This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Anderson, B.D.

Baldwin, A.R.

Foster, C.C.

Lamm, D.

Madey, R.

Manley, D.M.

Wang, Y.

Watson, J.W.

Wetmore, B.

Zhang, W.M.

English

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FRAGMENTATION OF HIGH-SPIN STRETCHED STATES 
IN THE (p,n) REACTION ON 36Ar AND 40Ca 
B.D. Anderson, B. Wetmore, A.R. Baldwin, D. Lamm, R. Madey, 
D.M. Manley, J.W. Watson, and W.M. Zhang 
Phys i c s  D e p a r t m e n t ,  K e n t  S t a t e  Univers i ty ,  K e n t ,  Oh io  4 4 2 4 2  
C.C. Foster and Y. Wang 
I n d i a n a  Un ive r s i t y  Cyc lo t ron  Facili ty,  B looming ton ,  I n d i a n a  474 08 
High-spin "stret ched-states" have been the object of many experimental and theoret- 
ical studies over the last decade. A "stretched-state" is one with a particle-hole configura- 
tion where the j's of the particle and hole are coupled to the maximum possible j-value. If 
both the particle and hole orbitals involve the largest j-values in their respective shells, then 
the state formed is the "maximum" stretched state. Because the j-value is the maximum 
possible involving a simple particle-hole excitation between these two shells, the st ate is 
unique within 2fiw of excitation. Reduction and spreading of this strength can arise from 
configuration mixing with more complicated configurations. These stretched states have 
been studied in (p,pl), (e,e1) and (p,n) reactions,'j2 and also in transfer  reaction^.^ 
One of the most studied stretched configurations is the (f712 ,d~;~)6- state observed 
with s-d shell nuclei targets. This 6- state has been observed in 24Mg and 28Si in both 
inelastic proton and electron ~ c a t t e r i n g . ~ > ~ j ~  The strength is observed to be predominantly 
concentrated in a single state; however, two problems arose concerning the 6- strength 
in s-d shell nuclei. The first problem was that only about 113 of the simple shell-model 
strength was observed in the reactions on 2 4 ~ g  and 28Si. The second problem was that 
this 6- strength seemed to disappear for s-d shell nuclei heavier than 28Si. Both of these 
problems have been addressed theoretically and experimentally. 
Large-basis shell-model calculations have been performed by Carr, e t  ~ l . , ~  which ap- 
pear to describe the reduction of 6- strength for 28Si. The reduction in strength comes 
from mixing with the more complicated configurations, even though this reduction is much 
larger than one expected from such effects. The "missing" 6- strength for heavier s-d shell 
nuclei is due to fragmentation of this strength in these nuclei. Earlier (p,n) experiments 
on 32S and 40Ca were able to see this fragmented This fragmentation appears 
as the d5/2 hole orbital is pushed down from the Fermi surface and the 2sl12 and d3/2 
orbitals are filled above it. The large-basis shell model calculations predict at least some 
of this fragment ation. 
The question is now to determine just how good are such large-basis shell-model 
calculations. The agreement with experiment for A = 28 where there is one predominant 
state is very impressive. In order to test these calculations for heavier s-d shell nuclei, we 
performed new (p,n) measurements on the self-conjugate nuclei 36Ar and 40 Ca. 
The experiment was performed using the beam-swinger system at the IUCF in January 
1993. The experiment used the "stripper loop" storage ring to achieve ~2 ps between beam 
bursts. This long time between beam bursts eliminated overlap background from earlier 
beam bursts and also greatly reduced backgrounds from cosmic rays (because the system 
is open for a smaller fraction of the total time). The net result was an improved signal-to- 
background ratio which is important in looking for the weak parts of the fragmented 6- 
strength. The flight paths to the large-volume, mean-timed neutron detectors was 128 m. 
We obtained an overall time resolution of 750 ps, which translates into an energy resolution 
of 260 keV. The 36Ar(p,n) data were obtained using a high pressure (6 Torr), 4 cm long gas 
cell with 0.5 mil Kapton windows. (Empty cell runs were used for background subtraction.) 
We see clearly fragmented 6- strength in the (p,n) reaction on 36 Ar and 40 Ca. The 
data are still in the preliminary stages of analysis. Comparisons will be made with large 
basis shell model calculations. lo 
1. R.A. Lindgren, et al., in Nuclear Structure at High Energies, Excitation and Mornen- 
turn Transfer, ed. H. Nann (AIP Conf. Proc. 142, New York, 1988) p. 133. 
2. B.D. Anderson, et al., ibid. p. 155. 
3. R.J. Peterson, et al., Phys. Rev. C 33, 31 (1986). 
4. G.S. Adams, et al., Phys. Rev. Lett. 38, 1387 (1977). 
5. C. Olmer, et al., Phys. Rev. C 29, 361 (1984). 
6. H. Zarek, et al., Phys. Rev. Lett. 38, 750 (1977). 
7. J.A. Carr, et al., Phys. Rev. C 45, 1145 (1992). 
8. N. Tamimi, et al., Phys. Rev. C 45, 1005 (1992). 
9. T. Chittrakarn, et al., Phys. Rev. C 34, 80 (1986). 
10. J .A. Carr, private communication. 


Fragmentation of High-spin Stretched States in the (p,n) Reaction on 36-Ar and 40-Ca

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Fragmentation of High-spin Stretched States in the (p,n) Reaction on 36-Ar and 40-Ca

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