SPIN FLIP STRENGTH AT HIGH EXCITATION ENERGY IN THE 90Zr([MATH]) 90Zr REACTION

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SPIN FLIP STRENGTH AT HIGH EXCITATION ENERGY IN THE 90Zr([MATH]) 90Zr REACTION

S. Nanda, C. Glashausser, K. Jones, J. Mcgill, T. Carey, J.B. Mcclelland, J.

Moss, S. Seestrom-Morris, J. Comfort, S. Levenson, et al.

To cite this version:

S. Nanda, C. Glashausser, K. Jones, J. Mcgill, T. Carey, et al.. SPIN FLIP STRENGTH AT HIGH

EXCITATION ENERGY IN THE 90Zr([MATH]) 90Zr REACTION. Journal de Physique Colloques,

1984, 45 (C4), pp.C4-459-C4-463. �10.1051/jphyscol:1984435�. �jpa-00224101�

JOURNAL DE PHYSIQUE

Colloque C4, suppl6ment au n03, Tome 45, mars 1984 page C4-459

90 + + 90

SPIN FLIP STRENGTH AT HIGH EXCITATION ENERGY IN THE

Zr(p,p') Zr

REACT

1

ON

S .K. Nanda, C . G l a s h a u s s e r , K.W.

l ones^,

J . S . McGill T . A . Carey*,

J . B . McClelland*, J .M. Moss*

,

S . J . ~ e e s t r o m - M o r r i s t f

*,

^{J . R .} Comfort**,

S . Levenson***, R. segel*** and H . Ohnuma****

R u t g e r s U n i v e r s i t y , New Brunswick, NJ 08903, U . S.A.

*Los A Z m o s N a t i o n a l Laboratory, Los AZamos, NM 87545, U.S.A.

* * ~ r i z o n a S t a t e U n i v e r s i t y , Tempe, AZ 85287, U.S.A.

***iVorthwestern U n i v e r s i t y , Evanston, I L 60201, U.S.A.

* * * * ~ o k ~ o I n s t i t u t e o f Technology, Tokyo, Japan

R6sum6

-

Des mesures de la section efficace, du pouvoir d'analyse, et de la probabilitg du renversement du spin sont en accord avec 131 pour le pic 5 9,O MeV dans la rgaction

9 0 Zr(p,p')90Zr 2 319 MeV, mais elles ne sont pas dgfinitives.

Des mesures de S montrent une section efficace d'excitation du spin qui est BGportante de 8

2

25 MeV.

Abstract - Ileasurements of the cross section, analyzing power, and spin-flip probability are consistent with an 1111 assignment for the 9.0 MeV peak in the g 0 ~ r ( p , p ' ) 9 0 Z r reaction at 319 MeV, but they are not definitive. The spin-flip measurements reveal a large spin-excitation cross section from 8 to 25 MeV ex- citation.

Measurements oE cross section angular distributions for 200 MeV pro- ton scattering on " ~ r have revealed a 2 MeV wide peak at about 9 MeV excitation which apoears to exhaust about thirky per cent of the ex- pected M1 strength-I The structure of the M1 region seen in electron scattering is rather different, ang the strength is less than about 15% of the pure shell model value. At higher excitation energies, only natural parity states have been observed; these natural parity resonances are not expected to involve spin transfer (AS=l). However, a number of calculations indicate that spin excitations should be ob- served also at excitation energies above 10 M ~ v . ~

Fig. 1 Spectrum

;:

^the

zr(p,p1) " ~ r reaction at 319 MeV at a labora- tory scattering angle of 3.5O.

E. (MeV)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984435

C4-460 JOURNAL

DE PHYSIQUE

Since cross section (o) measurements are generally not sufficient to distinguish between AS=O and AS=l excitations, we have measured also the analyzing power ( A ) and the spin-flip probability ( S ) in the

zr (p ,p

'

) zr2 reactign at 319 MeV. Like back-angle elegpron

scattering cross sections4 the spin-flip cross section as is a mea- sure of AS=1 excitations. Excitation of an M1 state, foFnexamPle, is expected to yield a large value of Snn at small momentum transfer q where distortions are not very important. Our data are concen- trated at small q to selectively populate low spin states.

The data were taken at High Resolution Spectrometer (HXS) at the Clinton P. Anderson Physics Facility (LAMPF) with a beam of 319 MeV protons polarized perpendicular to the reaction plane (n). Cross section and A measurements were carried out with the standard BRS system modifigd -Lo minimize the instrumental background inherent in small angle inelastic scattering. The polarization of the scattered protons was measured with the HRS focal plane polarimeter; this con- sis-ts of a 7.5 cm thick carbon analyzer followed by a system of eight multi-wire drift chambers and an array of scintillators to recon- struct trajectories of the scattered protons. The efficiency of the polarimeter was 10 to 15% over an excitation energy range of 11 MeV.

These measurements required a thick " ~ r target (250 mg/cm2), so the energy resolution was 180 keV and the instrumental bacl-,ground was decidely worse than that achieved in our pure cross section measure- ments.5 A spectrum at 3.5' is shown in Fig. 1 The peak at about

9 MeV corresponds to 'chat observed previously;i some structure is apparent even with the poor resolution. This structure has been studied further in cr measurements v~it1-1 60 keV r e s ~ l u t i o n . ~ Tlze large peak at about 16 MeV corresponds predominantly to excitation of the giant dipole resonance.

Pigs. 2 and 3. Cross section and analyzing power angular distri- butions, respectively, for the entire 9.0 MeV peak in the

9 0 ~ r ( p , p ' ) 9 0 ~ r reaction at 319 MeV after background subtraction. The solid curves represent DITIA calculations described in the text.

10.0\ I I J

9 0 ~ r ( p , ~ ~ ) ~ ~ r

-

T, =319MeV 0.0

L

b ~ r (a,p'd)90~r

- -

Tp = 319MeV

ln

-

Total "Mt" peak

\

-

Total "MI" peak

Y ^OWIA

-

E

-

^{O W I A}

-

^0.4

-

-

Z AY

-

p

1.0: 0.0 --

I- 1

B : ^-

cn - -

_-0.4-

-

cn -

cn -

0 -

a

U -0.8

- -

0.1 ^{t I I I}

0.0 2.0 4.0 6.0 8.0 10.0 12.0 0.0 2.0 4.0 6.0 8.0 I2.O

ern

(deg) 0, (deg)

) 0.0 10.0 20.0

E x (MeV)

F i g s . 4 and 5 . S p i n - f l i p p r o b a b i l i t y S and

sgin-flip

c r o s s s e c t i o n oS a t 3.5' and 5 . 0 ° , r e s p e c t i v e l y , f o F n t h e Z r ( p , p 1 )

" z r

r e a c t i o n atnn319 MeV, b e f o r e baclrground s u b t r a c t i o n .

C r o s s s e c t i o n s and a n a l y z i n g powers f o r t h e e n t i r e peak a t 9 MeV a r e shown i n F i g s . 2 and 3 ; a smooth background was s u b t r a c t e d . The c u r v e s r e p r e s n t DWIA c a l c u l a t i o n s w i t h t h e Love-Franey i n t e r a c t i o n f o r a ( v g

-7 ,

g 7 2)1-t c o n f i g u r a t i o n ; o p t i c a l p a r a m e t e r s were e x t r a - p o l a t e d £86; n e a r b d e n e r g i e s . The c r o s s s e c t i o n i s a b o u t 15% l a r g e r t h a n t h a t measured a t Orsay a t 200

M ~ v . ~

The s p e c t r a of S and oSnn a t 3.5O and 5.0' a r e shown i n F i g s . 4 and 5 w i t h o u t backgr8nnd s u b t r a c t i o n . A s e x p e c t e d , s p i n - f l i p c r o s s s e c - t i o n s a r e s m a l l a t low e x c i t a t i o n e n e r g y i n t h e r e g i o n o f i s o l a t e d s t a t e s ; t h e y r i s e t o s i g n i f i c a n t v a l u e s around 8 MeV. Compared t o t h e h i g h e r e x c i t a t i o n e n e r g y r e g i o n , however, t h e r e g i o n around 9 EeV i s no way r e m a r k a b l e . R a t h e r , s p i n e x c i t a t i o n s a r e o b s e r v e d up t o a t l e a s t 25 MeV wliere o n l y n a t u r a l p a r i t y s t a t e s have p r e v i o u s l y been s e e n . I f a s i m p l e background i s drawn i n t h e 9 MeV r e g i o n , t h e n a v a l u e o f 0.6210.20 i s o b t a i n e d f o r S a t 3.5' f o r t h e e n t i r e 9.0 MeV pealr; b e c a u s e of t h e u n c e r t a i n t y i n n p h e background, t h e s y s t e m a t i c e r r o r i n t h i s v a l u e i s l a r g e . ~t 5O, t h e c o r r e s p o n d i n g v a l u e i s 0.20?0.08.

I n o r d e r t o i n t e r p r e t t h e s e r e s u l t s more p r e c i s e l y , we have measured t h e c o r r e s p o n d i n g q u a n t i t i e s a l s o f o r t h e well-known 10.23 MeV 1+

s t a t e i n T h i s i s a n i s o l a t e d s t a t e , p r e d o m i n a n t l y a p u r e neu- t r o n ( v f

-

f )

+

t r a n s i t i o n , and it p r o v i d e s a good t e s t o f t h e DWIA d e s & ? p t i o n 5 ' z f 1 t h e r e a c t i o n mechanism. The r e s u l t s a r e shown i n F i g . 6 . The s h a p e s o f t h e o and A a n g u l a r d i s t r i b u t i o n s a r e w e l l - d e s c r i b e d by t h e DWIA and c o n s i s t e n t &it11 t h o s e measured f o r t h e 9.0 MeV peak i n " ~ r . The measured CJ i s a b o u t 30% of t h e f u l l s i n g l e p a r t i c l e DWIA p r e d i c t i o n , c o n s i s t e n t w i t h p r e v i o u s work.7 The v a l u e s of S a r e l a r g e , b u t n o t a s c l o s e t o t h e plane-wave v a l u e o f 0.67 a s w 0 u l 8 ~ b e d e s i r a b l e f a r maximum s e n s i t i v i t y

.

The v a l u e s o f Snn ob- s e r v e d f o r t h e 9.0 MeV peak i n ' ' ~ r a r e t h u s c o n s i s t e n t w i t h an M 1 a s s i g n m e n t . The p o o r r e s o l u t i o n o f t h e s e d a t a , h o ~ r e v e r , d o e s n o t p e r m i t i s o l a t i o n o f i n d i v i d u a l E l s t a t e s .

C4-462 J O U R N A L DE PHYSIQUE

The most s u r p r i s i n g a s p e c t o f t h e p r e s e n t work i s t h e o b s e r v a t i o n o f t h e s p i n t r a n s f e r s t r e n g t h above 10 MeV. I f a l l t h e AS=l s t r e n g t h h e r e corresponded t o M l e x c ' t a t i o n s , t h e c r o s s s e c t i o n would b e more

-i

t h a n t w i c e t h e f u l l ( ~ g ~ , ~) c r o s s s e c t i o n . The r e l a t i v e l y f l a t a n g u l a r d i s t r i b u t i o f l fronq3'!3O t o 5 . O 0 shows t h a t n o t a l l t h i s s t r e n g t h i s M 1 , b u t it i s n o t p o s s i b l e a t t h i s t i m e t o make more de- f i n i t e s p i n a s s i g n m e n t s . We have c l o s e l y examined many p o s s i b l e s o u r c e s o f e r r o r i n o b t a i n i n g t h e s p e c t r a of F i g s . 4 and 5 . The i n - c l u s i v e a n a l y z i n g power o f t h e carbon a n a l y z e r was c a l i b r a t e d w i t h t h e HRS a t O 0 w i t h a s t a t i s t i c a l a c c u r a c y of b e t t e r t h a n 0.01. F o r e l a s t i c s c a t t e r i n g from a s p i n z e r o t a r g e t , S = O and P=A ; t h e s e r e l a t i o n s were confirmed w i t h a s t a t i s t i c a l a55uracy o f b g t t e r t h a n 0.01, i n s e v e r a l r u n s w i t h a 2 0 8 ~ b t a r g e t f o r v a r i o u s f o c a l p l a n e p o s i t i o n s . Most r e s c a t t e r e d p a r t i c l e s ( i n s t r u m e n t a l background) c o u l d be e l i m i n a t e d from t h e s p e c t r a by a p p l y i n 9 p r o p e r s o f t w a r e c u t s i n r e p l a y ; d e t a i l e d examination of d a t a f o r d i f f e r e n t r e g i o n s of t h e f o c a l p l a n e w i t h o u t t h e s e c u t s showed t h a t t h e s p i n - f l i p p r o b a b i l i t y o f t h e r e s c a t t e r e d p a r t i c l e s was c l o s e t o z e r o . T h e r e f o r e , contami- n a t i o n o f good e v e n t s by such r e s c a t t e r e d p a r t i c l e s would n o t c o n t r i - b u t e t o oS ; i t erould merely lower t h e measured v a l u e of Snn. A s a f i n a l chec2: many measurements were r e p e a t e d a t a l a t e r t i m e w i t h d i f f e r e n t beam c o n d i t i o n s and d i f f e r e n t magnet s e t t i n g s ; t h e s e r e - s u l t s , i n c l u d e d i n F i g . 4 , confirmed t h e o r i g i n a l r e s u l t s . Because of t h e uniform s p r e a d i n g o f t h e s p i n - f l i p s t r e n g t h , it i s d i f f i c u l t t o be c e r t a i n t h a t a l l s o u r c e s o f e r r o r have been e l i m i n a t e d . Never- t h e l e s s , o u r r e s u l t s s t r o n g l y s u g g e s t t h a t t h e s p i n - f l i p c r o s s s e e - t i o n a t 3.5' i s a p p r o x i m a t e l y 0.8 mb/sr/MeV t h r o u g h o u t t h e r e g i o n from 8 t o 25 MeV e x c i t a t i o n . The 5' d a t a a r e l e s s e x t e n s i v e and t h e y have n o t y e t been checked i n a second r u n , b u t t h e v a l u e s of Snn a p p e a r t o b e s i m i l a r .

F i g . 6. Cross s e c t i o n , a n a l y z i n g

T~ ~ 3 1 9 MOV power, and s p i n - f l i p p r o b a b i l i t y

E x = 10.23 MeV a n g u l a r d i s t r i b u t i o n s f o r t h e

4 8 C a ( p , p l ) 4 8 ~ a (10.23 MeV) r e - a c t i o n a t 319 MeV. The s o l i d c u r v e s r e p r e s e n t DWIA c a l c u l a t i o n s d e s c r i b e d i n t h e t e x t .

0.0

0.2

0.0 0.0 1.5 3.0 4.5 6.0 7.5 9.0 8, (deg)

While no spin excitations have previously been observed in this re- gion, it is important to note that no experiments really sensitive to such stre gth have been performed; the large angle electron scatter- ing data,' for example, extend only up to 10 MeV. The uniform spreading of the oS strength is suggestive of the apparent quasi- free background obsgpved in the (p,n) reaction; 0sterfeld8 is able to explain this as AS=l excitations up to spin 3' even at 0'. Similar results have also been obtained by Izumoto. In addition, recent RPA calculations of the spin-excitation strength expected in (p,pl) and ( e r e t ) experiments by Auerbach and Klein reveal large broad structures for many 'J values between 10 and 30 MeV. To consider the present results in terms of a quasi-free mechanism, it is in- teresting to observe that the Snn predicted by Arndt's phase shift solutions for free p

+

p scattering at 3.5O is 0.37 and 0.44 at 5.0°;

for free p

+

n scattering, it is approximately constant at 0.20.

Comparison with Snn values in Figs. 4 and 5 shows that the nuclear response is not dramatzcally different from that of a Fermi gas, even though this is a region of high natural parit3 collectivity.

This disagrees with the conclusions of Iloss et al. for 2 a 8 ~ b at 400 MeV.

This initial measurement of spin observables in the giant resonance region of "zr is thus consistent with an unnatural parity assignment for the proposed M1 peak at 9.0 IIeV in 9 0 ~ r , but it is not defini- tive; high-resolution spin-flip experiments with high resolution are necessary. On the other hand, the spin-flip data reveal for the first time considerable spin excitation cross section over the entire re- gion from 8 to 25 ileV.

We are grateful to N. Anantaraman, U.E.P. Berg, G. Crawley, A.

Galonsky, N. Marty, and A. Richter for helpful correspondence and discussion. This work has been supported by the NSF and DOE.

t~resent Address: Los Alamos National Laboratory and University of California at Los Angeles

tt~resent Address: University of Minnesota References

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Bertrand

- -

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(1982) 15; G . ~ X r a w l e ~ _1., to be p z l z h e d in Phys. 9ev.

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