Charged particle spectroscopy using the (t,p) reaction has been employed for more than two decades to study the low-energy structure of nuclei. This reaction has contributed significantly to the elucidation of single-particle and collective phenomena for neutron rich nuclei in virtually every mass region. We have begun to use the (t,p) reaction in conjunctionuclei with in-beam ..gamma..-ray and conversion-electron spectroscopy to bring additional understanding to low-energy nuclear structure. In this report we briefly discuss the experimental considerations in using this reaction for in-beam spectroscopy, and present some results for nuclei with mass near 100

Decman, D. J.

Estep, R. J.

Henry, E. A.

Kantele, J.

Mann, L. G.

Meyer, R. A.

Sheline, R. K.

Stoeffl, W.

Ussery, L. E.

UNT Digital Library

tf*fr-ax2H&--fcb DCRL—93302 DE85 017471 • • * • • • IN-BEAM SPECTROSCOPY USING THE (t,p) REACTION: RECENT RESULTS NEAR A=100 P̂ CIr-iVED QV OliTI E. A. Henry R. J. Estep R. A. Meyer J. Kantele D. J. Decman L. G. Mann R. K. Sheline W. Stoffl L. E. Ussery This paper was prepared for submittal to the Symposium on Recent Advances in the Study of Nuclei Off the Line of Stability, American Chemical Society, Chicago, IL, Sept. 8-13, 1985 SEP 1 7 1 9 8 5 September A, 1985 Tfcji it i arcariat «f • aaatr i a t c M fe, a*kalka la a journal ar anct««ati. Siace •ay kc awac M m aaklkatiaa, lab anarlat It taait itaRakk with the m-imumtlmi that it win M I W d M *r rearaaac«4 without tk« atnaiiaiM «T the aariwr esngmi« « # " * * - 3 In-beam Spectroscopy Using the (t,p) Reaction: Recent Results Near A-100 E. A. Henry, R. J. Estep, R. A, Meyer, J. Kantele, D. J. Decman, L. G. Mann, R. K. Sheline, W. Stffffl, and L. E. Ussery** Lawrence Llvermore National Laboratory, Livermore, CA 94550. 'Department of Chemistry, Florida State University, Tallahassee, FL 32306. TDepartment of Physics, University of JyvUskyla", Jyva'skyla*, Finland. * LOB Alamos National Laboratory, Los Alamos, NM 87545. INTRODUCTION Charged particle spectroscopy using the (t,p) reaction has been employed for more than two decades to study the low-energy etructu: : of nuclei. This reaction has contributed significantly to the elucidation of single-particle and collective phenomena for neutron rich nuclei in virtually every mass region. We have begun to use the (t,p) reaction in conjunction with in-beam Y-ray and conversion-electron spectroscopy to bring additional understanding to low-energy nuclear structure. In this report we briefly discuss the experimental considerations in using this reaction for in-beam spectroscopy, and present some results for nuclei with mass near 100. EXPERIMENTAL METHODS Until now the only methods available for studying the beta unstable nuclei with a mass near 100 were the prompt Y-ray decay and beta decay of fission products, charged-particle spectroscopy using two-neutron transfer reactions, and, to a limited extent, in-beam spectroscopy using reactions like ( 1 80, Oy). In-beam spectroscopy using the (t,py) reaction has several features that make it an attractive technique to complement these methods. 1) Even-even nuclei that have two neutrons more than the heaviest target can be studied by the (t,py) reaction with useful cross sections. 2) The levels in the product nucleus are populated by both direct and compound nuclear reac­tions. Thus the set of levels that are identified at low excitation energies can be quite complete. 3) The spin distribution of levels populated is broader than is usually the case in beta decay. The ground state band is often populated up to the 8 member. In the same experiment 0 states can also be populated, probably by the direct reaction mechanism. 4) The (t,p) reaction has a unique signature, an energetic proton that identifies that particular channel. The disadvantages of this reaction place some real constraints on its use. 1) The <t,p) cross section is only about 5 percent of the total cross section. 2) The dominant reaction, usually (t,2n), produces abundant prompt "Y rays. 3) Reactions such as (t,n) and (t,d) [as well as (t,p)] often result in short-lived beta decaying products. 4) The usual in-beam techniques such as angular distributions are complicated by the necessity to use the outgoing proton to identify the reaction. As a result of the first three disadvan­tages, much of the y-ray and electron count rates are not from the (t,p) reaction and thus experiments of reasonable duration have limited statistics. We have developed Y-ray and conversion-electron spectroscopy techiques that take advantage of the energetic proton as an Indicator of the (t,p) Fsnwm er THIS HBCHSSKT IS aiawrraL ' ill'llB. * (10+)I 8 + ! 5 -2!_y 2 + 0 + ^ O CM in 35 » CNI oq w w S o) 2 K sr .2417.6 2018.1 {8 +) 1397.3 fl^s^ii/-^ : — © • p» *̂ o """* !* in Z; to e> « in El 1244.9 847.1 742.7 7C\ 696.6 t 8 295.7 a 1 0 2 Mo 42 M O60 • A • • A • • A • • A • • A • • A Fig. 1. Level scheme of Mo determined from (t.PYY) studies. Previ­ous studies are beta decay (circles), ( 8 0 , 0y) (squares), and (t,p) triangles. reaction. Our conversion-electron spectrometer and its use with the (t,p) reaction have been described in the literature [DEC84, ST084]. For y-ray spectroscopy, a l-mm thick cylindrical plastic scintillator detects the protons to gate germanium detectors. A thin tapered cylindrical aluminum absorber inserted into the scintillator prevents reaction deuterons and scattered tritons from reaching the scintillator, while allowing energetic protons to do so. The geometric solid angle for the scintillator Is about 30% of 4it. Typically, when the Y-ray singles rate Is 10,000 cps, the py coin­cidence rate is 20-80 cps, and the pyy coincidence rate is 1-5 cps. The y rays associated with the (t,p) reaction are the dominant ones in the spectrum that is gated by the proton detector, with those from the (t,2n) reaction being attenuated by a factor of fifty or more. NUCLEAR STRUCTURE STUDIES NEAR A-100 In 1970 Chelfitz et al. ICHE701 presented experimental evidence of rotation-like nuclear structure for *" Zr and several nearby nuclei. They found that as N«50 or Z-50 closed shells were approached, indicators of defor­mation (e.g. E.+/E2+) varied farther from the rotational limits, but were also significantly different from the values for spherical nuclei. Experimentally, many of the nuclei in this mass region display complex low energy structure. Thus the experimental knowledge of the structure of nuclei surrounding the deformed region near A-100 must be as complete as possible to confidently apply a theoretical description. We report here on preliminary results from four in-beam studies of nuclei in this mass region, and discuss the results briefly in the context of the nuclear structure of the region. 102, Mo. The '18ft 16, ( l o0, Oy) reaction has been used by Koenig et al. [K0E81] istablish the yrast levels in * Mo. Our pyy coincidence in an data attempt to estab —Mconfirm the Wt and 6* levels established In that study (See Fig. 1). However It is clear from our data that the 8t-»-6t transition Is at 691 keV, establish­ing the 8j" level at 2018 keV. The 655-keV y ray, assigned by Koenig et al. as 2 <8*)S< I 3 .8 (31) 2318.5 453 6 + ! —1 687.4 n + " \ - 1 s 8 »»2« . B S cnrr ? ^ 1 « . i l y = - * T^T r- ™ _ • - * CM T". •» IUWW.J 2 + 1 8 & 8 * 3 S 924.0 JK 39( _ _ _ _ 4 / ' 1 ' r» w A 736.8 A 5 348.7 _ _ . tJ 2 + ' " li o+ 8 1 0 t . i 1 12 46 Pd 56 Fie. 2. Level scheme of 1 1 2 P d determined from (t, PYY) studies. Previous studies are beta decay (circles), fission frag­ment decay (squares), and (t,p) triangles. the 8t+6t transition, actually feeds the 4t level directly from a new level at 1397 keV. We suggest that this new level has a J* value of 4 +. Gamma rays with energies of 398 and 401 keV are known from previous studies and are observed as a multiplet In the (t,py) data. These y rays depopulate the 2"jj and ot levels, respectively. However, a gate on these y rays reveals coincidences with all the members of the yrast band, not just the 2T*°T transition as expected. A narrower gate set at about 399 keV reveals weak coincidences with the 8**6p 6i+4*, and tf+2* transitions. Though the statistics are poor, the intensities for these transitions in the coincidence spectrum are consistent with the 399-keV Y ray feeding the St level. We suggest that this transition may be the 10J+8J yrast transition. If this assignment is confirmed, it will be the first experimentally observed backbend in the neutron rich nuclei near A-100. A backbend at the 10* level would agree with the calculation by Tripathi et al. [TRI84] of a pronounced backbend at the 10j" level in 1 0 2 M o . 112 -rtfc Pd. Little has been known experimentally about the level structure of Pd. Previous studies are in agreement only for the J* assignment of the it level at 349 keV [CAS72.CHE70]. We have established five new levels in 1 1 2 P d (see Fig. 2), and have been able to propose J* assignments for some of the I 1 2 P d levels from decay patterns. The coincidence results established a ground state transition depopulating the 736 keV level, indicating that it is a 2 + level. The level at 924 keV has a transition to the 2* level, and a y ray seen in the proton gated singles Is probably the ground state transition, suggesting a J* value of 2 for that level also. Our data confirm that the 4* level occurs at 882 keV, in agreement with the results of the fission fragment decay studies, but that the previous tentative assignment of the 6t level was incorrect. Instead, we establish the 6* level at 1550 keV, and have evidence that suggests that the 8* level occurs at 2319 keV. New levels at 1096, 1362, and 2002 keV are based on the coincidence results. 3 Stachel et al. [STA82] have interpreted the structure of Ru and Pd nuclei in terms of the transition between the S0(5) (vibrational) and 0(6) (y-unstable) limits of the Interacting Boson Model (IBM). The energy ratios EA /E 2 and E 6 + / E 2 are consistent with those of the 0(6) limit for the heavlejr Pd and ̂ Ru Ixiclei. Stachel et al. point out that the energy ratio E 2 / E 2 , is not well reproduced in their calculation. Experimentally^ that Tatio ks closer to the SU(5) limit than the 0(6) limit, even for ^ 1 2Fd. However, if the suggested 2% level at 924 keV in 1 1 2Pd is instead the it level within the model space, the E 2 + / E 2 "tio is just above that of tne 0(6) limit. In 1 1 2 P d the E Q + / E 2 + energy1 ratio is higher than for the lighter Pd nuclei, but still only mldwa^ between the SU(5) and the 0(6) limits. Stachel et al. suggest that the 0 2 level is an intruder level similar to those known in nearby Cd nuclei. Taken together, this evidence indicates that 1 1 2 P d can­not be described by the simple 0(6) limit of IBM alone. Recent calculations show that complete structures resulting from intruder states must be included with a large degree of mixing occurring between the two configurations [KUS85]. M M 0.16 I I I • Mo A Ru • Pd I I 1 0.12 — < i — 0.08 — 0.04 * i • . M I I t I • T 1 1 Fig. 3. Systematics of p 2 1 and X211 n e a r A"*00. Results for ^Mo, from literature data. 1 0 2 R u , and l° 2-106 p d a r e - 0.06 - 0.04 ->T - 0.02 -54 56 58 60 62 64 66 Neutron number 54 56 58 60 62 64 66 Neutron number 4 &#m .sitIons. We have measured EO transitions in the neutron rich nuclei lMo^ "™Ru, and 1 0 8 - i l 2 p < j using the (t,p) reactions and coincidence techniques. Listed in Table 1 are the values for the EO(K) branching ratios from our measurements. Where half-life information and E2 branching^ ratios are available from our own measurements or from the literature, p and X values have been determined (see Fig. 3) . The p|j and X211 values for 1 0 2 M o are larger than in our previous report [DEC83] because of the additional Y~ray recently observed at 399 keV in that nucleus. ] n? Mo has been described as a transitional nucleus which is quite close to being rotational, as its neighbor 1 M M o is. If we use Rasmussen's model [RAS601 for the X value (Xo^ « ^ ) , we find that the deformation parameter B for rjTlo is about 2.3 times that of 1 0 0 M o . This suggests that the oj level in 1 0 Mo might be characterized as the beta-vibrational level in a rotating nucleus. However, while the yrast levels of 1 0 Z M o appear to be evolving toward a rotational character, the lower energy levels might be better charac­terized by a model that mixes vibrational and y-unstable limits of the IBM-1 [SAM82]. From (t,p) charged particle studies it is known that the 0£ level has a substantial two-neutron component, however. This suggests that IBM-2 may be a better model with which to describe * Mo. Fig. A. Levels of 9 6Zr and ' 8Zr from (t.prr) studies. Levels are grouped according to their decay properties. 854 Table 1. E0(K) branch­ing ratios for Ot levels. •.el. ^ 0 0 . ( x l 0 6 ) total l°,°,Mo 71200 ± 3600 JJJfHo 4780 +, 780 1 0 6 R u 30 + 12 0 + 0 0 + 0 1 0 8 p d 4 3 + 1 9 'Pd 122 ± 29 ZPd 126 + 57 "Zr *»Zr " > 1? 2± H 40 56 40 58 5 The Measured pSj and X,il values for 1 U 6 R u and W°-U*?i a r e quite small. This would be consistent with the suggestion that these nuclei are evolving from the SU(5) and the 0(6) limit of the IBM as neutrons are added [STA82], since the O^+O* transition is forbidden in both limits. As pointed out earlier however, Stachel et al. suggest that the d£ levels in Fd nuclei may be due to intruder states since their energies are much to low compared to a calculation using the IBM. It may be that the somewhat higher vlaues of p|j and X 2 l l for 1 1 2 F d reflect this influence. Zr and 8Zr. The preliminary results of our studies for the levels in 9 6Zr and Zr are shown in Fig. 4. The levels for each nucleus are segregated into two groups. Levels within a group y decay with their largest reduced transi­tion rates to other levels within the group. For 9 8Zr, the strongest EO tran­sitions are between the two groups, rather within them, indicating possible shape differences between the 0 + levels in the two groups. In '"Zr, the (4 ) and (5**) assignments indicated are not definite, but these levels seem to be the lowest that could have these J* assignments. For some levels the group preference based on the Y decay criterion is modest. However, on the whole, a coherent pattern is developed. Bengtsson et al. [BEN84] show, that there is extra stability toward a spherical shape in the ' Zr ground state brought on by the 2d 5y 2 neutron subshell closure. Focusing on the group that includes the ground state in each nucleus, we see the repeated pattern of levels with j" of 2 , 3~, and, possibly, 4 and 5" at energies which are reasonably close in the two nuclei. The comparison in Fig. 5 suggests that, Insofar as excited states in Zr are concerned, there may be some additional stability toward spherical shapes afforded by filling the 3s, /» neutron orbital. The closeness of the energies of the two ground state groups should not be taken too seriously. The two accompanying "bands" must undoubtedly mix with the respective ground state groups to bring them closer together in energy. Nevertheless, this comparison graphically demonstrates the complex interplay between possible models for nuclear structure in this region. A portion of this work is part of the doctoral thesis of one of the authors (RJE). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. REFERENCES BEN84 R. Bengtsson, et al., Fhys. Scr. _24_, 402 (1984). CAS72 R. F. Casten, et al., Nucl. Phys. A184, 357 (1972). CHE70 E. Cheifitz, et al., Fhys. Rev. Lett. _2£, 38 (1970). DEC83 D. J. Decman, et al., Nuclear Chemistry Division Annual Report, FY83, Lawrence Livermore National Laboratory, Livermore, CA, UCAR 100062-83/1 pg 181 (1983). DEC84 D. J. Decman, et al., Nucl. Inst. Meth. 219, 523 (1984). K0E81 J. Koenig, et al., Fhys. Rev. C24, 2076 (1981). KUS85 D. F. Kusnezov and R. A. Meyer, Nuclear Chemistry Annual Report, FY85, Lawrence Livermore National Laboratory, Livermore, CA, UCAR 10062-85/1 (1985). RAS60 J. 0. Rasmussen, Nucl. Phys. _19_, 85 (1960). SAM82 K. Sambataro and G. Molnar, Nucl. Phys. A376, 201 (1962). ST084 W. Stffffl and E. A. Henry, Nucl. Inst. Meth. 227, 77 (1984). TRI84 P. Tripathi, S. Sharma, and S. Khosa, Phys. Rev. C29, 1951 (1984). 6 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use­fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe­cific commercial product, process, or service by trade name, trademark, manufac­turer, or otherwise does not necessarily constitute or imply its endorsement, recom­mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 

In-beam Spectroscopy Using the (t,p) Reaction: Recent Results Near A = 100

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In-beam spectroscopy using the (t,p) reaction: recent results near A = 100

In-beam spectroscopy using the (t,p) reaction: recent results near A = 100

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