPES phase 2
The goals of the SPES-2 design study are: a R&D in the solid neutron converters for high power beam, and the optimization of the UCx production target, both with primary beam energy less than 50 MeV. Two different solutions are possible: a) a direct beam on target b) protons or deuterons primary beam coupled with a target-based neutron converter configuration. The two options being named 1-Step and 2-Steps respectively, have shown in Figures 2-3.
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Figure 2. Scheme of the 1-Step configuration: the primary beam impinging on the production Target
Figure 3 . Scheme of the 2-Steps configuration: the primary beam full stopped in a neutron converter. The emitted neutrons induce fission inside the production target
In this paper three different parameters are analyzed, in order to have a comparison of the two solutions under study: the fission yields, the fission fragment distributions and the energy released in the medium. It is important to point out that higher energy beams even if involving less dissipated energy, would increase sensitively the cost of the apparatus. Moreover, the 238U(-> UCx) fission cross sections [4], as reported in Figure 4, shows a plateau, both for neutrons and protons for energies higher than 35MeV: between 35 and 50 MeV particle energy, there is almost the same values (above 1.5 barns) for deuterons, neutrons and protons [5], [ 6 ] . Since the fragment yield per incident particle is almost constant from about 35 MeV, the availability of a high intensity proton Linac will be sufficient for the isotope production as well for energy below 50 MeV.
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238
5
15
25
35
U Fission Cross Section
45
55
65
75
85
95
105
Energy (MeV)
Figure 4. 238UExperimental Fission Cross Section for neutron, proton and deuteron as a function of the incident particles energy.
The main problem in this scenario is the energy loss in passing medium by the incident beam (window, and target) due mainly to of electromagnetic interactions, which are rather high in this energy range. A direct, high intensity beam, fully impinging on the production target, although previously assessed as a possible solution, was at the beginning not considered, because of hard UCx material beam removal, when a 10 KW, or even higher power loads, should be employed. However if protons with higher fission cross sections only are driven in a thin production target, e.g. 10% of the total one, while the rest being lost inside a passive dump, an interesting way to yield enough fission residuals could be taken into account. In this paper, the neutron converter based configuration, named 2-Steps, the most exploited in the RIB'S projects (RIA, EURISOL, SPIRAL 2), and the 1-Step configuration using a thin target, are compared. All calculations presented here, are based on MCNPX code. 4. Target optimization
In all the simulation trials performed, a thin (200 pm) tungsten disk window layout has been introduced, just before the target system, in order to separate both the beam line and Target-Source void regions. This solution, far from being the optimal one (no light element, like carbon, has been assessed) has nonetheless been selected due to the tungsten high melting point (above 3000 "C).
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A proton beam, with energy between 20 and 50 MeV, in I-Step configuration, has at the beginning chosen mono-energetic and with a Gaussian spot of 1.5 cm. A (UC2) thin target (2 mm thickness), with the usual density of 2.5 g/cm3 has been considered. The total volume of the target is about of 5.6 cm3 with a total mass of approximately 14 g. In Table 1, the total fission fragment yield, the double-magic "'Sn isotope yield , and the power dissipated in the medium considered, is reported for each beam energy considered.. As expected (see Figure 4), the fission residuals production is not very sensitive to the proton energy, when the beam energy above 30 MeV is used. Another important parameter, is the energy dissipated in medium by the charged beam. For low energies particles, the beam energy loss due to the electromagnetic interaction is high, while this value decreases when the beam energy becomes higher. In the Table 1 the beam energy loss in the production target is about 4 MeV from 20 to 35 MeV, while it is only 1 MeV when passing from 35 to 50 MeV. Considering both the isotope production in the target and the dissipation of energy in the medium, it appears that the optimal solution is when a 35 MeV proton beam is employed. Table 1, Fission fragment yield and energy loss in 1-Step p configuration, for different incident proton energies Incident
Fission
'"Sn Yield
Proton Energy
Residual
(per incident
(MeV)
(per incident
proton)
(MeV)
(MeV)
Energy (MeV)
4. 8 . 10.' 9 '10" 8 .10" 7.
4.8 4 3.4 3 2.3
8.4 6.3 5.2 4.4 3.3
7 15 22 28 45
Energy Loss
Energy Loss
in W Window in UCx Target
Average Out coming Proton
proton)
20 25 30 35 50
9. 2.8 . lo" 3.5 4.lop3 4.3 .
The fission fragment mass distribution for the different proton bombarding energies is shown in Figure 5 . It appears that at 35 MeV incident energy the symmetric valley of the mass distribution has been already filled and the only difference with the 50 MeV incident energy is that in this case the distribution is wider. In table 2 the same quantities reported in the Table 1 are shown, for a 35 MeV primary proton beam, with intensities from 0.1 to 2 mA. When a ImA of beam current is used, a thermal power dissipation in the target of 4.5 kW is
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achieved, which corresponds to power density of approximately 800 W/cm3 in case of uniform beam distribution. Residual Yield: p-UCx
-s
1.00E-04 +ZO
c
MeV
e
1.00E-05 a,
9 E 1.00E-06 I
9 a,
> 1.00E-07
70
80
90
100
110
120
130
140
150
160
170
A
Figure 5. Fission Fragment mass distribution for 1-step p configuration for different incident protons energies.
The best solution is based on a 35 MeV 1mA current proton beam solution, which supplies the maximum fission yields, and, in the same way, provides the possibility to dissipate the power in the target and in the window. In fact, preliminary calculations using a similar UCx disk, with the same power dissipation reveals that the target mean temperature remains below from the melting point. Table 2. Fission residuals and power loss, in 1-Step p configuration, with E ~ 3 MeV, 5 varying the beam current
Incident Proton Current (mA) 2 1
Power Loss in the UCx Target (kW) 9
Power Loss in the Dump (kW)
3.5
4.5
27
1.7
2.2
13
0.3
0.4
2.7
Fission I3'Sn Yield Power Loss (l/s) in the W Residual Window (Us) (kW) 5 . loi3 1 . loii 7 2.5.loi3
5 .loio
0.5
1.2 . loi3 2.5 . loio
0.1
2.5 . loi2
5 . lo9
54
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Using this target system configuration, a large amount of the power impinges directly the dump (27 kW); 3.5 kW hits the window, while the remaining 4.5 kW hits the production target. The total fission residuals in UCx target is about 2.5 . lOI3 per atom/s. Nevertheless we have assessed other possible configurations using primary beam energy at 35 MeV. First of all, the 1Step configuration with deuterons has not been taken in account in this phase, since the d-238Ufission cross section is similar to the ones, at this energy. We have considered also a 2-steps configuration for both protons and deuterons. For both types of target systems, an easier and efficient neutron converter has been analyzed, and almost the whole beam power is dissipated inside. For the 2-Step solution, two different , namely the I3C and I2C, converters thick enough to completely stop the primary beam, have been considered. The emitted neutrons induce fissions in a cylindrical 1.5 Kg UCx target (with density of 2.5 g/cm3). The results obtained using the 2-step configuration (for d e p beam) point out a strong reduction of fission residual in both cases with respect to the 1-step option with proton beam. The reduction factor of 1-step configuration is of approximately 200 times using proton beam and almost 30 times using a deuterons one. In Figure 6 are plotted the fission spectra in the three cases considered above: in the 1-step p configuration the central region spectra shows flat trend (symmetrical), while in both the 2-steps configurations, an asymmetrical distribution still appears, due to the contribution caused by low energy neutron induced fissions, originated in the converter.
I
Residual Yield: 35 MeV Comparison
73
83
93
103
113
123
133
143
153
163
A
Figure 6. Fission Spectra for all configurations studied
-+-Yield
35 p-13C-U
+Yield
35 d-12C-U
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It is important to compare the total fission fragments given in all the above configurations studied above, for a fixed beam current of 1 mA. In 1-Step p configuration, the residual in the target is 2.5.1013 atoms/s. For 2-steps p is around 1.5.10" atoms/s, and for 2-Steps d is 8.5.10" atom/s.
5. Conclusion Some possible scenarios of a Target-System, aimed at producing exotic nuclei using an UCx target, with both proton and deuteron primary beam at energy less than 50 MeV, have been investigated. The results suggest that the 1-Step p solution appears to be the most efficient with respect to all others considered. For primary beam energy around 35 MeV and beam current of 1 mA, 2.5.1013 residualds might be produced in target. Preliminary numerical calculations also suggest that the power dissipated inside the production target (< 5 kW), might not represent a critical engineering issue. Further and detailed R&D study on the thermo mechanical behaviour of the Uranium-Carbides target is in any case required.
References 1. Status of the High Current Proton Accelerator for the Trasco Program INFN/TC 00/23 2. SPES Technical Design for an Advanced Exotic Ion Beam Facility - LNLINFN (REP)181/02 3. A. Andrighetto, et a1 , Nucl Instr. Meth. B204 (2003) 205 4. P. C. Stevenson et a1 ,Phys. Rev. 111,3 (1958) 886 5. S. Baba et al, Nucl Phys A175 (1971) 177 6 . B. L. Tracy et al, Phys Rev C5 (1972) 22
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679
THE RUSSIAN - GERMAN CO-OPERATION AT GSI AN EXAMPLE OF SUCCESS AND FRIENDSHIP
HELMUT ZEITTRAGER Gesellschaft fur Schwerionenforschung mbH, GSI Planckstr. 1, 64291 Damstadt, G e m a n y
May I first give some personal remarks: I want t o express my thanks to the organisators of this meeting and all the hospitality we received. It is a great honour and pleasure for me, to be invited to a such distinguished conference here at this historic site at Peterhof. In 1969 I stayed the last time at St. Petersburg. Since its 300th birthday St. Petersburg has revived again in a new splendour after all the indescribable and unimaginable nazi-terror, this beautiful city and mainly its inhabitants suffered for many years. Its 540 bridges seem t o me a symbol of cooperation also in the field of science to use the creative power of our scientists, seen here in the outstanding buildings and arts a t Peterhof and St. Petersburg. In a year of a very rare astrophysical spectacle with the transition of the planet Venus a t the sun, which fascinated us I think in a similar way it did the great Russian scientist Lomonosov around 250 years ago, I feel very proud to be here. I hope that the science to be discussed here will have also that importance like Lomonosov’s observation of the Venus atmosphere in 1761. Co-operation between Russian and German scientists and engineers are not a phenomena of today. One important early example is of course the founding of the Russian Academy of Sciences and Arts here in St. Petersburg, which must be a t least mentioned once in this talk. Via networks of scientists and royal families the Russian Zar and the German philosopher Gottfried Wilhelm Leibniz got involved in creating a concept for a Russian academy for Peter the Great. Also other German scientists participated in the planning of the academy. E.g. Christian Wolff the teacher of Lomonosov at the universities of Marburg and Halle
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Lomonosov observed
the Venus transit in 1761
Figure 1.
M. V. Lornonosov
contributed to the plans. After the opening of the academy by Katharina I in 1725, Wolff accompanied the construction of the Academy and continued his contacts to Lomonosov (Fig. 2). As one important interest of the academy, the "discovery journeys" were promoted. The discovery journeys should explore the Russian territory and establish collaborative scientific measurements. The traditions of the collaboration between Russian and German scientists, started with the academy and the universities, continued and strengthened more and more. Due to focus my talk to the last 30 years, I cannot mention all the highlights during that time, which I have to skip. Collaboration stopped during first World War, but revived shortly after its end again, but ended suddenly after nazis took over power in Germany and during the second World War. After that the scientific relations between the UdSSR and West-Germany were unfortunately transformed into the patterns of the cold war.
68 1
Figure 2.
Figure 3.
Professor Flerov
After the founding of GSI in 1969, the Gesellschaft fur Schwerionenforschung set out on another ”discovery journey” - a journey to unknown territory in the sea of instability where already the scientists of Dubna, Orsay, Berkeley, and other places were present. On the transparency - you all know - you can see Professor Flerov in 1973 standing in the GSI barracks (Fig. 3). He is showing at his famous map the ways to the island of
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the superheavy elements while he was giving a talk on the synthesis of the superheavy elements in the GSI seminar. It seems that Professor Flerov’s travel was part of the rapprochement of East and West, of the Federal Republic of Germany and of the UdSSR: In the ”MOSCOW contract” of 1970, as one important goal, the acceptance of the existing borders in Europe was declared. In 1973 the UdSSR and the Federal Republic of Germany agreed on co-operation in economy, industry and technology. The formal contacts between GSI and JINR got closer. Besides the formal contacts, there were already links to Russia before 1973: In 1970/71 Professor Yuri Oganessian was the first official Russian visitor at the new laboratory for heavy-ion research in Darmstadt, which existed only on plans and first construction that days. Also before 1973, the scientific contacts to JINR were already started. Via the co-operation with the French scientists in Orsay, the GSI got contacts to Dubna and scientists of the GSI were involved in the preparation and in experiments at Dubna. In 1972 the GSI series, the so-called ” GSI-translations” started. Until 1993 a t least about 350 Russian scientific papers were translated into German and some few into English. Because of special interests of an individual GSI-scientist or engineer mainly JINR papers were translated. In this way the GSI-translations reflect (along JINR classification) the fields of interests of GSI-scientists in the scientific work a t JINR. Beside heavyion research (more than 75 translations P7), other fields of major interest were nuclear spectroscopy and radio chemistry (more than 43 translations P6), chemistry (more than 29 translations P12), and low energy theoretical physics (more than 13 translations P4). On the next foto (Fig. 4) you can see my predecessor Hans Otto Schuff, again Professor Flerov, and on the right side, the first scientific director of GSI, Christoph Schmelzer, one of the builders of the proton synchrotron PS at CERN. Most of the contacts at the time had been managed via the international platform CERN, which was a fruitful contribution to the later co-operation between Russian institutions and GSI. The co-operation was further developed under Schmelzer’s successor Professor Gisbert zu Putlitz. In Fig. 5 Professor Bogoljubow on a tour at GSI, the wall papering of new exotic nuclei in the room of the scientific director a t GSI and Professor Flerov and Professor Bock are watching the scene. Of course Professor Yuri Oganessian tightened and tightens the ties of friendship t o GSI. I have selected two fotos of two birthday events: one
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Figure 4.
Figure 5.
of Professor Peter Armbruster and the second of 25th birthday of GSI in 1995. Professor Oganessian visited GSI also last year for the celebration
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Figure 6.
naming element 110 - Darmstadtium (Fig. 6). Russian scientists are a very important part of the discovery team of the element 110. This also reflects an increasing co-operation with Russian and GSI-scientists with highly estimated contributions by the Russian side (Fig. 7). An example for co-operation outside Dubna are the close contacts of GSI to Novosibirsk. In 1984/85 the electron cooling, invented at Novosibirsk, was integrated into the plans for an electron-cooled experimental storage ring. In that time the cooperation went by a rather roundabout way via CERN: the experience gathered by CERN with cooled particles and especially with cooled antiprotons for the famous AA-experiment had high relevance for GSI. Another important step for GSI was the delivery of an electron cooling
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Figure 7.
Discovery team of the element 110
system for the SIS synchrotron mid of the 9Os, which was built by the Budker Institute at Novosibirsk for GSI.
Figure 8. In 1986 the Ministers Riesenhuber, Gentscher, Schewardnadse and Jefremow signed the contract to the co-operation in science and technology (WTZ)
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In accordance with the 'CSCE process' (Conference on Security and Co-operation in Europe), in 1986 for the governments of the UdSSR and the Federal Republic of Germany, the Ministers Schewardnadse, Jefremow, Gentscher and Riesenhuber signed the contract to the co-operation in science and technology (WTZ), in which the GSI was bound into the agreement to the peaceful usage of nuclear energy. This was an important step for a closer and easier co-operation. One of the first co-operation in the WTZ framework started with the Institute for Theoretical and Experimental Physics ITEP in Moscow. Today GSI has 7 running projects with Russian partners in the bilateral "WTZ co-operation". Once it had been up to twenty, but unfortunately funds had been cut. One project together with St. Petersburg, just running deals with the mass measurement of exotic nuclei.
Figure 9. The WTZ Committee meeting in 1999 and the last meeting of the Joint Steering committee in 2001
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It should also not be unmentioned that there were strong links between East-German scientists t o Dubna also in the field of the superheavy elements, because the German Democratic Republic was a founding member state of the JINR. After the German unification, the formal links to Dubna have been strongly renewed since 1991. Two examples are the WTZ Committee meeting in 1999 with Viceminister Alimpijew and State Secretary Catenhusen (in Bonn) and the last meeting of the Joint Steering committee 2001 in Leipzig with the JINR Director Professor Kadyshevsky and the co-chair Dr. Wagner from BMBF, now also chairman of GSI’s supervisory board together with Professor Sissakian. This photo (Fig. 9) and the next (Fig. 10) were taken from the JINR annual report 2001.
Figure 10. Contacts between the heads of JINR and GSI to the FAIR project
An another photo from that report shows the contacts between JINR and GSI to the FAIR project from Mai 2001. After the German unification also new programmes were started. One is the TRANSFORM programme of the German government, in which the physical research played a dominant role. FAIR Project is now a new field of close collaboration. A new significant step in the co-operation between Russian institutions and GSI are the contributions of Russian Scientists in the fields of magnet design, super conducting technologies, and cooling techniques. Russian representatives are also members in the International Steering Committee for the planning period of FAIR. I want t o present you very fast the important co-operation-partners within some FAIR-collaborations.
688 Information Technologies jt
Material and Physical Technologies
3% Geo Sciences Plant Security
.--
15% Pnysical Fundamental Research 14% Environmental a Sea Research 15%
Life Science 17%
F i g u r e 11.
The
TRANSFORM
Energy 7%
programme
CBM hosts 4 partner institutions from St. Petersburg, 4 from Moscow, different institutes in Dubna, in Obninsk and Protvino. At the Proton Antiproton Darmstadt Experiment, PANDA, of course Novosibirsk is involved. Another collaboration is the PAX group with partners from Dubna, St. Petersburg, Moscow and Protvino. Of course there are also further collaborations like NUSTAR, FLAIR or SPARC with Russian participation. Very successful are the joint applications at the EC's INTAS programme (17 projects). As you see, a lot of Russian institutions are involved into the latest programme of 2003, which was a big success. Throughout the last years, the contracts with Russian scientists increased. Per year around more than 50 guest scientists are working a t GSI. For GSI this scientific contribution is very essential, as you see and the serious question is: what should GSI do without all these famous Russian scientists? My last transparency shows the Russian-GSI network spread widely over the landscape. Thank you for the fruitful co-operation during so many years and for the existing strong scientific network between many scientists and research institutes from Russian and GSI. I hope this collaboration, grown and intensified over so many years could still be strengthened for a bright future for the benefit of both partners! Thank you for your attention.
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Figure 12
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JINR: INTERNATIONAL SCIENTIFIC CENTRE BRINGING NATIONS TOGETHER
V. M. ZHABITSKY Joint Institute for Nuclear Research, Joliot-Curie 6, 141980, Dubna, Moscow region, Russia E-mail: [email protected]
1. Introduction
The Joint Institute for Nuclear Research (JINR) is an international intergovernmental organization' located in Dubna, Russian Federation, about 120 km north of Moscow. The Joint Institute for Nuclear Research was established on the basis of two soviet laboratories2: - Institute for Nuclear Problems (founded in 1946) with a 680 MeV synchrocyclotron operating, headed by M.G. Meshcheryakov and V.P. Dzhelepov, - Electrophysical Laboratory (founded in 1952) with a synchrophasotron under construction, headed by V.I. Veksler. The USSR Ministry of Atomic Energy built the town and the institute infrastructure. The USSR Academy of Sciences organized the scientific staff, worked out of the first research programme. The Agreement on the establishment of the Joint Institute was signed on 26 March 1956 in Moscow. Investigations in many fields of nuclear physics of interests for research centres of the JINR Member States were launched here. D.I. Blokhintsev (USSR) was the first Director of JINR. M. Danysh (Poland) and V. Votruba (Czechoslovakia) were the Vice-directors. The history of JINR is associated with outstanding scientists and high quality scientific schools of N.N. Bogoliubov, D.I. Blokhintsev, G.N. Flerov, I.M. Frank, B.M. Pontecorvo, V.I. Veksler, V.P. Dzhelepov, M.G. Meshcheryakov and other outstanding physicists. At present the founders' names are kept in the alley names at the Laboratories' sites. It was in many ways due to their selfless labour at JINR that a high qualification community of scientists, engineers and workers has been formed at the Institute who manage to cope with any task.
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At JINR, many scientists of different countries received the highest professional qualification in the field of modern nuclear physics. Among those who worked at JINR are Academician A. Logunov, Scientific Leader of the Institute for High Energy Physics (Protvino, Russia), Academician A. Tavkhelidze, who in the immediate past was the President of the Academy of Sciences of Georgia, Professor I. Wilhelm, Rector of Charles University (Prague, Czech Republic), Academician Nguyen Van Hieu, President of the National Centre for Science and Technology of Vietnam (Hanoi), and other outstanding scientists.
2. JINR Today
JINR has at present 18 Member States: Armenia, Azerbaijan, Belarus, Bulgaria, Cuba, Czech Republic, Georgia, Kazakhstan, D.P. Republic of Korea, Moldova, Mongolia, Poland, Romania, Russian Federation, Slovak Republic, Ukraine, Uzbekistan, and Vietnam. The JINR Member States contribute financially to the Institute’s activity and have equal rights in its management. The Joint Institute was created in order to unify the intellectual and material potential of the Member States to study the fundamental properties of matter. This goal was proclaimed in the Charter of Institute. The JINR Charter3 was adopted in 1956 and amended in 1992. In accordance with the Charter, the Institute’s activity is realized on the basis of its openness, mutually beneficial and equal co-operation for all interested parties to participate in research. A very important role is played by the Agreement between the Government of the Russian Federation and the Joint Institute for Nuclear Research4, which was signed by the President of Russia V. Putin on 2 January 2000. The Federal Law confirms the legal capacity of JINR. It includes the obligations that Russia will follow to ensure JINR’s successful activity on the territory of the Russian Federation. Russia is the host country of the Joint Institute. JINR’s co-operation with Russian scientists is naturally most extensive: about 150 scientific centres, universities and organizations from 40 Russian cities are partners of Dubna physicists. Broad international co-operation is one of the most important principles of the JINR activity. Almost all investigations are carried out in close collaboration with JINR Member-State scientific centres as well as with international and national institutions and laboratories all over the world.
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Co-operation with Germany is a brilliant example of the JINR’s scientific links. Research activities are regulated by the Governmental level Agreement concluded in 1991 between the German Ministry of Education and Research and JINR. The joint programme are being continuously improved to meet the current interests of the participating institutions. Today JINR carries out research activities with 71 institutions in 45 German cities. JINR has bilateral agreements with many international organizations, including the United Nations Educational, Scientific and Cultural Organization, the European Organization for Nuclear Research, and the LatinAmerican Centre for Physics. In general JINR has agreements, protocols and other documents concluded with about 700 institutions in 60 countries. JINR has grown into a large multi-branch physics centre, and its structure is determined by scientific specialization. The Committee of Plenipotentiaries of the governments of the Member States is the supreme body governing the Institute. The Committee of Plenipotentiaries has its own advisory bodies: the Scientific Council and the Finance Committee. The Scientific Council organized three Programme Advisory Committees as expert bodies. The permanent administrative body is the JINR Directorate responsible for all research, financial, administrative and social activities on the basis of the Committee of Plenipotentiaries’ decisions and recommendations, the existing agreements and adopted responsibilities. JINR has 7 Laboratories, 1 Division and University Centre: - Bogoliubov Laboratory of Theoretical Physics (BLTP), - Veksler-Baldin Laboratory of High Energies (VBLHE), - Laboratory of Particle Physics (LPP), - Dzhelepov Laboratory of Nuclear Problems (DLNP), - Flerov Laboratory of Nuclear Reactions (FLNR), - Frank Laboratory of Neutron Physics (FLNP), - Laboratory of Information Technologies (LIT), - Division of Radiation and Radiobiological Research (DRRR), - University Centre (UC). JINR employs about 5500 people, including about 1100 scientists from the Russian Federation and about 400 inviting scientists from the JINR Member States5. Among the scientists there are full members (academicians) and corresponding members of Academies of Sciences, more than 260 Doctors of Sciences and 650 Candidates of Sciences. Joint Institute for Nuclear Research is now a large multidisciplinary world-recognized international scientific centre performing basic research of
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the structure of matter, development and application of high technologies, and university education in the relevant fields. Three main avenues can be singled out from JINR’s wide range of investigations. The most direct way for studying the structure of matter is performed in particle physics experiments. JINR scientists carry out research in this field not only at the accelerators in Dubna, but also at CERN, IHEP (Protvino, Russia), Fermilab (Batavia, USA), DESY (Hamburg, Germany) and in other scientific centres. Another avenue of investigations is nuclear physics. Here one studies nuclear properties, nuclear reactions, new elements, including superheavy ones. JINR is one of the world leaders in this field. The third avenue is condensed matter physics. This field of fundamental science has shown rapid progress. Experimental methods of nuclear physics are applied to study physical phenomena in solids and liquids, new properties of materials. Five facilities (superconducting relativistic nuclei synchrotron Nuclotron, heavy-ion cyclotrons U400 and U400M, neutron pulsed source IBR-2, and proton accelerator Phasotron) are used for experimental studies in particle and nuclear physics, and condensed matter research. The JINR’s largest accelerator is the nuclotron6.This accelerator was constructed in the tunnel under the synchrophasotron operated at JINR in 1957 - 2002. Research programme at the synchrophasotron was terminated in 2002, and all experiments with relativistic nuclear beams are conducted now at the nuclotron. Nuclotron is a super-conducting strong focussing synchrotron. This complex7 allows accelerating multicharged ions with the energy of 4 - 6 GeV per nucleon. Polarized deuteron beams are accelerated also at the nuclotron. A number of research facilities are operated at the nuclotron, including DELTA-SIGMA, DELTA-2, MARUSYA, STRELA, GIBS, FASA and others. Further development of the existing research facilities and creation of new ones are planned. Over the last several years, the model of a users’ centre at the nuclotron has been continuously and successfully developed. U400 is a heavy ion isochronous cyclotron’. It produces ion beams of atomic masses from 4 to 100 and maximum energy up to 25 MeV/nucleon. The maximum ion beam intensity is presently 2. 1014 s-l for Ne, and drops to 2 . 1013s-l for Ca. The experimental devices are placed in 12 extracted beam channels, the ions are extracted from the accelerator in two directions. U400M is an isochronous cyclotrong to accelerate heavy ions from He to Ar with energies up to 50MeV/nucleon (maximum energy is 100 MeV/nucleon).
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IBR-2 is a pulsed reactorlo. Power pulses are obtained by mechanical modulation of the reactivity by means of a movable neutron reflector. The average power of IBR-2 is 2 MW. The pulse power is 1500 MW, pulse duration is 220 microseconds, the frequency of pulse repetition is 5 per second, pulse density of the neutron flux is 10l6n.cm-2+,-1. The IBR-2 reactor is competitive with the best world sources in most areas of application. Over 150 experiments are being carried out annually at the IBR-2. They involve physicists, biologists and chemists from the scientific centres of the JINR Member States and other countries. Major contributions are made by the physicists from Russia, Poland and Germany. Modernization of IBR-2 is in progress. The result of the modernization is that JINR will have operating pulsed neutron source of the world class.Its parameters will be unique in many aspects, which will make it possible for IBR-2 to remain one of the best neutron sources for physical research for another 20 - 30 years. Phasotron is an accelerator of 680 MeV protons. 10 beam channels are available at this machine, which are used to carry out experiments with pions, muons, neutrons and protons. 5 beam channels are designed to carry out medical investigations. The intensity of the extracted proton beam is 2pA. The hadron beams at the Phasotron are used for medico-biological and clinical research on treatment of cancer patients". So, during 2003 a total of 95 patients were fractionally treated with the medical proton beam. Also, some new basic facilities are being developed and constructed at JINR for investigations that will be able to compete in future with other first-class accelerator centres. These new facilities under construction are IREN (Intense Resonance Neutron Source") and DFUBs (Dubna Radioactive Ion Beams13). Creation of the Synchrotron Radiation Source is discussed14. In view of its wide scope of research, as well as the world-wide collaboration, JINR is intensively developing its networking, information and computing centre. JINR's Computer Centre and powerful net of servers, workstations and PC-farms allow carrying out numerical simulations of complicated multifactor problems of nuclear and particle physics, creating modern computational tools for data processing and data analysis. All computer units in JINR are integrated into the local internal net and the world computer nets15.
A fruitful scientific co-operation is under way with CERN, especially during the last years, as well as with many physics laboratories in the USA, France, Germany, Italy, Switzerland and other countries. Dubna specialists
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participate in experiments performed at the CERN and U.S. accelerators. New detectors for the LHC (CERN) are being designed. Many first-class achievements belong to Dubna physicists. As a recognition of the outstanding contribution of JINR scientists to modern physics and chemistry, the General Assembly of the International Union of Pure and Applied Chemistry (IUPAC) named element 105 of the Periodic Table Dubnium16. In 1999 - 2003, scientists of the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in collaboration with colleagues from the Lawrence Livermore National Laboratory (USA) synthesized new superheavy elements17 with atomic numbers 114, 116,118, 115, and 113. Pure research has always been a rich source of new ideas. Many of the technologies we now take for granted had their origins in basic science. In the past, such discoveries made their way from laboratories to the wider world largely of their own accord. But as the pace of progress increases, new technologies may need a helping hand. That’s why JINR takes permanent efforts to ease the transition of ideas from the Laboratory to industry, to the general public, and eventually into our everyday lives in the form of new technologies. JINR’s basic mission remains fundamental research. But the tools it uses, particle accelerators and detectors, push technology to its limits and beyond. The nuclear track membranes, medical applications and advanced informatics techniques are just a few of the many recent spin-offs from fundamental research at JINR. JINR means high skill of specialists and good conditions for training and education of talented young people. The Joint Institute gradually changes now from a purely scientific research institution to an international centre, where fundamental science, engineering and applied researches are closely connected with training. Structurally, it takes the form of a new satellite “students” laboratory which is the currently operating University Centre of JINR. This new training function of JINR is supposed to be oriented t o international demand. The Institute’s Laboratories provide good opportunities for the students’ initial research. About 270 students from higher education institutions of JINR Member States attended studies at the JINR University Centre in 2004. JINR continued its postgraduate programmes in 10 specialities of physics and mathematics. In 2004, the total number of PhD students at the UC was 70. The UC regularly organizes schools for young scientists and joint seminars in research centres of Member States. An interesting educational project is realized by JINR and BNL (USA).
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The training program on some subjects of physics, mathematics and other basic subjects for schoolchildren was created using Internet technologies.
3. Conclusion
The opportunities available at JINR for performing research at the inhouse facilities in Dubna, for participating in experiments at facilities in world’s leading laboratories, and for training specialists are widely used by the Member States of JINR and scientific partners of JINR in many countries. Since the very beginning JINR has participated in organizing important international scientific conferences. With their close co-operation, even in the gloomiest years of the Cold War, the scientists of JINR were performing a noble mission: contributing to the mutual understanding of people from different countries. Russian Empress Elizabeth Petrovna was right when she said that “Enlightenment of mind eradicates evil.” The Joint Institute is known not only for its excellent achievements in fundamental science. JINR is a model of what can be done when a group of nations joins forces with a common goal. Through collaboration with different countries, JINR has given the world a real example of peaceful coexistence. Scientific collaborations have always been a partnership of equals. The study of fundamental physics requires stable conditions and peaceful relations. It thrives on pooling resources and freely exchanging information. Researchers in this atmosphere have taken their new-found view of the world back to share with neighbours, civil servants, and ministries in their home countries. Their impact on the friendly atmosphere of community is significant. This extremely important role of JINR in bringing nations together by joining the efforts of scientists from different countries in peaceful goals of nuclear science is world recognised. The best illustration of the last words is the scientific collaboration of JINR and CERN. The achievements of the joint work of JINR and CERN scientists, scientific partners of these international organizations, scientific societies of different countries and scientists with no regard to their political views are presented in the joint photo exhibitions “Science Bringing Nations Together”18. These photo exhibitions were held in Oslo University, the UN European department in Geneva, UNESCO (Paris), in the European Parliament in Brussels, at the State Duma of Russia, at the Ministry of Education and Sciences in Bucharest (Romania) and at the Yerevan University in Armenia. In October 2003 the exhibition “Science
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Bringing Nations Together” was demonstrated at the Diplomatic Academy in Moscow. T h e Directorates of JINR and CERN believe that the organization of these exhibitions contribute t o the popularization of t he scientific achievements of the two international centres and hope t o continue this joint effort in th e future.
In th e XXI century the Joint Institute for Nuclear Research continues t o fulfil its mission successfully at the frontier of scientific research, introducing progressive technology and training staff of highest qualification, primarily for its Member States. JINR is a model of what can be done when a group of nations joins forces with a common goal. With all their activities JINR scientists prove th at the joint scientific centre functions in favour of its Member States and Science. References 1. Joint Institute for Nuclear Research. http://www.jinr.ru/. 2. M. G. Shafranova. JINR: Informational and Biographical Booklet. Fizmatlit. Moscow (2002). 3. Charter of the Joint Institute for Nuclear Research. JINR, 11-7696. Dubna (1999). 4. Agreement between the Government of the Russian Federation and the Joint Institute for Nuclear Research. Journal of international agreements. The Presidential Executive Office in the Russian Federation. 5, 53 (2000). 5. Joint Institute for Nuclear Research. Annual Report 2003. Dubna (2004). 6. A.D. Kovalenko. Proc. of EPAC’94. London. v.1, 161 (1995). 7. V. Volkov et al. Proc. of EPAC 2004. Lucerne. 2718 (2004). 8. 0. Borisov et al. Proc. of EPAC 2000. Vienna. 1468 (2000). 9. B. Gikal, G. Gulbekian, V. Kutner. Proc. of Int. Conf., Cyclotrons and Their Application. Cam. 587 (1998). 10. V. D. Ananiev et-al. Communication of the JINR, P13-2004-156. Dubna (2004). 11. G. V. Mytsin et al. Proc. of the 3rd Russian Sci. Conf. ((Radiologyand Radiotherapy in Clinic of X X I Centwy”. Moscow, 109 (2002). 12. W. Furman et al. 11th International Conference on Nuclear Engineering. Tokyo, JAPAN, April 20-23, 2003, ICONE 11 - 36318 (2003). 13. G. M. Ter-Akopian, Yu. Ts. Oganessian et al. Nucl. Phys. A . V.704, 295 (2004). 14. I. N. Meshkov et al. Proc. of EPAC 2000. Vienna. 660 (2000). 15. V. Ilyin, V. Korenkov, A. Soldatov. Open Systems. No.1, 56 (2003). 16. G. Audi, A.H. Wapstra and C. Thibault. Nuclear Physics A729, pp. 337-676 (2003). 17. Yu. Ts. Oganissian et al. Phys. Rev. C 69. 021601 (2004). 18. The CERN - JINR photo exhibition “Science Bringing Nations Together”. http://sbnt.jinr.ru/.
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JINR UNIVERSITY CENTRE SVETLANA IVANOVA University Centre, Joint Institute for Nuclear Research Dubna, Moscow Reg., 141 980, Russia [email protected] More than ten years ago, joint Order of the Ministries of Education and Atomic Energy No. 28/33 was signed on January 16, 1991. It was titled "On Providing Staff for Research and Applied Work in Nuclear Physics, Elementary Particle Physics, Condensed Matter Physics, and High-Temperature Superconductivity." This Order initiated the specialized education on the basis of JINR of graduate students from Moscow State University (MSU), Moscow Engineering Physics Institute (MEPI), and, a little later, Moscow Institute of Physics and Technology (MIPT). On the grounds of this Order, the University Centre (the UC) was established, whose 10th anniversary was celebrated on March 21, 200 1.
1.
Introduction
The UC was established to provide students of the 4th, 5th, and 6th years completing their graduate programmes in physics [ 13.The curricula are prepared jointly with the respective departments of the UC founder institutions of higher education and are supplemented according to the scientific research of the JINR laboratories The main aims of the UC are the following: Development and update of the curricula and programmes for physics students. Support of the postgraduate studies. Organization and conduction of international schools for students and postgraduates. Development of the cooperation with international Funds (DAAD, EMSPS, etc.) aimed at organizing student and postgraduate exchanges between the UC, universities and research centres regulated by special agreements. Development of the system of the specialized training of highly skilled specialists for the JINR Member States. Organization of bilateral supervision of postgraduates by scientists of JINR and its Member States. Development of the UC's specialized practicum for students and secondary school students. Organization of a secondary school lecture course in physics. Creation of a system of raising the professional skills of JINR's engineering
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and technical staff. Development of a computing and information technology complex for the university-type educational process and a database of the UC courses. Over the past five years, the UC’s total graduate enrolment was 600 students from higher education institutions of JINR Member States.
Main activity of the UC
2.
The University Centre offers the following full-time programmes: - Nuclear Physics - Particle Physics - Condensed Matter Physics - Theoretical Physics; - Technical Physics - Radiobiology
There are about 80 students from different institutions of higher education of JINR Member States at the UC every year. The UC develops new programmes of the special target tuition of students from JINR Member-States. As an example, in 1998-2003, groups of students of the University of Bratislava were being trained to become specialists for the Slovak cyclotron facility to be built with JINR’s support. This special programme included an intensive course of Russian before lectures. Noted should be the active participation of JINR’s scientists in the education process. The UC’s teaching staff is about 50. In recent years appeared young lecturers, which is especially important in rapidly developing areas like modern computing and microprocessor systems. Some lectures and seminars were held by the UC postgraduates. In 1995, post-graduate studies opened at the UC. In 1998, JINR was certified by the Russian Federation’s Ministry of General and Professional Education to conduct educational activity in postgraduate professional education. The opening of the postgraduate studies attracted both the UC graduates and young scientists from JINR Member States. The postgraduate students are trained in the following specialties: 0
Physics of nuclei and elementary particles High energy physics Theoretical physics Charged particle beam physics and accelerator technique Computational mathematics
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Solid state physics Physical experiment technique, instrument physics, and automation of physical research Radiobiology Mathematical support of computers, computational complexes, and networks Mathematical modelling, numerical methods, and software complexes The UC's total annual enrolment of postgraduate students is about 60; they come from a number of JINR Member States. In 1997, the UC started a lecture cycle for graduate and postgraduate students under a general title "Modern Issues of Natural Sciences." Every year scientists from different countries are invited to the UC to lecture on modern scientific topics. Over these years, more than 20 lecture cycles were given at the UC. This can be exemplified by the following lectures: Yu.P. Gangrsky (JINR), ((Atomic Electron Shell Structure and its Relation to the Nucleus Structure)); R.V. Jolos (JINR), ((Nuclear Dynamics in Near-Equilibrium States, and NonEquilibrium Processes in Nuclei)); D.I. Kazakov (JINR), ((Beyond the Standard Model, or What Kind of Physics We are to Face in the Next Decade Accelerators)); P. Spillantini (Italy), ((Nuclear and Subnuclear Astrophysics)); R.Kragler (Germany), ((Muthematica Tutorial Course)); J. Le Duff (France), ((Beam Dynamics in the Presence of Synchrotron Radiation)); E. Kapuscik (Poland), ((Introduction to the Theory of Open Systems)); G. Stratan (Romania), ((Selected Issues of the History of Physics)); A. Sobiczewski (Poland), ((Properties of Superheavy Nuclei)); F. Dydak (CERN), ((Neutrino Oscillations: Status and Prospects)); and D. Blaschke (JINR), ((Contemporary Problems in Quantum Field Theory of Dense Nuclear/ Quark Matter)). Since its foundation, the UC has been publishing its own textbooks and manuals. Some of the lectures given within the lecture cycle "Modern Issues of Natural Sciences" has been published [2]-[7]. 3.
International cooperation
Keeping in line with JINR's international character, the UC actively develops its international cooperation. Especially busy are the UC's relations with universities of Belarus, Bulgaria, the Czech Republic, Poland, Romania, Russia, Slovakia, and Ukraine. On the basis of the UC, institutes and universities of JINR Member Sates unite their efforts in education activities. One of the UC's missions is the organization and conduction of international
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scientific schools and training courses. For students and postgraduates from both the UC and JINR Member States, schools, which have now become regular, proved to be very useful. Those are the Schools in Memory of B. Pontecorvo and International Student School on Nuclear Physics Methods and Accelerators in Biology and Medicine. 3.1 Summer student practice
In 2004, for the first time in its history, the UC hosted a Summer Student Practice in JINR Fields of Research, which was organized jointly with the Czech Technical University in Prague, Technical University of Bratislava (Slovakia), Adam Mickiewicz University (Poznan, Poland), Moscow State University, Moscow Institute of Physics and Technology, and Moscow Engineering Physics Institute. During a month - from June 29 to July 29 - the Practice was attended by 36 students from Bulgaria, the Czech Republic, Poland, Romania, Russia, Slovakia, and Ukraine, who had passed competitive selection. The main aim that the International Practice organizers set before themselves was active involvement of students in the work of experimental and theoretical research teams at JINRs facilities. Therefore, the Practice was arranged in such a way that in the morning they attended lectures, and in the afternoon they worked with research teams at JINRs Laboratories. On the day of the Practice opening, the students got acquainted with their supervisors; then began their daily work at the Laboratories, where they were to be absorbed in real research - that is, to immediately participate in the fulfilment of specific scientific tasks under the supervision by JINR's leading specialists. Thus, at the Frank Laboratory of Neutron Physics students were broken up in twos and threes and were assigned to the following fields: the study of n - e interaction, methodology of correlation gamma-spectroscopy, study of moderated neutrons, fission physics in the experiments at the IBR-2 reactor, study of ultra-cold neutrons, and neutron activation analysis at IBR-2. At the Dzhelepov Laboratory of Nuclear Problems, students worked with a group concerned with thermal multifragmentation at the 4n-facility FAZA, and studied scintillation spectrometry of different kinds of nuclear radiation. At the Flerov Laboratory of Nuclear Reactions, the Practice participants worked with groups performing laser spectroscopy research, studying nuclear reaction mechanism, and carrying out experiments to study exotic nuclei at the AKULINA highresolution channel and heavy element properties at the VASILISA separator.
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During the last ten days of the Practice, its participants, as envisaged by the Practice programme, attended the International Student School on Selected Issues of Theoretical Nuclear Physics, which was held on July 20 - 29 at the Bogoliubov Laboratory of Theoretical Physics and focused on the results of the latest research into nuclear structure and nuclear reactions, theoretical methods, and their use in astrophysics and mesoscopic systems. As lecturers, besides the Laboratory's leading specialists, invited were prominent scientists of the Czech Republic, Germany, Russia, and Ukraine. The School programme included the following topics: nuclear excitations at different energies, nuclear structure and nuclear reactions at the stability border, astrophysical aspects of nuclear structure, double beta-decay and the neutrino mass problem, and hypernuclei. Also, much attention was paid to the study of the properties of radioactive nuclei and the reaction mechanisms through which they are produced, including the fusion reactions leading to the formation of massive nuclear systems. On July 20 - the day of the School opening - Acad. Yu. Oganessian gave a lecture on superheavy elements. All the Practice participants submitted written reports on their work under supervisors at the Laboratories and received certificates from the Organizing Committee. 4.
International funds
The UC has been actively developing its well-established relations with foreign institutions of higher education. It received several times grants from the European Physical Society, which allowed the UC students and postgraduates to have practical experience at European universities (in Germany and Italy). A notable example is the UC's participation in the Leonard Euler Scholarships programme of the German Service of Academic Exchanges (DAAD). Thus, a joint project by the UC and the Institute of Theoretical Physics at the University of Giessen (Germany) has been supported since 1998. Within this project, graduates and postgraduates, together with scientists of the UC and Laboratory of Theoretical Physics, have been performing research in heavy ion physics. They get additional scholarship and go on missions to Germany. A great contribution to the development of this cooperation was made by Prof. W. Scheid, Head of the mentioned Institute at Giessen and a JINR Honorary Doctor. The UC is today an acknowledged educational centre. It is one of the Russian Federation's coordinators in the European Mobility Scheme for Physics Students (EMSPS) and is included in the EMSPS database, where the information on the UC has been available since 1995.
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In 1998, JINR signed an Agreement on Partnership with the European Physics Education Network (EUPEN). This Agreement allows the UC students and lecturers to be involved in the exchange programmes organized by the European Physical Society. Information on the Uc‘s activities is published at its Internet site, http://uc.jinr.ru
References 1. Leading article “Joint Institute for Nuclear Research - University Centre”. Nucl. Phys. News 8,3,39-40 (1998). 2. K.Junker “Introduction to theoretical intermediate energy nuclear physics”, UC 95-3, JINR, Dubna (1 995). 3. YuGangrsky et al., “Interaction of heavy ions with nucle?’, UC-97-4, JINR, Dubna (1997). 4. R. Kragler, “MathemuticaTutorial Course”, UC-200-8, JINR, Dubna (2000). 5. Yu.Gangrsky, “Atomic andplasma physics”, UC-2001-12, JINR, Dubna (2001). 6. G. Stratan, “Selected Issued of the History of Physics“ Part 1, UC-2003-18, JINR, Dubna (2003). 7. DKazakov, ‘LSupersymmetricgeneralization of the standard model of fundamental interactions; Textbook”, UC-2004-23, JINR, Dubna (2004).
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PUBLIC AWARENESS OF NUCLEAR SCIENCE IN EUROPE A.KUGLER for Nuclear Physics Board of European Physical Society Nuclear Physics Institute A S CR, Czech Republic, [email protected]
1.
Introduction
Recent outreach activities of nuclear scientist originated from the idea that it is important to convey to the general public important issues associated with Nuclear Science. The assumption is that an informed public is better able to come to sensible judgments based on knowledge instead of prejudice. The image of anything "nuclear" is presently viewed in Europe with suspicion mainly due to the public perception being influenced by sensational negative media coverage rather than a balance view across the whole field. Let us give some examples of recent evidence for this negative image: The term Nuclear Magnetic Resonance (NMR) has been changed to Magnetic Resonance Imaging A survey among school children (14-18 yr) resulted in the following ranks for the most interesting subjects in nuclear science, its applications & consequences: Nuclear weapons (62%), Environmental risks (45%), Health risks & social risks (44 %), Universe (37%), Radiation biology (37%) To conclude, public associated Nuclear science mainly with atomic bombs (Hiroshima & Nagasaki) and with reactor accidents (Tschernobyl, Three Miles). Even scientists from other fields (biology, chemistry . . ..) have negative opinion about consequences of Nuclear sciences for society. Therefore, if nuclear science is to have a healthy future, then it is important that we initiate activities that can start to bring about a more balanced view. This is the basic remit of PANS. It is an activity that was started several years ago by two bodies, which represent Nuclear Physics community in Europe, i.e. by the Nuclear Physics Board of European Physical Society (NPB EPS) and by Expert Committee of the European Science Foundation (NuPECC ESF). 2.
European Physical Society
As one of European self-funded scientific society, EPS is funded by national physical societies based in particular European countries. Member fees of national societies represent the major part of EPS budget. Each member of any national society can associated hedhimself with one of EPS divisions, which represent specific interest of the members and they are another organization
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structure of EPS going across national aspect and joining professionals on European scale. Both national societies and divisions are than represented in the EPS Council, which meets regularly to approve EPS budget, to elect EPS Executive board and to approve any changes in EPS constitution. For details see http://www.eps.org To commemorate the centennial of Einstein’s famous three publications on the photo effect, the Brownian motion and special relativity - and actually two more, also published in 1905: Einstein’s doctoral thesis on a new determination of the size of molecules and Avogadro’s number, and another one complementing his article on special relativity, and presenting the most famous formula in physics, E = mc2, WYP2005 is supported by other physical societies outside Europe as well as by UNESCO, see htt~://www.w~12005.argJoverview.html. Particularly, EPS will hold its next general meeting in July 2005 at Bern organized into three parallel conferences, each on one of the major papers of Einstein. Nuclear Physics Division of EPS is represented by Nuclear Physics Board consisting from nuclear physicist from member societies. Some of NPB activities are: Divisional conferences to cover different activities of European Nuclear Physics Community NPDCl7 “Nuclear Physics in Astrophysics”, Debrecen, 30.9. - 3.10.2002 NPDCl8 “Phase transitions in strongly interacting matter “, Prague, 23.8.29.8. 2004., NPDCl9 “New Trends in Nuclear Physics Applications and Technology”, Pavia, 5.9.-9.9.2005., http://www.pv.infn.it/-npdc19 East-West Task Force To discuss some of the problems of the nuclear physics community in the Central and East of Europe EPS Nuclear Physics Division Prize “Lise Meitner Prize” for outstanding work in the fields of experimental, theoretical or applied nuclear science. “IBA-EPS Prize in Applied Nuclear Science and Nuclear Methods in Medicine “ Position Paper of EPS on the Future of Nuclear Energy http ://www.kv i .n If-ep s-npl European Science Foundation The other body that represents European Nuclear Physics, is Nuclear Physics European Collaboration Committee (NuPECC), which is an Expert Committee of the European Science Foundation (ESF) in the field of Physical and Engineering Sciences. Its members are nominated by its Subscribing Institutions
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which are in general Member Organizations of the ESF involved in nuclear science research or research facilities. The objective of NuPECC is translated into the following aims: to define a network of complementary facilities within Europe and encourage optimization of their usage; to provide a forum for the discussion of the provision of future facilities and instrumentation; to provide advice and make recommendations to the ESF and to other bodies on the development, organization and support of European nuclear research and of particular projects. 4.
PANS
Public Awareness of Nuclear Science (PANS) is a joint initiative by the Nuclear Physics Board of the European Physical Society and NuPECC. The objective is to co-ordinate and stimulate a European wide network with the goal to enhance the knowledge of Nuclear Science, the achievements, techniques and applications, within the broad public of non-specialists. The PANS network was formed at a workshop in Louvain-la-Neuve in April of 1998 with a board then made up by J. Deutsch, A.C. Shotter and A. Van der Woude. The present board members are C. Leclerq-Willain and H. Freiesleben (from the NPB EPS) and G.-E. Korner, H. Leeb and A. Eiro (from NuPECC ESF). Today the network consists of approximately 35 scientists and teachers from 17 European countries. Twenty-one months after its formation the network received funding for its activities from the EU under the 5" framework program. This, together with additional funding from the EPS, NuPECC and several major Nuclear Physics laboratories throughout Europe, has enabled PANS to organize two workshops a year for scientists engaged in public awareness activities, teachers and experts on outreach work and to support transnational projects bringing Nuclear Science closer to the European public. There were several projects furthered by PANS. The first three of these included the creation of a website, accessible to everyone, with resource material for lectures aimed at the broad public (PANS-WEB), the production of a book at the level of high-school pupils and the design and realization of a traveling exhibition. The exhibition received funding from the EU under a separate contract for the European Science Week 2000. 5.
Exhibition
First activity, the exhibition about radioactivity, was motivated by finding, that most of the public is afraid of radioactivity as it :
JCannot be seen, JCannot be smelt
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4 s very dangerous
The exhibition, Radioactivity: a facet of nature, was produced in three languages, Italian, French and German by A. Pascolini, G. Edelheit, K.-D. Gross and H. Leeb. The three editions were simultaneously on display in Milan, Paris and Wiesbaden. The main topic presented were: a) Radioactivity in the universe
p) Radioactivity in nature
y) Radioactivity used by man
Since then these exhibitions have been on display also in Turin, Bressanone, Pisa, Florens, Gorizia, Pordenone, Perpignan and Caen. There is now also an edition in Hungarian, first on display in Debrecen one and a half years ago and since then in Paks and Budapest. Recently edition in Portuguese was in display in Lisbon. 6. Book A group of four active nuclear physicists, R. Mackintosh, J. Al-Khalili, B. Jonson and T. Pena, took on the challenge to write the book. Their beautiful result, Nucleus - a trip into the hart of matter (ISBN 0 9537868 3 8) appeared in November 2001. The main idea of the book is that The nucieus is a very tiny, but extremely massive object situated in the center of the atom. It is the nucleus that decides the properties of the atoms which constitute our material world. The-book gives a clear and fascinating account of the important field of nuclear science, its history and iis applications. Foreword is written by the nobel laureate Ben Mottelson, Contents of each from ten chapters can be represented by: 1: From the immensity of space to the invisible world of the nucleus 2: How a new world was revealed by the discovery of radioactivity 3: Strange laws at the heart of matter 4: Determining the shape and size minute objects. 5: The content of a nucleus. 6: The variety and abundance of nuclei 7: Interactions with everyday life 8: From hydrogen to neutron stars 9: The stars and the birth of elements 10: Cosmology and the nuclear processes in the Big Bang
The first translation has appeared in print 2003 in the Czech Republic followed by editions in Hungary and Sweden and this year in Portugal (2004). Next editions are in preparation in Serbia and Montenegro. Negotiations are continuing with publishers in several other countries.
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7.
NUPEX
In the beginning of 2003 a project targeted primarily at teachers as science communicators received funding from the EU. The project, Nuclear Physics Experience (NUPEX), was originally formulated by a working group within PANS chaired by H. Oberhummer and is now co-ordinated by a commercial partner (www.nupex.org). The aim is to develop, within two years, a web-based science communication system (webSCS) communicating nuclear science and its applications to the public and to achieve: High quality web-based one-stop shop for contents in nuclear science and applications in at least 5 European languages Primary target group are schools: Teachers as science communicators and their pupils Innovative features, e-didactics and role of teacher communities essential 1 1 involved institutions: 2 companies, 4 research institutions, 3 universities and 2 outreach institutions from A, B, D, I, EL, HU, MT, PL, UK Duration of project: 24 months For details and current status of NUPEX see http://www.nupex.net 8.
New Projects
The pans-info project, a development of the original PANS-WEB, aims at creating a network of web sites and scientists, accessible to everyone. The aim of the project is to provide, in the local language, up-to-date information on Nuclear Science and its applications and simultaneously to provide an interface between the public and active scientists. The material on the sites, therefore, has to be carefully prepared and linked to individual European scientists. The first one of these sites (in English) is in principle working (www.pans-info.org). What is needed are resources to host the site and to supervise and edit the content that has already to some extent been provided by individual scientists along with their e-mail address.
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HEAVY METALS ATMOSPHERIC DEPOSITION STUDY IN POZNAN USING THE MOSS TECHNIQUE
z. BLASZCZAK, I. CISZEWSKA, M.V. FRONTASYEVA*, O.A. CULICOV‘ Faculty of Physics, Adam Mickiewicz University,Poznati, Poland *FrankLaboratory of Neutron Physics of the Joint Institutefor Nuclear Research, Dubna, Russia
The paper reports preliminary results on the content of heavy metals in the atmospheric air in the city of Poznan. Measurements have been made by the method of active biomonitoring with the use of neutron activation analysis. This method based on the use of moss, is non-destructive and characterized by high sensitivity of 10.” kgkg. Standard samples of living moss distributed at different sites in the city were exposed to the atmospheric air at different altitudes for a few weeks, to accumulate heavy metals from the air. Analysis of the gamma radiation from unstable radioisotopes of different lifetimes formed on exposing the sample to a beam of neutrons from the reactor IBR-2, permitted unambiguous identification of the accumulated elements. The results were used for analysis of distribution of heavy metals in the Poznan atmosphere. Maps illustrating the level of air pollution with heavy metals Cs, Sb, V, Zn and trace elements Al, Ca, Cr, Na were made.
1. Introduction Air pollution is one of the most important sources of the environment pollution as the pollutants easily penetrate water and soil and through them get into the living organisms. In cities the main sources of air pollution are the transportation, heat and electricity producing plants, industrial plants, construction industry and corrosion of construction materials. Among many methods proposed for monitoring of the atmospheric air pollutions there is an interesting one based on the use of bioindicators, absorbing and accumulating potentially toxic substances from the air, in particular specific bioindicators [ 1, 21. When monitoring heavy metals an important bioindicator is the moss, effectively accumulating from the air the majority of metals and other trace elements. In this study the method of active biomonitoring with the use of neutron activation analysis was applied for determination of the type and concentration of heavy metals in the air [3,4]. The study was performed in the city of Poznan (261 km2), which is situated in the Wielkopolska region in the middle course of the Warta river. In geographical coordinates its position is delimited by 52O17’34” N 52’30’27” and 16O44’08”E 17’04’28”Poznan is an important centre of transport and
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communication, with its own airport serving domestic and international connections. It is a large industrial centre with dominant branches of food industry, machine industry, motor vehicle production and chemical industry. There are also some production plants in the vicinity of the city. The energy demand of the city is covered with the heat and electricity producing central plant working on fine coal and some local heat producing plants working on gas or coal. 2. Experimental
The moss samples used in the study were collected from the vicinity in Dubna (Russia) a region characterised by very low pollution of natural environment. Dried samples of moss were placed in nets suspended at 44 different localities to be exposed to the air pollution (Fig.1). After a certain time of exposure the level of the heavy metals accumulated in the samples was determined by the method of neutron activation analysis.
Figure1 . Distribution of moss samples over the area of Poman
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3. Sample Preparation
After moss samples lyophilisation from each sample two subsamples were collected of 0.3 g for analysis of the content of isotopes of long lifetime (4 -5 days exposure) and short lifetime (3-5 minutes exposure). Previous experience from the use of NAA in moss biomonitoring surveys has shown that samples of 0,3 g are sufficiently homogeneous to be used without homogenization [5]. The samples were processed into tablet forms. The tablets were weighted to an accuracy o f f 0,0001 mgkg and wrapper in the foil, and subjected to neutron irradiation in the reactor together with the reference standard sample. The content of the isotopes were calculated directly from the ratio of activity in the sample and in the reference standard. NNA was performed at the IBR-2 in the Frank Laboratory of Neutron Physics. The analytical procedures and the basic characteristics of the pneumatic system employed for fast removal of the samples from the reactor are described in details elsewhere [6]. 4. Results and Discussion
Table I shows the minimum, maximum and mean elemental content and median values of 29 elements detected in moss samples from 44 sites in the Poznah city. The local background levels for most of the elements were calculated as an average of values from the 3 sites showing the lowest content of heavy metals. Table I. Element content (in mgkg) in moss samples distributed in Poznah city area.
Minimum
I
Maximum mgk
Median
Mea
Concentration amplitude
Na
209
3630
341
574,6
17,4
Mg
882
13500
1430
2413,3
15,3
A1
414
4900000
651,5
374871,3
11835,7
CI
160
2520
1045
1160,7
15.7
K
I
1490
1
15200
I
7075
I
7172,2
I
10.2
0,064
0,311
0,115
0,128
4,s
Ca
2740
49 100
4465
8177
17,9
Cr
0,767
62
198
2,14
8.1
sc
V
0,727
255
136
23,76
350,7
Mn
33.5
717
484,51
476,7
21.4
As
0,209
1,16
0,375
0,415
5,55
Br
3,17
13,5
434
5,62
42
Sr
8344
89,2
15,35
19,89
10,6
I
712 Rb
1
I
1
90,6
Sm
0,037
0,186
0,066
0,079
5
Hf
0,034
0.27
0.08
0,092
7,9
Ta
I
0,007
1
0,043
I
48’15
0,015
I
46,03
I
3,82
0,017
I
23.7
6.1
1
I
In order to illustrate graphically the spatial distribution of particular heavy metals determined from the moss exposed to air at different locations, the factor analysis procedure [7, 81 was used. The maps illustrating the distribution of concentration of 29 elements over the area of Poman are shown in Fig.2. In general higher concentrations of the elements determined occurred in the western and south-western parts of the city.
5. Conclusions The contents of particular heavy metals in the moss samples exposed to air pollution in Poznan showed considerable variations (Table I.). The greatest amplitude of variation was recorded for aluminium, vanadium and cerium, as for them the differences between the minimum and the maximum value were of 11835, 350 and 138 times, respectively. The analogous differences for the other metals were smaller from 10 to 20 times, except for antimony whose concentration differences were of 49 times. Because of specific geographical localisation and the absence of large industrial plants in the main directions of the winds the air quality in Poznan depends mainly on the emission sources in the city. In the southern and south-western parts of the city the air quality is affected by the chemical plant in Lubon whose emission reaches sometimes the central part of the city. The Central part of the Old City and large adjacent parts (Jeiyce, Grunwald and W ilda) are covered with compactly arranged apartment houses with small shops or workshops and the buildings are heated by small heat producing coal combusting plants. Compact arrangement of buildings and relatively narrow streets mean that the main source of air pollution are motor vehicles (low emission sources). The northern and eastern parts of the city are
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V - wanad
Na - s6d
Cs - cez
Ca - waDn
[mg/kgl IO-500 0 500-1000 1000-1500
1500-2000
I >zoo0
Sb antvmon
bwkl 0-0 2 0 2-0 4 0 4-06
-06-08 I > 0 8
Figure 2. Maps showing distribution of the heavy metals Cs, Sb, V, Zn and trace elements Al, Ca, Cr, Na in the Poznan area.
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taken by large housing estates (Piqtkowo, Winogrady, Rataje, Zegrze) heated by the municipal heating network, the level of pollution emission there is rather low. Significant emission of different type comes from small sources in the central part of the city (Wilda, Franowo, Podolany).The most polluted districts are the centre and fragments of western and south-western parts of the city. The main source of lead pollution are exhaust gases from motor vehicles. It is supposed that the sources of copper emission are industrial plants, although it is difficult to identify exactly which ones. Analysis of the distribution of zinc pollutants has shown a coincidence with the areas polluted mainly with lead. The distribution of elevated level of lead pollution coincides with the main transition roads and great communication junctions in Poznan.
References 1. A. Ruhling, G. Tyler, Water, Air, Soil Pollut., 2,445 (1973). 2. 8.Ruhling, G. Tyler, Water, Air, SoilPollut., 22, 173 (1984). 3. E. Steinnes., M.V. Frontasyeva, Analyst, 120, 1437 (1995). 4. M.V. Frontasyeva, T.Ye. Galinskaya, M. Krmar, M. Matavuly, S.S. Pavlov, D. Radnovic, E. Steinnes, J. Radioanal. Nucl. Chem., 259, 141 (2004). 5. E.Steinnes, J. E. Hanssen, J. P. Rambaek, N. B. Vogt, Water, Air, Soil Pollut., 74, 121 (1994). 6. M. V. Frontasyeva, S. S. Pavlov, Analytical Investigations at the IBR-2 Reactor in Dubna, JINR Preprint, E14-2000-177, (2000). 7. J . Schaug, J. P. Rambaek, E. Steinnes, R. C.Henry, Atmos.Environ., 24A, 2625 (1990). 8. T. Berg, 0. RQyset, E. Steinnes, M. Vadset, Environ. Pollut., 88,67 (1995).
715 LIST OF PARTICIPANTS Yasuhisa ABE Yukawa Institute for Theoretical Physics, Kyoto University Kitashirakawa, Sakyou-ku, Kyoto 606-8502, JAPAN abev@,yukawa.kvoto-u.ac.jv Nicolas ALAMANOS Commissariat A L'energie Atomique, (CEA-Saclay) Service de Physique Nucleaire CEA/DSM/DAPNWSPhN Saclei 9 1191 Gif Sur Yvette FRANCE alamanos@,cea.fr Marcos ALVAREZ Commissariat A L'Energie Atomique (CEA-Saclay) Service De Physique Nucleaire CENSACLAY -Batiment 703 DSM/DAPNIA/SPhN 9 1191 Gif-Sur-Yvette FRANCE malvarez@,cea.fr Albert0 ANDFUGHETTO Laboratori I "di Legnaro Via dell universita' 2 Legnaro (PD) ITALY
andriahetto@,lnl.infb.it Remy ANNE GANIL, Bd Henri Becquerel - BP 55027 14076 Caen cedex 5 FRANCE anne@,aanil.fr
Anatoli ARTUKH Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA artukh62iinr.m Georges AUDI Centre National de la Recherche Scientifique (CNRS) C.S.N.S.M. (IN2P3-CNRS &. UPS) Batiment 108 91405 Orsay FRANCE audi0,csnsm. in2p3.fi Nassima ALLAL Faculte De Physique Universite Des Sciences Et De La Technologie Houari Boumediene 16111 Bab-Ezzouar ALGERIA allaln@,vahoo.com Olivier BAJEAT Institut de Physique Nucleaire F-9 1406 Orsay FRANCE bai eat0,ivno.in2v3.fi
Jon BATCHELDER IHIWORNL, P.O. Box 2008, Bld 6008 Oak Ridge TN 37831-6374 USA [email protected] Alexey BOGACHEV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA bogachev@,nrmail.iinr .m
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Virginie BOUCHAT Universite Libre de Bruxelles 1050, Bruxelles BELGIUM vbouchat@,ulb.ac.be Francesco CAPPUZZELLO Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud (INFNLNS) Via S.Sofia 62,95123, Catania, ITALY cappuzzello0.lns.infn.it Francesco CATARA Department of Phisics and Astronomy University of Catania Via S. Sofia, 64,I-95 123 Catania ITALY catara@,ct.infn.it Evgeni CHEREPANOV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA cher@,iinrm Grigory CHUBARIAN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA and Cycloron Institute, Texas A&M University, College Station, TX 77843, USA [email protected]
Joseph CUGNON University of Liege Physics Department B5 Allee du 6 Aout 17,batBS B-4000 Liege 1 Belgium cugnon0.plasma.theo.Dhys.ulg.ac. be Caros DASSO University of Seville Faculty of Physics Avda. Reina Mercedes sfn Apdo. 1065 - 41080 Sevilla SPAIN dasso@,us.es Jean-Michel DAUGAS Commissariat a 1'Energie Atomique DAM Ile de France CEA/DIF BP 12 9 1680 Bruyeres le Chatel FRANCE jean-michel.daugas(cea.fr Alla DEMYANOVA Kurchatov Institute, 123182 Moscow, RUSSIA adem@,dni.polvn.kiae.su Zdenek DLOUHY Nuclear Physics Institute, ASCR CZ-25068 Rez Czech Republic dlouhv@,uif.cas.cz Sergei DMITRIEV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA dmitriev@,flnr.jinr.ru
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Piotr DECOWSKI Clark Science Center Smith College Northampton, MA 01063, USA pdecowski@,smith.edu
Hans FELDMEIER Gesellschaft fuer Schwerionenforschung Planckstr. 1,64291 Darmstadt GERMANY [email protected]
Tatiyana DONSKOVA International Department Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA tatyana. donskova@,iinr.ru
Yogy GAMBHIR Department of Physics IIT-Powai, Mumbai 400 076, INDIA [email protected] .ac.in
Francois De OLIVEIRA SANTOS GANIL, Bd H.Becquere1 BP 5027 CAEN Cedex 5 FRANCE Oliveira@,aanil.fr Antoine DROUART (CEA-Saclay) Service De Physique Nucleaue CENSACLAY -Batiment 703 DSM/DAPNIA/SPhN 9 1191 Gif-Sur-Yvette FRANCE adrouart@,cea.fr Zoltan ELEKES Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI), 18/c. Bem ter Debrecen H-4026 HUNGARY elekes@,atomki.hu Hans EMLING Gesellschaft fuer Schwerionenforschung Planckstr. 1, D-6429 1 Darmstadt, GERMANY h.emling@,gsi.de
Yuri GANGRSKY Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA ganar@,iinr.ru Hans GEISSEL Gesellschaft h e r Schwerionenforschung Planckstr. 1,64291 Darmstadt GERMANY h.geissel@,gsi.de Stefan GMUCA Institute of Physics Slovak Academy of Sciences Institute of Physics Dubravska cesta 9 SK-845 11 Bratislava 45, SLOVAKIA gmuca@,savba.sk Friedrich GOENNENWEIN University of Tuebingen Physikalisches Institut Auf der Morgenstelle 14 72076 Tuebingen GERMANY poennenwein@,uni-tuebingen.de
718 Vladlen GOLDBERG Cycloron Institute, Texas A&M University, College Station, TX 77843, USA poldberg@,comtxtamu.edu Philip Marshall GORE Vanderbilt University Department of Physics and Astronomy Vanderbilt University W Station B 351807 Nashville, TN 37235 USA phi1ip.m.Pore@,vanderbilt.edu Walter GREINER Institute for Theoretical Physics Johann Wolfgang Goethe- University Frankfurt am Main Robert-Mayer Str. 8-10, D-60054 Frankfurt am Main GERMANY
[email protected] Konstantin GRIDNEV St. Petersburg State University St. Petersburg RUSSIA gridnev@,nuclpc1 .phvs.spbu.ru Georgy GULBEKIAN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA ge0rgymflnr.i inr.ru
Dorninique GOUTTE GANIL, Bd Henri Becquerel - BP 55027 14076 Caen cedex 5 FRANCE [email protected] Hans GEISSEL Department KPII, Gesellschaft fiir Schwerionenforschung mbH, Planckstrasse 1 D-6429 1 Darmstadt GERMANY [email protected] Stefan GMUCA Institute of Physics Slovak Academy of Sciences Dubravska cesta 9 SK-84228 Bratislava SLOVAKIA pmuca@,savba.sk Stephane GREVY Laboratoire de Physique Corpusculaire de Caen, 6, bd du Ma1 Juin 14050 Caen FRANCE grevv@,in2p3.fr_ Dominique GUILLEMAUDMUELLER I Institut de Physique Nucleaire F-91406 Orsay FRANCE guillema@,in2p3 .fr Mohini GUPTA Manipal Academy of Higher Education University Building, Madhav Nagar, Manipal 576 119, INDIA nuclear@,rolta.net
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Joseph HAMILTON Vanderbilt University Box 1807-Station B Nashville, TN 37235 USA
Julia ITKIS Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA
i.h.hamilton@,vanderbilt.edu
iitkis@,m.iinr.ru
Sigurd H O W A " Gesellschaft fiir Schwerionenforschung mbH, Planckstrasse 1, D-6429 1 Darmstadt, GERMANY [email protected]
Svetlana IVANOVA JINR University Center, Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA ivanova@,uc. jinrm
Masayasu ISHIHARA Hirosawa 2-1,. Wako, Saitama 351-0198, RIKEN, JAPAN ishihara@,rarfaxp.riken.no.ip
Igor IZOSIMOV V.G.Khlopin Radium Institute 194021 St.Petersburg, 2nd Murinski ave.28 RUSSIA izosimov@,atom.nw.ru
Fadi IBRAHIM Institut de Physique Nucleaire 9 1406 Orsay Cedex, FRANCE Ibrahim@,ipno.&?.p3.fr
Elizabeth JONES Vanderbilt University Department of Physics and Astronomy Vanderbilt University W Station B 351807 Nashville, TN 37235 USA elizabethfi@,earthlink.net
Bruno IGEL Instituto De Fisica Da Usp Departamento De Fisica Nuclear Universidade de Sao Paul0 Caixa Postal 663 18 BRASIL szanto@,dfn.if.usp.br Mikhail ITKIS Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA itkis0.flnr.i inrm
Rumiana KALPAKCHIEVA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA kalpakaflnr. iinr.ru Alexei KORSHENINNIKOV Hirosawa 2-1, Wako, Saitama 351-0198, RIKEN, JAPAN [email protected]. ip
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Dmitri KAMANIN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA karnanin0,nr. iinr.ru Sarkis KARAMYAN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA karamianmnrmail .i inr.ru Jan KLIMAN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA kliman @,fl nr. i inr .m Sergei KLYGIN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA Sergey.Klvpin@,iinr.ru Galina-KNIAJEVA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA
galina.kniaieva@,rnail.ru
Lubos KRUPA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA krupa@,nrmail.iinr.ru Toshiyuki KUBO RIKEN (The Institute of Physical and Chemical Research) 2-1 Hirosawa, Wako, Saitama 351-0198, JAPAN kubo@,rarfaxp.riken.go.ip Kairat KUTERBEKOV Institute of Nuclear Physics, 1, Ibragimov str., 480082, Almaty, REPUBLIC OF KAZAKHSTAN kuterbekovminp. kz Andrej KUGLER Nuclear Physics Institute ASCR 25068 Rez CZECH REPUBLIC kugler@,uif.cas.cz Yuliya LASHKO 14-b, Metrolohichna str., Bogolyubov Institute for Theoretical Physics, Kiev, 03 143, UKRAINE lashko@,univ.kiev.ua Alinka LEPINE-SZILY Instituto de Fisica da USP Caixa Postal 663 18 Sao Paulo, Brazil alinka@,if.usp.br
72 1
Rubens LICHTENTHALER Instituto De Fisica Da Usp Departamento De Fisica Nuclear Universidade de Sao Paulo Caixa Postal 663 18 BRASIL rubens@,if.usv.br Jonathan LEE Physics Department McGill University 3600 University St.Montrea1, Que., CANADA jlee@,vhvsics.mci).pill.ca Marek LEWITOWICZ GANIL, Bd Henri Becquerel - BP 55027, 14076 Caen cedex 5 FRANCE lewitowicz@uanil. fr Alexander LISETSKIY NSCL Michigan State University 1 Cyclotron Laboratory East Lansing, MI 48823-1321 East Lansing, USA lisetski@,nscl.msu.edu Yuri LOBANOV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region, RUSSIA lobanov@,sunms.iinr.ru Sergei LUKYANOV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA lukyanov@,lnr.iinr.ru
Vladimir MASLOV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA maslov vova@,mail.ru Milan MATOS Gesellschaft fur Schwerionenforschung GSI Planckstr. 1,64291 Darmstadt GERMANY rn.matos@,i).psi.de Hiroari MIYATAKE High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Ibaraki 305-0801, JAPAN hiroari .mivatake@,kek.ip Tohru MOTOBAYASHI RIKEN Wako Institute, Discovery Research Institute 2-1 Hirosawa, Wako, Saitama 351-0198, JAPAN motobaya@,riken.ip. Kosuke MOIUTA RIKEN (The Institute of Physical and Chemical Research) Hirosawa 2-1, Wako-shi, Saitama 351-0198, JAPAN morita@,rarfaxv.riken.eo.iv Gottfried MUENZENBERG Gesellschaft fiir Schwerionenforschung mbH, Planckstrasse 1, D-6429 1 Darmstadt, GERMANY G.Muenzenberg@,gsi.de
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Francois MARECHAL Institut de Recherches Subatomiques 23, Rue du Loess BP 28 F-67037 Strasbourg Cedex 2 FRANCE fkancois.marechal@,ires.in2r,3.fr Yuri NOVIKOV PNPI, Gatchina 188300, RUSSIA novikov@,r,nr,i.svb.ru Yuri OGANESSIAN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA oganessian@,iinr.ru Alexei OGLOBLIN Kurchatov Institute, 123182 Moscow, RUSSIA aoglob@,dni.volyn.kiae.su Takaharu OTSUKA Department of Physics University of Tokyo Hongo, Bunkyo-ku, Tokyo 113-0033 JAPAN otsuka@,r,hvs.s .u-tokvo .ac.i v Yuri PENIONZHKEVICH Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA pyuer@,nrsun.i inr.ru
Valeria PERSHINA Gesellschaft fur Schwerionenforschung Planckstr. 1, D-64291 Darmstadt GERMANY V .Pershina@,,asi.de Andrei POPEKO Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA poveko@,iinr.ru Galina POPEKO Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA gsr,ooeko@,iinr.ru Yuri PYATKOV Moscow Engineering Physics Institute Kashirskoe sh. 3 1 115409 Moscow RUSSIA yvr, nov@,mai.ru Igor POKROVSKIY Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA igom@,nr .iinr .ru Elena PROMOROVA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA lena@,nrmail.iinr.ru
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Emilie RICH I Institut de Physique Nucleaire F-91406 Orsay 91406 FRANCE rich@i~no.in2~3 .fi Maria Valentina RICCIAFtDI Gesellschaft fur Schwerionenforschung (GSI) Planckstr. 1, D-64291 Darmstadt GERMANY m.v.ricciardi@,gsi.de Yevgeny RYABOV 644121, Omsk. prospekt Mira %-a, Omsk State University, department of theoretical Physics RUSSIA ryabov@,org.omskreg.ru
Hewe SAVAJOLS GANIL, Bd Henri Becquerel - BP 55027 14076 Caen cedex 5 FRANCE [email protected] Yuri SEREDA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region, RUSSIA Y uri. Sereda0.i inr.ru Susumu SHIMOURA University of Tokyo Wako branch at RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, JAPAN
shimourak3cns.s.u-tokvo.ac.iv Hiroyuki SAGAWA Center for Mathematical Sciences University of Aizu Aizu-Wakamatsu, Fukushima 965-8580 JAPAN sagawa@,u-aizu.ac.ip Roman SAGAIDAK FLN Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA sagaidak0flnr.i inr.ru Hiroyoshi SAKURAI Departent of Physics, University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-0033 JAPAN [email protected]
Igor SHIROKOVSKY Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA Olga SEMCHENKOVA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA osem@,lnr.iinr.ru Hans-Joachim-SCHOETT Gesellschaft fur Schwerionenforschung (GSI) Planckstr. 1, D-6429 1 Darmstadt GERMANY h. i.schoettO.gsi.de
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Sergei SIDORCHUK Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA sid@,lnr.iinr.ru Cosimo SIGNORINI Physics Department, University of Padova, via Marzolo 8, 35 131 Padova, ITALY signorini@,pd.inf.it Nikolai SKOBELEV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA skobelev@,jinr.ru Adam SOBICZEWSIU Soltan Institute for Nuclear Studies Hoza 69, PL-00-68 1 Warszawa, POLAND [email protected]
Christelle STODEL Grand Accelerateur National &Ions Lourds GANIL B.P. 5027 F-14076 CAEN Cedex 5 FRANCE [email protected] Alexander SVIRIKHIN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA sasha@,sunvas.jinr.ru Toshio SUZUIU College of Humanities and Sciences, Nihon University Department of Physics, Sakurajosui 325-40, Setagaya-ku, Tokyo 156-8550, JAPAN
suzuki@,chs.nihon-u.ac.ip Alejandro SZANTO DE TOLEDO Instituto de fisica da usp Departamento de fisica nuclear CAIXA POSTAL 663 18 05315-970 - Sao Paulo-SP BRASIL Szanto@,dfn.if.usp.br
Yuri SOBOLEV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA sobolev yuri@,mail.ru
Eloisa SZANTO Instituto de fisica da usp Departamento de fisica nuclear CAIXA POSTAL 663 18 053 15-970 - Sao Paulo-SP BRASIL eloisa.szanto@,dfn.if.usp.br
Sergei STEPANTSOV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA stepan@,suntimpx.iinr.ru
Igal TALMI The Weizmann Institute of Science Rehovot 76100 ISRAEL igal.talmi@,weizmann.ac.il
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Oleg TARASOV NSCL, Michigan State University 1 Cyclotron Laboratory East Lansing, MI 48823-1321 East Lansing, USA tarasov@,nscl.msu.edu Tacashi TERANISHI Department of Physics, Kyushu University 6- 10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan teranisi0.cns.s.u-tokyo.ac.ip
Wladyslaw Henryk TRZASKA Department of Physics of University of Jyvaskyla P.O.Box 35 FIN-40014 Jyvaskyla FINLAND trzaska@,tlhys.ivu.fi Vladimir TISHENKO Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA tishenko@,nrsun.iinrm
Gurgen TER-AKOPIAN FLNR, JINR 141980 Dubna, Moscow region RUSSIA
Andreas TUEFUER Technical Uneversity of Munich Institute for Radiochemistry Walther-Meissner-Str. 3, 85748 Garching GERMANY
Gurgen.TerAkopian@,iinr.ru
Andreas.Tuerler@,radiochemie.de
Christophe THEISEN DSM/DAPNIA/SPhN CEA Saclay, Bat 703 9 1191 Giflvette Cedex FRANCE ctheisen@,cea.fr Catherine THIBAULT CSNSM Bat. 108 F-9 1405 Orsay-Campus FRANCE Thibault@,csnsm.in2p3 .fr
Vladimir UGRUMOV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA
Svetlana TRETYAKOVA Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA tsvetl@,iinr.ru
Vladimir UTYONKOV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA utyonkov@,sungns.iinr.ru David VERNEY Institut de Physique Nucleaire F-9 1406 Orsay France FRANCE vernev@,ipno.in2~3.fi
726 Tamas VERTSE Institute of Nuclear Research of the Hungarian Academy of Sciences Bem ter 18/C, Debrecen HUNGARY vertseaatomki. hu
Philip Arthur WILK Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 9455 1 USA wilk2@,llnl.gov
Andrea VITTURI Dipartimento di Fisica and INFN Universita’ di Padova Dipartimento di Fisica Via Marzolo 8,I-3513 1 Padova ITALY vitturi@,Dd.infn.it
Roman WOLSKI Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA wolski@,lnr.iinr.ru
Andrei VORONTSOV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA donklod@,cv.iinr.ru
Alexander YAKUSHEV Technical Uneversity of Munich Institute for Radiochemistry Walther-Meissner-Str. 3, 85748 Garching GERMANY
alexander.vakushev@,radiochemie.de
Alexei VOINOV FLNR Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA voinov0,sungns. i inr.ru
Alexander YEREMIN Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA eremin@,sunvas.iinr.ru
Michiharu WADA The Institute of Physical and Chemical Research 2-1, Hirosawa, Wako, Saitama, 351-0198 JAPAN mw@,riken.go.ip
Arkadi WKHIMCHUK FSNC-VNIIEF 607 188, Sarov, Nizhni Novgorod Region RUSSIA arkad@,triton.vniief.ru
Takahiro WADA Department of Physics, Konan University 8-9-1 Okamoto, Kobe 658-8501, JAPAN wada0,konan-u.ac.i p
Valeri ZAGREBAEV Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA valeri.zagrebaev@,iinr.ru
727 Helmut ZEITTRAEGER Gesellschaft f & u d ; r SchwerionenforschungmbH Dannstadt, Planckstrasse 1 D-64291 Darmstadt GERMANY H.Zeittraeger@,gsi.de
Andrey Zubov JINR University Center, Joint Institute for Nuclear Research 141980 Dubna Moscow region RUSSIA zua@,uc.jinrm
Vyacheslav ZHABITSKY Joint Institute for Nuclear Research, J.-Courie 6; 141980, Dubna, Russia V.Zhabitskvk2jinr.m
Lina ZUFFI Dipartimento Di Fisica Via Celoria 16, 20 133 Milano ITALY [email protected]
Victor ZLOKAZOV FLNF'h Joint Institute for Nuclear Research, J.-Courie 6; 141980, Dubna, Russia zlokazov@,nf.iinr.ru
Ivo z v m Flerov Laboratory of Nuclear Reactions Joint Institute for Nuclear Research 141980 Dubna, Moscow region RUSSIA zvara@,nrmail.iinr.ru
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729
AUTHOR INDEX Abdullin, F. Sh. 168 Abe,D. 467 Abe,Y. 241 Ackermann, D. 157 Added,N. 651 Adeev, G. D. 495 Akaishi, Y. 467 Aksenov, N. V. 285 Alamanos, N. 121 Aleshin, V. P. 424 Alexandrov, A. A. 351,588 Alexandrova, I. A. 351,588 Allal, N. H. 548 Amar,N. 180 Amato, A. 662 Amza1,N. 198 Andrighetto, A. 668 AngClique, J. C. 29,489,559 Angu10,C. 29 Anne,R. 180 Antilopov, V. V. 592 Aoi, N. 64 Aoust, Th. 439 Apasov, V. A. 592 Arazi,A. 651 Amtkh, A. G. 424,579 Asahi, K. 84 Ashwood, N. 29 Astabatyan, R. 128,524 Astabatyan, R. A. 404 Astier, A. 339 Attallah, F. 96 Audi, G. 11, 17 Auger, F. 121 Auger, G. 180, 198 Azaiez, F. 630 Baba, H. 58,64 Baby, L. T. 524 Bachelet, C. 17 Baiborodin, D. 23, 108 Bajeat, 0. 630 Balabanski, D. L. 524 Ban,G. 489 Barth, W. 157 Bastin, J. E. 198
Bastug,T. 309 Basybekov, K. B. 414 Batchelder, J. 479 Baumann, P. 489,559 Beaumel, D. 36 Becheva, E. 36 Beck,C. 520 Beckert, K. 90,96 Beghini, S. 3 17,333 Behera, B. R. 333 Bekhterev, V. V. 592 BClier, G. 524 Beller, P. 90, 96 Belozerov, A. V. 206, 575 Benhamouda, N. 548 Benjamim, E. A. 65 1 Benjelloun, M. 108 Benrachi, F. 489 Bersillon, 0. 11 Beyer, C. J. 357 Bhagwat, A. 261,265 Bialkowski, A. 133 Bingham, C. R. 479 Blachot, J. 11 Blaszczak, Z. 709 Blumenfeld, Y. 36 Bogachev, A. 339 Bogachev, A. A. 325 Bogatchev, A. A. 317,343 Bogomolov, S. L. 168,592,643 Bohlen, H. G. 400,520 Borcea, C. 489, 559 Borge, M. J. G. 29 Borremans, D. 524 Bosch, F. 90,96 Bouchat, V. 29,325 Bouchez, E. 198 Bourgeois, C. 630 Bouriquet, B. 180, 198,241 Boutin, D. 90,96 Brant, S. 511 Briancon, Ch. 206 Brown, B. A. 542 Bruchertscifer,H. 285 Briichle, W. 301, 305 Budzanowski, A. 133,579
730 Buklanov, G. V. 168 Burvenich, T. J. 212 Buta, A. 489, 559 Butler, P. A. 198 Calabretta, L. 662 Camargo Jr., 0. 651 Canchel, G. 559 Cappuzzello, F. 582,662 Casandjian, J. M. 180, 198 Catford, W. 23,29, 559 Catford, W. N. 108 Caurier, E. 489 Cee, R. 180, 198 Celona, L. 662 Chartier, M. 23, 108, 536 Charvet, J. L. 192 Chatillon, A. 180, 198 Chaturvedi, L. 357 Chbihi, A. 192 Cheikh Mahmed, M. 630 Chelnokov, M. L. 52,206,575 Chepigin, V. I. 206, 575 Chepygin, V. I. 285 Cherepanov, E. 271 Chevallier, M. 192 Chines, F. 662 Chizhov, A. Yu. 317, 333 Chubarian, G. 3 17 Chubarian, G. M. 343 Ciszewska, I. 709 Clarke, N. 29 Cltment, E. 180, 198 Cole, J. D. 357,365, 530 Corradi, L. 3 17,333 Cosentino, L. 662 Courtin, S. 489,559 Croizet, H. 630 Csatlos, M. 58 Csige, L. 58 Cugnon,J. 439 Culicov 0. A. 709 Cunsolo, A. 582 Curien, D. 339 Curtis,N. 29 Cuttone, G. 662
Dahl,L. 157 Daniel, A. V, 365,357,530 Danilyan, G. 374 Daugas, J. M. 524,559 Dauvergne, D. 192 Dayras, R. 180, 192, 198 de Angelis, G. 520 de Faria, P. N. 65 1 de France, G. 180, 198 de Francesco, A. 385 de Oliveira Santos, F. 180, 524 de Oliveira, F. 559 de Saint Simon, M. 17 de Tourreil, R. 180, 198 Delaunay, F. 36 Deloncle, I. 339 Demichi, K. 64 Demin, A. M. 592 Demonchy, C. E. 23, 108 Demyanova, A. S. 392,400 Denikin, A. S. 404 Denisov, S. V. 588 Denke, R. 651 Descouvemont, P. 29 Dessagne, P. 559 Dessagne, Ph. 489 Dimitrov, V. 357 Dlouhy, Z. 23, 108, 128,559 Dmitriev, S. N. 285,567,643 Dombradi, Zs. 58, 64 Donangelo, R. 357,365,530 Dorvaux, 0. 198,206,317,325, 339,343 Drigert, M. W. 357 Drouart, A. 180, 192,198 Duchene, G. 339 Ducourtieux, M. 630 Ebbinghaus, B. B. 567 Eeckhaudt, S. 198 Efimov,G. 520 Eichler, R. 285 Elekes, Z. 58,64 Emling, H. 100 Emori,S. 84 Escano Rodriguez, C. 192 Essabaa, S. 630
73 1 Fallon, P. 357, 530 Feldmeier, H. 136 Fellah, M. 548 Fifi, R. 630 Filippov, G. F. 142 Finocchiaro, P. 662 Fioretto, E. 333 Florko, B. V. 351 Fornichev, A. 36 Fornichev, A. S. 45,52,365 Fong, D. 357,365 Fong, D. J. 479 Fortier, S. 36 Foti, A. 582 Franchoo, S. 630 Franczak, B. 90,96 Frankland, J. D. 192 Franzke, B. 90,96 Frascaria, N. 36 Frauendorf, S. 357 Freer,M. 29 Frontasyeva, M. V. 709 Fujirnoto, R. 467 Fukuda,N. 58 Fukushirna, A. 255 Fulop, 2s. 58,64 Gacsi,Z. 58 Gadea,A. 333 Gagarski, A. 374 Gaggeler, H. W. 285 Gales, S. 36 Gall, B. 198 Gall, B. J. P. 339 Garnbhir, Y. K. 261,265 Garnmino, S. 662 Gangrsky, Yu. P. 553 Garufi,D. 662 Gaubert, G. 630 Gaulard, C. 17 Gebauer, B. 520 Geissel, H. 68,90,96 Gelberg, A. 357 Georgiev, G. 524 Giardina, G. 3 17, 325 Gibelin, J. 64 Gikal, B. N. 168,643 Gilat, J. 357
Gillibert, A. 23, 36, 108, 180 Ginter, T. N. 357,479 Giot, L. 23, 108 Girod,M. 524 Glodariu, T. 385 Glukhov, Yu. A. 392,400 Goldring, G. 524 Golovkov, M. S. 45, 52 Golubkov, A. N. 592 Gomi,T. 64 Goncharov, G. N. 279 Goncharov, S. A. 392,400 Gdnnenwein, F. 374 Gorbunov, N. V. 579 Gore,P. 365 Gore, P. M. 357, 530 Gorgen, A. 198 Gornostaev, Ye. V. 592 Gorshkov, V. A. 52,206, 575 Goutte,H. 524 Grahn, T. 198 Grawe, H. 467,542 Greenlees, P. T. 198 Greiner, W. 212,233,365,485 Grkvy, S. 180, 198,489,559 Gridnev, D. K. 485 Gridnev, K. A. 485 Grigorenko, L. V. 45 Grishechkin, S. K. 592 Gross, C. J. 479 Grzywacz, R. K. 479 Guenaut, C. 17 Guillemaud Mueller, D. 559 Guillot, J. 36 Guirnariies, V. 65 1 Gulbekian, G. G. 168,592 Gulbekyan, G. G. 643 Gulyas, J. 58 Gupta, J. B. 530 Gupta, M. 261,265 Hagino, K. 479 Hamilton, J. 485 Hamilton, J. H. 357,365,479,530 Hammache, F. 36 Hanappe, F. 29, 180,3 17,325, 339 Hannachi, F. 180
732 Hasegawa, H. 64 Haseyama, T. 84 Hass, M. 524 Hassan, A. A. 128,404 Hauschild, K. 180, 198,206 Hausmann, M. 90,96 Hellstrom, M. 96 Herfurth,F. 17 Herzberg, R-D. 198 Hessberger, F. P. 157, 180, 198 Himpe,P. 524 Hirata, D. 108 Hofmann, S. 157, 180 Horoi, M. 542 Huguet,Y. 630 Hiirstel, A. 198 Hussein, M. S. 651 Hussonnois, M. 285 Hwang, J. K. 357,365,530 Ibrahim, F. 630 Id Betan, E. 148 Ignatyuk, A. V. 432 Ikin, P. J. C. 198 Iliev, S. 168 Iliev, S. N. 571 Imai,N. 64 Inglima, G. 385 Ishihara, M. 64, 84 Isolde Collaboration 17 Itkis, I. 317 Itkis, I. M. 325, 343 Itkis, M. G. 168, 285,317, 325, 333, 339,343,643 Ivanova, S. 698 Iwasa,N. 58 Iwasaki, H. 64 Izadpanakh, A. 400 Izosimov, I. N. 503 Jacquet, D. 192 Jager, E. 301,305 Jandel, M. 325,365 Jesinger, P. 374 Jiang, Z. 357 Jollet, C. 489 Jones, E. F. 357, 530 Jones, G. D. 198
Jones,P. 198 Julin,R. 198 Jurado,B. 23 Juutinen. S. 198 Kabachenko, A. P, 206,575 Kalagin, V. V. 643 Kalpakchieva, R. 128, 133,404 Kamanin, D. 520 Kamanin, D. V. 351,588 Kameda,D. 84 Kaminski, G. 424,579 Kanno, S. 64 Karaivanov, D. V. 553 Karamian, S. A. 451 Karny,M. 479 Kartavenko, V. G. 485 Kato, G. 84 Kawai, S. 64 Kaza, E. 90,96 Keeley, N. 121 Kelik, A. 432 Kenneally, J. M. 168,285,567 Kettunen, H. 198 Khalfallah, F. 198 Khan,E. 36 Khelfallah, F. 339 Khlebnikov, S. 343,374 Khlebnikov, S. V. 133,351,400,499, 588 Khouaja, A. 23, 108, 180 Kijima, G. 84 Kindler, B. 157 Kinnard, V. 29 Kinugawa, H. 58 Kirsch, R. 192 Kishida, T. 64 Klepper, 0. 90,96 Kliman, J. 3 17,325, 343, 365 Kluge, H. J. 96 Kluge, H.-J. 90 Klygin, S. A. 424, 579 Kniajeva, G. N. 343 Knipper, A. 559 Knyazheva, G. N. 3 17,325,333 Kobayashi, Y. 84 Kokalova, Tz. 520 Kolata, J. 651
733 Kondratiev, N. A. 3 17,325,333, 343 Korichi, A. 198,206 Kormicki, J. 357,365, 530 Korsheninnikov, A. A. 3,45, 52 Korten, W. 198 Korzyukov, I. V. 3 17 Kosenko, G. 241 Kotzle, A. 374 Kozhuharov, C. 90,96 Kozulin, E. 339 Kozulin, E. M. 3 17,325,333, 343 Krasmahorkay, A. 58 Kratz, K. L. 90,96 Kratz, K.-L. 559 Krolas, W. 479 Krupa, L. 317,325,339,343,365 Kubo,T. 64 Kubono,S. 58 Kugler, A. 133,704 Kukhtina, I. N. 408,414 Kulko, A. A. 128, 133 Kurarnoto, R. Y. R. 651 Kurita, K. 64 Kurokawa, M. 58 Kuryakin, A. V. 592 Kushniruk, V. F. 133 Kuterbekov, K. A. 133,408,414 Kuzrnina, T. E. 588 Kumetsov, I. V. 133,404 Kuznetsova, E. A. 351,588 La Cornmara, M. 385 Labiche, M. 29 Laget, M. 192 Landingham, R. L. 567 Lapoux, V. 36, 121 Lashko, Yu. A. 142 Latina, A. 333 Lau,C. 630 Lautesse, P. 192 Lazzaro, A. 582 LeCoz,Y. 198 Le Scomet, G. 489 Leblanc, F. 630 Lecesne, N. 630 Lecolley, F. R. 489, 559
Lecomte, P. 630 Lecouey, J. L. 559 Lee, I. Y. 357,365,530 Lefort,H. 630 Leino, M. 198 Lenske, H. 582 LCpine-Szily, A. 23, 108, 536,651 Leporis, M. 592 Leppanen, A-P. 198 Leroy,R. 630 Levtsov, V. G. 592 Lewitowicz, M. 524, 559 Lhersonneau, G. 559 Li, K. 357,530 Lichtenthaler, R. 180, 536, 65 1 LiCnard, E. 489,559 Lima, G. F. 536,65 1 Lirna,V. 36 Liotta, R. J. 148 Lisetskiy, A. F. 542 Litovchenko, P. G. 579 Litvinov, S. 90 Litvinov, Yu. 90,96 Liu,X. 58 Lobanov, Yu. V. 168,285 Lobastov, S. 128 Lobastov, S. P. 133,404 Loginov, V. N. 592 Lojek, K. 180 Lornrnel, B. 157 Lopez-Martens, A. 180, 198,206 Lougheed, R. W. 168,567 Lucas,R. 339 Lukianov, S. 559 Lukyanov, S. 23,524 Lukyanov, S. M. 128, 133, 198,404 Lunney, D. 17 Luo, Y. X. 357,530 Lyapin, V. G. 333,343,351 Ma, W.C. 530 Maanselka, V. 198 Malkov, I. N. 592 Malyshev, 0. N. 206,575 Mann,R. 157 MarCchal, F. 489, 559 Marinova, K. P. 553 Markaryan, E. 128
734 Markaryan, E. R. 404 Markov, B. N. 553 Marques, F. M. 29 Maslov, V. 128 Maslov, V. A. 133,400,404 Matea, I. 524 Materna, T. 29, 3 17,325,343 MatoS, M. 90,96 Matsuo, T. 467 Matsuyama, Y. 64 Mazzocchi, C. 479 Mazzocco, M. 385 Mcewan,P. 29 Menna,M. 662 Mtot,V. 524 Messina, G. 662 Meyer,M. 339 Mezentsev, A. N. 168 Michimasa, S. 58,64 Miehe,C. 559 MiChe, Ch. 489 Mikhailov, L. 404 Minemura, T. 58,64 Mirea, M. 630 Mitrofanov, S. V. 351,588 Mittig, W. 23, 108,536 Miyatake, H. 636 Miyoshi, H. 84 Montagnoli, G. 317,333 Moody, K. J. 168,567 Morita, K. 188 Morjean, M. 192 Motobayashi, T. 58,64. 622 Mrazek, J. 23, 128,559 Mukhambetzhan, A. 408 Muntian, I. 249 Munzenberg, G. 90, 96, 157, 614 Musyaev, R. K. 592 Mutterer, M. 374 Nalpas, L. 36, 192 Napolitani, P. 432 Neff,T. 136 Negoita, F. 489, 559 Nesvishevski, V. 374 Neyens, G. 524 Nieminen, P. 198
Nikolskii, E. Yu. 45,52 Nilsson, T. 29 Ninane,A. 29 Nizery, F. 630 Nociforo, C. 582 Nolden, F. 90,96 Normand,G. 29 Notani,M. 64 Novikov, Yu. 90,96 Nowacki. F. 489 Obertelli, A. 36 Oganessian, Yu. Ts. 45,52, 168,206, 285,317,325,357,365, 530, 567, 592,643 Ogawa,H. 84 Ogloblin, A. A. 392,400,499 Ohnishi, T. K. 64 Ohta, M. 255 Ohtsubo, T. 90,96 Ong,H. J. 64 Orr, N. A. 23,29, 108,, 489,536,559 Orrigo, S. E. A. 582 Ostrowski, A. N. 90 Ota, S. 64 Otsuka, T. 467 Oudih, M. R. 548 Ozawa, A. 58,64 Pain, S. 29 Pakarinen, J. 198 Pakou, A. 121 Pantelica, D. 559 Papka,P. 520 Pashchenko, S. V. 592 Passarello, S. 662 Patin, J. B. 168, 567 Patyk, Z. 90,96 PCghaire, A. 180 Pellemoine, F. 630 Penionzhkevich, Y. 23, 108 Penionzhkevich, Yu. E. 128, 133, 198, 351,400,404,408,414,524, 553, 559,588 Perego, C. 651 Perevozchikov, V. V. 45 Perkowski, J. 198 Perrot, F. 489
735 Pershina, V. 309 Peter, J. 180, 559 Petrov,G. 374 Pfeiffer, B. 90,96,559 Phan Viet, C. 630 Piechaczek, A. 479 Pierroutsakou, D. 385 Pietri, S. 559 Piqueras, I. 339 Pita, S. 23, 108 Placco, V. 65 1 Poirier, E. 489,559 Pokrovski, I. V. 3 17,325,343 Pollacco, E. C. 36 Polyakov, A. N. 168,571 Popeko, A. 305 Popeko, A. G. 206,575 Popeko, G. S. 365 Porquet, M. G. 339 Portillo, M. 90 Pothet, B. 36 Potier, Jc. 630 Pougheon, F. 559,630 Povtoreiko, A. A. 579 Pritchard, A. 198 Prokhorova, E. V. 29,3 17,325 Pustovoi, V. I. 592 Pyatkov, Yu. V. 35 1, 588 Rahkila, P. 198 Raia, G. 662 Ramayya, A. V. 357,365,479,530 Ramdhane, M. 489 Rasmussen, J. 0. 357, 365, 530 Ratzinger, U. 157 Ray,C. 192 Re,M. 662 Redon,N. 339 Reitmeier, S. 305 Rejmund, F. 432 Rejmund, M. 198 Remy Anne 605 Ribas, R. V. 630 Ricciardi, M. V. 432 Rich,E. 36 Ridikas, D. 630 Rifbggiato, D. 662 Rodin, A. M. 45, 52,365, 592
Roig, 0. 524 Romoli, M, 385 Roth, R. 136 Rousseau, M. 23,198,339,520 Roussel-Chomaz, P. 23, 36, 108 Roussiere, B. 630 Rovelli, A. 662 Rowley, N. 3 17,325 Rubchenia, W. 343 Rubchenya, V. A. 333 Rusanov, A. Ya. 3 17,325 Rusek, K. 121 Ryabov, E. G. 495 Ryabov, Yu.V. 351,588 Rykaczewski, K. P. 479 Sagaidak, R. 305 Sagaidak, R. N. 206,3 17,333,499, 575 Sagawa,H. 78 Saint-Laurent,M. G. 180,630 Saito, A. 58, 64 Sakai, H. K. 64 Sakurai, H. 64 Samarin, V. V. 420 Sandoli, M. 385 Sandulescu,N. 148 Santonocito, D. 36 Saren, J. 198 Sauvage, J. 630 Savajols, H. 23, 108 Sawicka, M. 524 Scarlassara, F. 3 17, 333 Scarpaci, J-A. 36 Schldel, M. 301,305 Schatz, H. 90,96 Scheidenberger, C. 90,96 Schempp, A. 157 Schillari, G. 662 Schimpf E. 301,305 Schmidt, K, 374 Schmidt, K.-H. 432 Schmitt, C. 325 Scholey, C. 198 Schulz, C. 520 Scuderi, V. 662 Sereda, Yu. M. 424, 579 Shaughnessy,D. A. 168,285,567
736 Shimada, K. 84 Shimoura, S. 58, 64 Shirokovsky,I, V. 168 Shishkin, S. V. 285 Shumann, D. 285 Shutov, A. 305 Shutov, A. V. 206,575 Sidorchuk, S. I. 45, 52 Signorini, C. 385 Simenel, C. 198 Skaza,F. 36 Skobelev,N. K. 128, 133,369,404 Skwirchinska,I. 133 Slepnev, R. S. 45, 52 Slyusarenko, L. I. 414 Smirnov, Yu. I. 592 Smolin, D. A. 579 Sobiczewski, A. 249 Sobolev, Yu. G. 128, 133, 198, 400, 404,414 Sokol, E. A. 285,351,575,588 Sokolov, V. A. 643 Soramel, F. 385 Sorlin, 0. 36,559 Sosin, Z. 180 St Laurent, M. G. 198 Stadlmann, J. 90,96 Stanoiu, M. 559 Steck, M. 90,96 Stefan, I. 489, 559 Stefanini, A. M. 3 17, 333 Stepantsov, S. V. 36,45, 52 Stezowski, 0. 339 Stodel, C. 192, 559 Stodel, Ch. 180, 198 Stolz, A. 479 Stoyer, M. A. 168,285,357,365, 567 Stoyer,N. J. 168 Stoyer,M. 530 Stuttge, L. 29, 180, 317, 325, 339 Subbotin, V. G. 168, 571 Subotic, K. 168 Sukhov, A. M. 168,571 Suzuki, T. 78,445,467 Svirikhin, A. I. 206, 575 Szilner, S. 333 Szmider, J. 579
Takeshita, E. 64 Takeuchi, S. 58,64 Tamaki,M. 64 Tanihata, I. 58 Tantawy, M. 479 Tarasov, 0. B. 428 Ter-Akopian, G. M. 45, 52, 357, 365, 530,592 Testa, E. 192 Teterev, Yu. G. 424,579 Theisen, Ch. 180, 198 Thibault, C. 11, 17 Thirolf, P. 58 Thummerer, S. 520 Timis, C. 29,559 Tinschert, K. 157 Tiourine, G. 374 Tishchenko, V. G. 351,588 Tjukavkin, A. N. 35 1 Togano,Y. 64 Tokarevsky, V. V. 414 Torilov, S. Yu. 485 Tretyakova, S. P. 499 Trotta,M. 317, 333 Tryggestad, E. 36 Trzaska, W. 343,351, 374,392,400, 499 Trzaska, W. H. 133,333 Tsukui,M. 84 Tsurin, I. P. 588 Tsyganov, Yu. S. 168,285,571 Tiirler, A. 301,305 Tyukavkin, A. N. 588 Tyurin, G. P. 400 Ueno,H. 84 Ugryumov, V. Yu. 133,404,414 Utyonkov, V. K. 168,285 Uusitalo, J. 198 Vakhtin, D. 343 Van de Vel, K. 198 Vardaci, E. 385 Vemey,D. 630 Vertse,T. 148 Vieira, D. 90 Vieira, N. 17 Villari, A. 198
737 Villari, A. C. C. 23, 108, 180, 536 Vincour, J. 404 Vinogradov, Yu. I. 45,592 Vinsour, I. 128 Voinov, A. A. 168,571 Volant, C. 192 Von Kalben, J. 374 Von Oertzen, W. 400,520 Vorobjev, G. 90,96 Vorontsov, A. N. 424,579 Voskresenski, V. M. 325 Voskressenski, V. M. 317, 333, 343 Vostokin, G. K. 285
Yamaguchi, T. 90,96 Yamaletdinov, S. R. 351,588 Yanagisawa, Y. 58,64 Yang, L. M. 357 Yazvitski, N. Yu. 592 Yefkmov, A. A. 592 Yeremin, A. 305 Yeremin, A. V. 206,285,567, 575 Yoneda,K. 64 Yordanov, 0. 432 Yoshida, K. 58 Yoshida, N. 5 1 1 Yoshimi, A. 84 Yu,C.-H. 479 Yukhimchuk, A. A. 45,592
W. C. M. A,, 357 Wada,M. 656 Wada,T. 255 Wapstra, A. H. 11 Watanabe, H. 84 Weick, H. 90,96 Wieleczko, J. P. 180 Wieloch, A. 180 Wierczinski, B. 301,305 Wild, J. F. 168,285, 567 Wilk, P. A. 168,285, 567 Wilson, J. 198 Winfield, J. S. 582 Winger, J. A. 479 Winkler, M. 90, 96 Wollnik, H. 90, 96 Wolski, R. 36,45, 52 Wu, S. C. 357,365
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