Anewµs isomerin 136 Sb produced in the projectile fission of 238 U - PDF

Eur. Phys. J. A 11, 9 1 (2001) THE EUROPEAN PHYSICAL JOURNAL A c Società Italiana di Fisica Springer-Verlag 2001 Anewµs isomerin 16 Sb produced in the projectile fission of 28 U M.N. Mineva 1,a, M. Hellström

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Eur. Phys. J. A 11, 9 1 (2001) THE EUROPEAN PHYSICAL JOURNAL A c Società Italiana di Fisica Springer-Verlag 2001 Anewµs isomerin 16 Sb produced in the projectile fission of 28 U M.N. Mineva 1,a, M. Hellström 1,2, M. Bernas,J.Gerl 2,H.Grawe 2,M.Pfützner 4, P.H. Regan 5, M. Rejmund 6, D. Rudolph 1, F. Becker 6, C.R. Bingham 7, T. Enqvist 8, B. Fogelberg 9, H. Gausemel 10, H. Geissel 2, J. Genevey 11, M. Górska 2,R.Grzywacz 7, K. Hauschild 6, Z. Janas 4,I.Kojouharov 2, Y. Kopatch 2, A. Korgul 4,W.Korten 6, J. Kurcewicz 4, M. Lewitowicz 12,R.Lucas 6,H.Mach 9, S. Mandal 2,P.Mayet 2, C. Mazzocchi 2,1, J.A. Pinston 11, Zs. Podolyàk 5, H. Schaffner 2, Ch. Schlegel 2, K. Schmidt 2,K.Sümmerer 2, and H.J. Wollersheim 2 1 Division of Cosmic and Subatomic Physics, Lund University, SE Lund, Sweden 2 Gesellschaft für Schwerionenforschung mbh, D Darmstadt, Germany Institut de Physique Nucléaire d Orsay, IN2P-CNRS, F Orsay Cedex, France 4 Institute of Experimental Physics, Warsaw University, PL-00861, Poland 5 Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom 6 CEA Saclay, DSM/DAPNIA/SPhN, F Gif-sur-Yvette Cedex, France 7 Department of Physics, University of Tennessee, Knoxville, TN 7996, USA 8 Oliver Lodge Laboratory, University of Liverpool, Liverpool, L69 7ZE, United Kingdom 9 Department of Radiation Science, Uppsala University, S Nyköping, Sweden 10 Department of Chemistry, University of Oslo, Blindern, N-015 Oslo, Norway 11 Institut des Sciences Nucléaires, IN2P-CNRS/Université Joseph Fourier, F-8026 Grenoble Cedex, France 12 Grand Accélérateur National d Ions Lourds, BP 5027, F Caen Cedex, France 1 Università degli Studi di Milano, I-201 Milano, Italy Received: 2 April 2001 Communicated by D. Schwalm Abstract. The neutron-rich isotope 16 Sb has been produced following the relativistic projectile fission of 28 U in an experiment performed at the Fragment Separator at GSI, Darmstadt. Delayed γ-ray spectroscopy of the fission products has been performed after isotope separation. A new isomeric state in 16 Sb has been populated, and its lifetime measured as T 1/2 = 565(50) ns. Realistic and empirical shell-model calculations have been performed and are compared to the experimental observables. PACS Tg Lifetimes Cs Shell model q Relativistic heavy-ion collisions 1 Introduction In the vicinity of the magic Z =50andN = 82 shell closures, the coupling of valence particles and holes in relatively high-j orbitals frequently gives rise to isomeric states withmoderate spin. Indeed, the presence of more than one β-decaying state in given nuclei is a characteristic feature for this region, and several nuclei are also known to have isomeric states with half-lives in the microsecond regime. Withits 51 protons and 85 neutrons, 16 Sb forms an interesting system withone proton and three neutrons outside the doubly magic 12 Sn core. Because of its oddodd character, information about its lower-lying excited structure should give direct access to the proton-neutron interaction in this rather neutron-rich region of the nuclidic chart. Of special interest is the question of how these a low-lying configurations develop as more protons and neutrons are added to 12 Sn. In analogy withthe situation in the 208 Pb region, as investigated by Alexa and Sheline [1], it is expected that mixing will increase with the number of valence particles until, ultimately, quadrupole-deformed collective states form. 16 Sb was first observed as a β-delayed neutron precursor [2] produced in thermal neutron-induced fission of 25 U, and its βn-emission half-life was determined to be T 1/2 =0.92(14) s []. Hoff et al. [4] studied the β-decay in detail and concluded from the population pattern of states in the daughter nucleus 16 Te that the 16 Sb ground state most likely has I π =1 and is dominated by the πg 7/2 νf7/2 configuration. Interestingly, Hoff et al. report only one β-decaying state, and no other excited states in 16 Sb have been observed. In contrast, 14 Sb, which has two neutrons less than 16 Sb, has two β-decaying states: the I π =0, T 1/2 = The European Physical Journal A 0.85 s ground state and a I π = 7, T1/2 = 10 s isomer. Both levels are members of the (πg7/2 νf7/2 )0,...7 multiplet [5]. Although the excitation energy of the 7 state and the placement of excited states between the 0 and 7 are still uncertain [6], a number of levels built on top of the 7 isomer are known [5]. The 208 Pb region counterpart of 16 Sb is 212 Bi, which has the same number of protons and neutrons outside the doubly magic core. 212 Bi has an I π = 1, T1/2 = 60.5 min ground state and an I π = 8 (9 ), T1/2 = 27 min β-decaying isomer, both with the configuration πh9/2 νg9/2 [7]. In principle, spectroscopic information about excited states in 16 Sb could be obtained by studying the β-decay of 16 Sn. Indeed, first results from the β-decay of very neutron-rich Sn isotopes have been obtained in a laser ionsource experiment at CERN/ISOLDE [8]. No γ-ray transitions in 16 Sb have been observed. It should be noted that in the β-decay the mother nucleus 16 Sn has ground-state spin and parity I π = 0+. Hence, population of high-spin states in the daughter 16 Sb is very unlikely. Recently, particle-tagged delayed-coincidence spectroscopy of isomers produced directly in nuclear reactions has emerged as a successful alternative method to study excited states in nuclei far from the valley of β-stability [9,10]. We have applied this new method to produce and investigate microsecond range isomers in the region around 12 Sn using projectile fission as a population mechanism. With the present study, 16 Sb becomes the most neutronrich Sb isotope from which γ-rays have been observed. As pointed out in ref. [11], spectroscopy of exotic nuclei offers a unique test of the components of the effective interactions that depend on the isospin degree of freedom. Continued investigations of 16 Sb and other neutron-rich nuclei beyond 12 Sn will enhance the understanding of how the shell structure and nuclear mean field develop when approaching the neutron drip-line. 2 Experiment The experiment was performed at the FRagment Separator (FRS) at GSI, Darmstadt [12]. Neutron-rich nuclei around 12 Sn were produced in the projectile fission of 28 U at the relativistic energy of 750 MeV/u impinging on a 1 g/cm2 9 Be target. The average beam intensity from the SIS heavy-ion synchrotron was U ions per second. The ions of interest were separated by combining magnetic analysis with energy loss in matter [12]. The FRS was operated in a standard achromatic mode and a wedgeshaped degrader with thickness set to 50% of the range of the selected fragments was placed at the central focal plane of the spectrometer. Over a four days running period, we covered three different settings optimized for 11 Sn, 14 Sn, and 10 Cd, respectively. For each production setting of the FRS, between 12 and 20 different fragment species were transmitted to the final focal plane, and in total some 0 isotopes in the neutron-rich Cd to Te with neutron numbers N varying from 76 to 87 were observed Proton number 10 Sb A/q Fig. 1. Particle identification spectrum showing the proton number Z vs. the mass-to-charge ratio A/q for fragments reaching the final focal plane of the separator during the setting optimized for 14 Sn. Each group represents a single isotope, as schematically indicated for 16 Sb. Due to the specific kinematics of in-flight fission, the total transmission (including nuclear absorption as well as straggling effects) from the production target to the final focal plane was 1%. The separated ions were identified event by event via combined time-of-flight, position tracking, and energy loss measurements. The FRS detectors were calibrated using the 28 U primary beam at different velocities. Figure 1 presents the particle identification plot of the second setting, in which 16 Sb was transmitted. The transmission losses [10], due to the reaction products picking-up electrons (thereby altering their A/q ratio) while passing through the different layers of matter in the separator, was very small: 99.0% of the Sn and 98.7% of the Te fragments were fully stripped in the second stage of the FRS. It should be noted here that the relativistic character of the in-flight fission process introduces a gain factor [1] with respect to the fission at rest: the high laboratory velocity of the fissioning projectile nucleus ( 8% of the speed of light) leads to an optimal transmission for those fragments that are emitted in a forward or backward direction with respect to the beam. The resulting kinematic boost implies that for equal fission rates, nuclei produced with cross-sections up to three orders of magnitude smaller can be separated and studied [1], thus significantly extending the possibilities for spectroscopy in very neutron-rich regions of the nuclidic chart [14]. At the final focus, the fragments of interest were slowed down using an adjustable aluminum degrader (6. g/cm2 for the setting in which 16 Sb was identified) and subsequently implanted in a 6.5 mm thick aluminum catcher. By comparing the energy loss signal from plastic scintillator counters placed just in front of and after the adjustable degrader, those fragments (up to 0% for a given isotope!), M.N. Mineva et al.: Anewµs isomer in 16 Sb produced in the projectile fission of 28 U 11 Counts/keV Counts/keV Eγ (kev) Eγ (kev) Fig. 2. γ-ray spectra measured in delayed coincidence with implanted ions of (a) 15 Te (delay interval µs), and (b) 16 Sb ( µs). The energy labels are in kev (a) (b) Table 1. Summary of determined half-lives Isotope I π T 1/2 (ns) T 1/2 (ns) present work previous work 14 Te (6) 164 (1) [17] 15 Te 19/2 512 (22) 510 (20) [18] 16 Sb (50) - undergoing charge-changing reactions during the slowingdown process, could be rejected. In addition, a veto scintillator placed after the catcher helped to reject ions that were not implanted correctly. The catcher was surrounded by four segmented Clovertype Ge detectors [15], in which delayed γ-rays emitted by the implanted ions were detected. The Ge detectors were energy and efficiency calibrated using standard sources. The flight time of the ions through the separator is about 00 to 400 ns, which typically sets a lower limit of about 100 ns for the lifetimes which can be studied. γ-rays from the decay of any isomeric state were recorded within a80µs time gate, which was started with the detection of a heavy-ion event. This slow coincidence technique allowed us to correlate the detected γ-rays withthe implanted heavy ions. The time interval between the implantation and delayed γ-ray was measured by using bothtdcs (8 µs range) and TACs (80 µs range). More details about the experiment can be found in ref. [16]. Analysis and results By selecting events associated witha single isotope species, very clean γ-ray spectra can be obtained. Figure 2 shows delayed γ-ray spectra measured in coincidence withimplanted ions of 15 Te and 16 Sb, respectively. The previously reported isomers in 14 Te (I π =6 +, T 1/2 = 164 ns) [17] and 15 Te (I π =19/2, T 1/2 = 510 ns) [18] have half-lives in the range ( 100 ns 1 ms) that can be studied using the described technique (see table 1). They were used as reference both for the particle identification in the first fragment setting, and to verify the procedure for the lifetime determination. The particle identi- Fig.. Time distribution curves of the (a) 25 kev line from the decay of the I π = 19/2 isomer in 15 Te, and (b) the 17 kev line identified in 16 Sb. fication of the second setting (cf. fig. 1), where the 16 Sb isomer was observed, was firmly established using the well known I π =8 +, T 1/2 =2.0 µs isomer in 12 Sn [19]. Time spectra from the particle-delayed γ-coincidence measurements for the 25 kev γ-ray in 15 Te and the 17 kev transition in 16 Sb are shown in fig.. The prompt part of the decay time distributions have been subtracted. The half-lives were extracted by fitting the slope, assuming a single component decay. The results are summarized in table 1 and compared withprevious measurements of known isomers. 12 The European Physical Journal A Excitation energy (MeV) Sb KHSM πg 7/2 ν f 7/2 ESM Excitation energy (MeV) Sb π g 7/2 ν f 7/2 KHSM ESM Spin (h) 0.0 Fig. 5. KHSM and ESM shell-model calculations for the πg 7/2 νf 7/2 multiplet in 14 Sb, see the caption of fig. 4 Experimentallevelenergiesfromref.[6]areindicatedbyopencircles Spin (h) Fig. 4. Comparison of realistic (KHSM, thin lines) and empirical (ESM, thick lines) shell-model calculations for the πg 7/2 νf 7/2 multiplet in 16 Sb. The two different approaches give very similar results. All states below 1 MeV have negative parity in the full (KHSM) calculation. 4 Discussion Since the residual interaction and the quadrupole properties of the 12 Sn and 208 Pb regions are rather similar [17, 20], and because the E2 effective charges are essentially equal, we can expect the shell-model descriptions for the lowest-lying πν multiplets to be very similar [21]. This is especially true when comparing nuclei with ground-state configurations involving bothprotons and neutrons in orbitals with the same number of nodes in their radial wave functions (π1g 7/2 and ν2f 7/2 in the 12 Sn region, corresponding to π1h 9/2 and ν2g 9/2 in the 208 Pb region). Indeed, after appropriate scaling with A 1/, interaction strengths and matrix elements obtained around 208 Pb can be applied also for nuclei close to 12 Sn [17,20,21]. (Note however that, as the collective negative-parity states in 12 Sn are higher than in 208 Pb, the octupole properties of the two regions may differ significantly.) The observation of only a single delayed γ-ray transition associated with 16 Sb makes it difficult to reconstruct the properties of excited states in this nucleus. In order to provide more information, we have therefore performed spherical shell-model calculations, using two different sets of interactions, to estimate the excitation energies of the lowest-lying states. The latest experimental information on single-particle energies in the 12 Sn region were included [22, 2]. The Kuo-Herling shell model (KHSM) uses realistic two-body matrix elements calculated for the 208 Pb region and scaled down by A 1/ in the full Z =50to82and N = 82 to 126 model space withmixing [24 26]. The empirical shell model (ESM), on the other hand, uses twobody matrix elements (TBME) scaled from the πh 9/2 νg 9/2 multiplet in 210 Bi to πg 7/2 νf 7/2, and f7/2 2 TBME obtained from 14 Sn. Pure configurations are assumed. The results of the calculations for 16 Sb are summarized in fig. 4. The two models predict very similar behaviour of the πg 7/2 νf7/2 multiplet members, and both correctly reproduce the observed ground-state spin and parity I π =1 [4]. Based on the calculations, several scenarios for the observed isomeric level in 16 Sb are possible. A high-spin (I 7) isomer is highly unlikely due to the lack of observation of other, higher energy ( 1 MeV) γ-ray transitions. In principle, such a high-spin isomer may be possible if the isomeric structure was built on top of a β-decaying isomer. However, this is at variance with the fact that only one β-decaying state has been observed so far [4]. As a second scenario, favored by the ESM calculations we propose a 4 isomer decaying to the 2 state via the observed 17 kev E2 transition. The ESM predicts that the 6 state would be a β-decaying isomer, which is again in disagreement withref. [4]. The experimental B(E2) value of the 17 kev isomeric transition is then deduced as 0.125(11)Wu, which appears low in comparison with B(E2) = 4.5 Wu given by the ESM model for an isomeric 4 to 2 transition. To solve the discrepancy a third scenario can be considered. We assume a low energy ( 50 kev) unobserved primary transition 4 to 2, and the detected 17 kev γ- ray would then be a 2 to 1 M1 transition. The conversion coefficient for a 50 kev E2 transition in 16 Sb is α tot =20.2, which leads to B(E2) =.6() Wu. However, this scheme is at variance with the large energy splitting predicted for the 4 to 2 states in bothmodels. We, therefore, propose as the most probable scenario that the observed isomer results from the small energy spacing between the 6 and 4 members of the πg 7/2 νf7/2 multiplet. This scheme is favored by the more elaborate KHSM model, and is consistent withthe predicted level spacing, the B(E2) values, and the non-observation of a M.N. Mineva et al.: Anewµs isomer in 16 Sb produced in the projectile fission of 28 U 1 high-spin (6 ) β-decaying state. The observed 17 kev γ- ray would in this case be the 4 to 2 E2 transition, and the low energy 6 to 4 and 2 to 1 transitions are not observed due to electron conversion and/or absorption in the aluminum catcher. Using the measured half-life, we deduce B(E2) = 7.7(7) Wu for a 26 kev (α tot 250) 6 to 4 transition the level spacing predicted by KHSM. This value is in good agreement with the KHSM theoretical prediction of 5.6 Wu (corresponding to a half-life of 780 ns), and is not strongly dependent on the precise energy of the unobserved transition. We have also performed the corresponding calculations for the lowest-lying πg 7/2 νf 7/2 multiplet in 14 Sb, as shown in fig. 5. Included for reference are the experimental level energies from ref. [6]. The agreement between the calculations and experiment is striking, as is the close similarity between the two models. A comparison of the calculated 0 7 level energies in the two even-mass antimony isotopes shows rather different trends. In 14 Sb, the lowest-lying multiplet members are those with the lowest and highest spins, whereas the intermediate-spin levels lie at significantly higher excitation energies. In contrast, the corresponding levels in 16 Sb are predicted (in bothmodels) to have a muchflatter distribution. A very similar behavior is also observed in the 208 Pb region, as exemplified by the corresponding nuclei 210 Bi and 212 Bi [1]. We interpret this flattening out to be a natural consequence of the presence of two additional neutrons. As the occupation of the neutron f 7/2 orbital increases, the πν particle-particle interaction in 14 Sb should change into a particle-hole interaction in 140 Sb. The signature of such a Pandya transformation is a gradual change of the excitation vs. spin distribution shape from a parabola in 14 Sb to an inverted parabola in 140 Sb, via more flat distributions as the one predicted for 16 Sb. Boththe expected gradual onset of collectivity as well as effects related to the lowering of the neutron binding energy could influence this simple shell-model picture. It is therefore of great interest to further explore the evolution of low-lying nuclear structure in the antimony isotopes as a function of the neutron number. 5 Summary γ-rays from an excited isomeric state in the very neutronrichfission fragment 16 Sb have been observed for the first time, and the half-life of the isomer has been determined to be T 1/2 = 565 (50) ns. To interpret the experimental observations, we have performed spherical shell-model calculations using two different sets of interactions. The results indicate that the isomer most likely is the I π =6 member of the πg 7/2 νf7/2 multiplet. However, although 16 Sb has only one proton and three neutrons outside the doubly magic 12 Sn core, the gradual onset of collectivity, as more valence particles are added, could influence the level ordering and spacing. Systematic studies, both experimental and theoretical, of the structure of 16 Sb and its neutron-richneighbors are clearly needed. The authors would like to thank LO Norlin, Stockholm, J. Nyberg, Uppsala, and K. Rykaczewski, Oak Ridge, for making electronic modules available. MN Mineva and M. Hellstr
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