51 Heavy-Ion-Induced Transfer Reactions A Spectroscopic Tool for High-Spin States P. D. Bond
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Brookhaven National Laboratory, Upton, NY 11973
One of the early hopes of heavy ion induced transfer reactions was that new states in nuclei would be preferentially populated. The fact that this hope diminished was due primarily to insufficient understanding of the reaction mechanism, but also to the generally poorer energy resolution ob tained with heavy ions as compared to light ions. In this paper, I hope to demonstrate that with the proper kinematical conditions there is a remarkable selectivity which can be obtained with a proper choice of the reaction and that these reactions can be valuable spectroscopic tools. The data in this talk have been taken using beams from the Brookhaven National Laboratory double MP tandem facility with particles identified in the focal plane of a QDDD spectrometer. Shown in Fig. 1 are spectra for three single neutron transfer reac tions leading to known states in the same final nucleus Sm. The ( C, C) in reaction in Fig. 1 populates final states much as the (d,p) reaction and because of the worse energy resolution this heavy ion reaction is thus not very useful for spectroscopic purposes. The ( C, C) and ( 0, 0) reactions on the other hand show a very strong selectivity for high spin states with the latter reaction also showing a strong preference for j»£,+l/2 final states. The reasons for this selectivity are discussed else where [B0N83], the purpose here is to use this selectivity for spectroscopic studies. Shown in Fig. 2 is a schematic representation of the shell model states for the region near * Gd. There has been a great deal of interest in this mass region since it was proposed [OGA78] that a shell closure occurs for Z-64. 2ηο As in the *"°Pb region high spin single particle states are available so that simple configurations are likely to con tribute to the structure of high spin states. In particular, near *^ Gd the proton hjj/2 orbital and the neutron i / 2 orbital should play an important role, however, there is little direct evidence about the states based on these orbitals. For the Nd isotopes considered Fig. 1. Single neutron transfer here the protons have partially filled reaction for Sm the 87/2*^5/2 levels and the neutrons * Sm for three dif are beginning to f i l l orbitals above ferent projectiles. N-82. Reproduced with permission from [B0N83]. Copyright 1983 Gordon & Breach Science Publishers. 149
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0097-6156/ 86/ 0324-0335S06.00/ 0 © 1986 American Chemical Society Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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NUCLEI OFF THE LINE OF STABILITY
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KVI 3146
Fig.
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Schematic r e p r e s e n t a t i o n
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of s h e l L model o r b i t a l s near
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* Gd.
We f i r s t focus on the p r e v i o u s l y unknown i / neutron strength i n the N=84 nucleus ^ S ï d . An e a r l i e r (d,p) study [RAM76] observed no 1χ3/2 strength except i n the low l y i n g 3" s t a t e . Gamma ray experiments f o l l o w i n g compound nucleus formation [BER76, GEE76, QUA82] found s e v e r a l negative p a r i t y states but a l l with odd spin ( n a t u r a l p a r i t y ) . The f i n d i n g s are con s i s t e n t with the supposition that the negative p a r i t y states are formed by a 3~ core e x c i t a t i o n coupled to two f / 2 neutrons producing spins of 3~, 5~, 7~, 9~. S i n g l e nucléon t r a n s f e r would only weakly populate these states through m u l t i s t e p processes. I f , on the other hand, these negative p a r i t y states were due p r i m a r i l y to v f / * ^ 1 3 / 2 c o n f i g u r a t i o n s the ordering of the n a t u r a l p a r i t y s t a t e s would be the same but t r a n s f e r to these states should be strong. In a d d i t i o n , the unnatural p a r i t y states ( J * 4~,6",8", 10") of t h i s m u l t i p l e t should a l s o be seen with the strongest state i n the t r a n s f e r spectrum being the unnatural p a r i t y s t a t e J ^ I O " . This state would not be e a s i l y seen i n the xn experiments because i t i s not y r a s t . The s e l e c t i v i t y shown i n F i g . 1 demonstrates the ( 0 , 0 ) t r a n s f e r s t r o n g l y favors t r a n s f e r to states of f 7 / 2 i-13/ 2 character. In bom bardment of a * Nd t a r g e t , which has J «7/2~, the states i n *Nd which should be populated are therefore ( vf / ) 0 , 2 , 4 , 6 and v f / * v i / 2 = 3~,...,10~. In F i g . 3 the spectrum of the * Nd( 0 , 0 ) *Nd r e a c t i o n [COL85] i s shown with the l o c a t i o n of known states i n d i c a t e d on the f i g u r e . For e x c i t a t i o n energies up to about 1.5 MeV there i s no ambiguity i n assign ments since other states are not present. However, above that e x c i t a t i o n energy the r e s o l u t i o n of roughly 100 keV i s not s u f f i c i e n t to uniquely iden t i f y the s t a t e s . In order to help determine the spin and e x c i t a t i o n of the l e v e l s , gamma rays i n coincidence with the p a r t i c l e peaks have been measured with an i n t r i n s i c Ge detector. 1
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Shown i n F i g . 4 are two gamma ray spectra [C0L85] one f o r the peak l a b e l e d 9" i n F i g . 3 and one for the p a r t i c l e peak at about 3.8 MeV, which was p r e v i o u s l y unknown. The upper gamma ray spectrum i n F i g . 4 confirms that the peak at 2.902 MeV i s indeed the 9~ state seen i n previous work [BER76, GEE76, QUA82]. The p a r t i c l e peak at 3.8 MeV shows a nearly i d e n t i c a l gamma ray spectrum with the a d d i t i o n of one gamma ray o f 900 keV, j u s t the d i f f e r ence i n energy between the 9" state and t h i s s t a t e . Since t h i s peak i s so
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
51.
Heavy-Ion-Induced Transfer Reactions
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Ο» ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι 4 3 2 EXCITATION ENERGY ( M e V ) F i g . 3.
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Spectrum of the
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Nd( 0 , 0 ) ^ N d r e a c t i o n .
Reproduced with permission from [BON83]. & Breach Science P u b l i s h e r s .
Copyright 1983 Gordon
s t r o n g l y populated i n p a r t i c l e t r a n s f e r and decays p r i m a r i l y to the 9" state i t i s almost c e r t a i n l y the p r e v i o u s l y unobserved 10" s t a t e . Gamma rays i n coincidence with the other labeled peaks i n the spectrum confirm they are indeed the s t a t e s i n d i c a t e d on F i g . 3.
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Gamma rays i n coincidence with a) the state at 2.9 MeV and b) the s t a t e at 3.8 MeV.
Reproduced with permission from [BCN83]. & Breach Science P u b l i s h e r s .
Copyright 1983 Gordon
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
338
NUCLEI OFF THE LINE OF STABILITY 11+l
The l e v e l scheme of states i n *Nd seen i n t h i s experiment i s shown i n F i g . 5. The s t r u c t u r e of the negative p a r i t y states i s as f o l l o w s . The low l y i n g 3" and 5~ states appear to be rather complicated as t h e i r population does not follow the (2J+1) population expected for a simple m u l t i p l e t . In c o n t r a s t , the 7~, 9" and 10" appear to have equivalent f / 2 * *l3/2 strength and thus have rather simple s t r u c t u r e . On the ^other hand, no e v i dence i s found f o r the 4~ or 8" states and only weak evidence that the s t a t e at 3.3 MeV i s a 6~ s t a t e . Configuration mixing i s a p o s s i b l e , though not a c e r t a i n , reason for the absence of these s t a t e s . In any case, there does not appear to be a simple v f / * v / multiplet. For the p o s i t i v e p a r i t y s t a t e s , the population of the 0 • 6 states follows (2J+1) which i n d i c a t e s that these states are consistent with being an ( f / ) m u l t i p l e t . As can be seen i n F i g . 1, the spectrum of the ( C C) r e a c t i o n leading to *Nd enhances h / t r a n s f e r . Comparison of the ( 0 , 0 ) and ( C , C ) spectra c l e a r l y shows that the 8+ state at 2.709 MeV i s p r i m a r i l y an f / * h / c o n f i g u r a t i o n . Unfortunately, there are no extensive s h e l l model c a l c u l a t i o n s with which to compare these r e s u l t s . We turn now to the proton l e v e l s i n the Ν-82 nucleus Nd. S i m i l a r spectra to those of F i g * 1 f o r proton t r a n s f e r demonstrate that the ( 0 , N ) r e a c t i o n s t r o n g l y enhances t r a n s f e r to d / and h n / 2 proton o r b i t a l s . ι ι Since the ground state of ** Ϋτ i s 5/2 the ( 0 , N ) r e a c t i o n on that nu cleus s t r o n g l y enhances f i n a l states i n * Nd of ( i f d / ) - 0 + , 2+, 4+ and i r d / * *hn/ 2 • 3". ...8". The s i n g l e proton t r a n s f e r spectrum [B0N85] to Nd i s shown i n F i g . 6. Gamma rays i n coincidence with these p a r t i c l e peaks have been measured and lead to the l e v e l scheme shown i n F i g . 5. 7
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