28 In-Beam Spectroscopy Using the (t,p) Reaction Recent Results near A = 100 1
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E. A. Henry , R. J. Estep , Richard A. Meyer , J. Kantele, D. J. Decman, L. G. Mann , R. K. Sheline, W. Stöffl , and L. E. Ussery 2
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Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550 Department of Chemistry, Florida State University, Tallahassee, FL 32306 Department of Physics, University of Jyväskylä, SF-40100, Jyväskylä, Finland Los Alamos National Laboratory, Los Alamos, NM 87545
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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 conjunction with in-beam γ-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 γ-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 ( O, Ογ). In-beam spectroscopy using the (t,pγ) 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 (ί,ργ) reaction with useful cross sections. 2) The levels in the product nucleus are populated by both direct and compound nuclear reac tions. 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. 18
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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 γ 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 disadvan tages, much of the γ-ray and electron count rates are not from the (t,p) reaction and thus experiments of reasonable duration have limited statistics. We have developed γ-ray and conversion-electron spectroscopy techiques that take advantage of the energetic proton as an indicator of the (t,p) 0097-6156/86/0324-0190$06.00/0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
28. HENRY ET AL.
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F i g . 1. Level scheme of Mo determined from (t,ργγ) s t u d i e s . P r e v i ous studies are beta decay ( c i r c l e s ) , ( 0 , 0γ) (squares), and (t,p) t r i a n g l e s .
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reaction. Our conversion-electron spectrometer and i t s use with the (t,p) r e a c t i o n have been described i n the l i t e r a t u r e [DEC84, ST084]. For γ-ray spectroscopy, a 1-mm t h i c k c y l i n d r i c a l p l a s t i c s c i n t i l l a t o r detects the protons to gate germanium d e t e c t o r s . A t h i n tapered c y l i n d r i c a l aluminum absorber i n s e r t e d i n t o the s c i n t i l l a t o r prevents r e a c t i o n deuterons and scattered t r i t o n s from reaching the s c i n t i l l a t o r , while allowing energetic protons to do so. The geometric s o l i d angle f o r the s c i n t i l l a t o r i s about 30% of 4π. T y p i c a l l y , when the γ-ray s i n g l e s rate i s 10,000 cps, the ργ c o i n cidence rate i s 20-80 cps, and the ργγ coincidence rate i s 1-5 cps. The γ rays associated with the ( t , p ) r e a c t i o n are the dominant ones i n the spectrum that i s gated by the proton detector, with those from the (t,2n) r e a c t i o n being attenuated by a f a c t o r of f i f t y or more. NUCLEAR STRUCTURE STUDIES NEAR A=100 In 1970 C h e i f i t z et a l . [CHE701 presented experimental evidence of r o t a t i o n - l i k e nuclear structure f o r Z r and several nearby n u c l e i . They found that as N=50 or Z=50 closed s h e l l s were approached, i n d i c a t o r s of defor mation (e.g. E/+/E2+) varied f a r t h e r from the r o t a t i o n a l l i m i t s , but were a l s o s i g n i f i c a n t l y d i f f e r e n t from the values f o r s p h e r i c a l n u c l e i . Experimentally, many of the n u c l e i i n t h i s mass region d i s p l a y complex low energy s t r u c t u r e . Thus the experimental knowledge of the s t r u c t u r e of n u c l e i surrounding the deformed region near A=100 must be as complete as p o s s i b l e to c o n f i d e n t l y apply a t h e o r e t i c a l d e s c r i p t i o n . We report here on preliminary r e s u l t s from four in-beam studies of n u c l e i i n t h i s mass region, and discuss the r e s u l t s b r i e f l y i n the context of the nuclear structure of the region. 2
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Mo. The ( 0 , 0 γ ) r e a c t i o n has been used by Koenig et a l . [K0E81] i n an attempt to e s t a b l i s h the yrast l e v e l s i n Mo. Our ργγ coincidence data confirm the 4* and 6* l e v e l s established i n that study (See F i g . 1). However, i t i s c l e a r from our data that the 8t>6"J" t r a n s i t i o n i s at 691 keV, e s t a b l i s h ing the 8* l e v e l at 2018 keV. The 655-keV γ ray, assigned by Koenig et a l . as
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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the e^^oj t r a n s i t i o n , a c t u a l l y feeds the 4Ϊ l e v e l d i r e c t l y from a new l e v e l at 1397 keV. We suggest that t h i s new l e v e l has a J value of 4 . 1 7
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Gamma rays with energies of 398 and 401 keV are known from previous studies and are observed as a m u l t i p l e t i n the (ί,ργ) data. These γ rays depopulate the 2"£ and 0* l e v e l s , r e s p e c t i v e l y . However, a gate on these γ rays reveals coincidences with a l l the members of the yrast band, not just the 2"}+0"£ t r a n s i t i o n as expected. A narrower gate set at about 399 keV reveals weak coincidences with the 8"J"-»-6~J\ 6"j^4"j\ and 4*-»·2* t r a n s i t i o n s . Though the s t a t i s t i c s are poor, the i n t e n s i t i e s f o r these t r a n s i t i o n s i n the coincidence spectrum are c o n s i s t e n t with the 399-keV γ ray feeding the 8* l e v e l . We suggest that t h i s t r a n s i t i o n may be the 10*+8"[ yrast t r a n s i t i o n . If this assignment i s confirmed, i t w i l l be the f i r s t experimentally observed backbend i n the neutron r i c h n u c l e i near A=100. A backbend at the 10~|" l e v e l would agree with the c a l c u l a t i o n by T r i p a t h i et a l . [TRI84] of a pronounced backbend at the 10+ l e v e l i n M o . 102
112 Pd. L i t t l e has been known experimentally about the l e v e l s t r u c t u r e of Pd. Previous studies are i n agreement only f o r the J assignment of the 2^ l e v e l at 349 keV [CAS72,CHE70]. We have e s t a b l i s h e d f i v e new l e v e l s i n Pd see F i g . 2 ) , and have been able to propose J assignments f o r some of the Pd l e v e l s from decay patterns. The coincidence r e s u l t s e s t a b l i s h e d a ground s t a t e t r a n s i t i o n depopulating the 736 keV l e v e l , i n d i c a t i n g that i t i s a 2 level. The l e v e l at 924 keV has a t r a n s i t i o n to the 2+ l e v e l , and a γ ray seen i n the proton gated s i n g l e s i s probably the ground s t a t e t r a n s i t i o n , suggesting a J value of 2 f o r that l e v e l a l s o . Our data confirm that the 4+ l e v e l occurs at 882 keV, i n agreement with the r e s u l t s of the f i s s i o n fragment decay s t u d i e s , but that the previous t e n t a t i v e assignment of the 6+ l e v e l was incorrect. Instead, we e s t a b l i s h the 6+ l e v e l at 1550 keV, and have evidence that suggests that the 8+ l e v e l occurs at 2319 keV. New l e v e l s at 1096, 1362, and 2002 keV are based on the coincidence r e s u l t s . 7 1
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Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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S t a c h e l et a l . [STA82] have i n t e r p r e t e d the s t r u c t u r e of Ru and Pd n u c l e i i n t e r m s o f the t r a n s i t i o n between t h e SU(5) ( v i b r a t i o n a l ) and 0(6) (γunstable) l i m i t s of the I n t e r a c t i n g Boson Model ( I B M ) . The e n e r g y r a t i o s 4 / 2 6 / 2 s i s t e n t w i t h t h o s e of the 0 ( 6 ) l i m i t f o r t h e h e i v i e i Pd and *Ru \ i u c l e i . S t a c h e l e t a l . p o i n t out t h a t the e n e r g y r a t i o E2 /E2 i s not w e l l r e p r o d u c e d i n t h e i r c a l c u l a t i o n . Experimentally, that r a i i o hs c l o s e r t o the SU(5) l i m i t t h a n the 0(6) l i m i t , even f o r Pd. However, i f the s u g g e s t e d 2^ l e v e l a t 924 keV i n Pd i s i n s t e a d the 2"£ l e v e l w i t h i n the model s p a c e , the E 2 / E r a t i o i s j u s t above t h a t of the 0(6) limit. In Pd the E Q / E energy r a t i o i s h i g h e r t h a n f o r the l i g h t e r Pd n u c l e i , but s t i l l o n l y mldwaV between t h e SU(5) and the 0(6) l i m i t s . Stachel e t a l . s u g g e s t t h a t the 0 l e v e l i s an i n t r u d e r l e v e l s i m i l a r t o t h o s e known i n n e a r b y Cd n u c l e i . Taken t o g e t h e r , t h i s e v i d e n c e i n d i c a t e s t h a t Pd c a n not be d e s c r i b e d by the s i m p l e 0 ( 6 ) l i m i t of IBM a l o n e . Recent c a l c u l a t i o n s show t h a t c o m p l e t e s t r u c t u r e s r e s u l t i n g f r o m i n t r u d e r s t a t e s must be i n c l u d e d w i t h a l a r g e degree of m i x i n g o c c u r r i n g between the two configurations [KUS85]· E
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3. S y s t e m a t i c s of p j n e a r A=100. R e s u l t s 102-106 Pd Mo, Ru, and from l i t e r a t u r e d a t a .
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Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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su red EO t r a n s i t i o n s i n the neutron r i c h n u c l e i We have 'Mo, R u , and ~ ^ P d using the (t,p) reactions and coincidence techniques. L i s t e d i n Table 1 are the values f o r the EO(K) branching r a t i o s from our measurements. Where h a l f - l i f e information and E2 branching: r a t i o s are a v a i l a b l e from our own measurements or from the l i t e r a t u r e , ρ and 1X values have been determined (see F i g . 3 ) . The P J and X values f o r M
