Inelastic electron scattering from 16O and the radiative widths of the 6.9 and 11.5 MeV levels

Volume 22, number 4

INELASTIC

PHYSICS LETTERS

1 September 1966

ELECTRON SCATTERING FROM WIDTHS OF THE 6.9 AND 11.5

160 AND THE MeV LEVELS

RADIATIVE

M. S T R O E T Z E L and F. GUDDEN Institut f a r Technische Kernphysik der Technischen Hochschule, Darmstadt Received 28 July 1966

Levels in 160 between 5 and 14 MeV excitation energy have been studied by inelastic electron scattering. Ground state radiative widths for the 2+ levels at 6.9 and 11.5 MeV are 0.100 ± 0.015 and 0.52 + 0.13 eV, respectively. Upper limits for some other transitions are given. The transition radius of the 6.9 MeV level is 3.82 ± 0.46 fm.

R e c e n t l y Brown and G r e e n [1] have g i v e n a d e s c r i p t i o n of the l o w - l y i n g e v e n - p a r i t y l e v e l s of 160 by i n t r o d u c i n g d e f o r m e d s t a t e s . A c h e c k of t h e s e c a l c u l a t i o n s is p r o v i d e d by the c o m p a r i s o n of c a l c u l a t e d E 2 - t r a n s i t i o n p r o b a b i l i t i e s with e x p e r i m e n t a l v a l u e s . We have i n v e s t i g a t e d the e x c i t a t i o n of l e v e l s in 160 by i n e l a s t i c s c a t t e r i n g of e l e c t r o n s f r o m the D a r m s t a d t l i n e a r a c c e l e r a t o r [2]. F i g . 1 shows a t y p i c a l s p e c t r u m taken with a BeO t a r get. All the p eak s a r i s e f r o m the e x c i t a t i o n of l e v e l s in 160. M e a s u r e m e n t s with a p u r e Be t a r get [3] showed no s h a r p r e s o n a n c e in the r a n g e of 5 to 14 MeV e x c i t a t i o n e n e r g y which had to be taken into a c c o u n t . The i n e l a s t i c c r o s s s e c t i o n s w e r e d e t e r m i n e d r e l a t i v e to the e l a s t i c ones. The e l a s t i c c r o s s s e c t i o n s w e r e c a l c u l a t e d by p h a s e shift a n a l y s i s with o s c i l l a t o r p a r a m e t e r s a = 1.82 f m and ot = 1.6 as g iv e n by E h r e n b e r g et al. [4]. The p e a k s l a b e l e d E x = 6.9 MeV and E x = 11.5 MeV in fig. 1 c o r r e s p o n d to e l e c t r i c q u a d r u pole t r a n s i t i o n s to the 2 + l e v e l s at 6.92 and 11.52 MeV, r e s p e c t i v e l y . In o r d e r to d e t e r m i n e the t r a n s v e r s e c o n t r i b u t i o n to the c r o s s s e c t i o n , a n g u l a r d i s t r i b u t i o n s w e r e m e a s u r e d at two d i f f e r ent v a l u e s of m o m e n t u m t r a n s f e r q. As a l r e a d y s u g g e s t e d by Bishop, B~tourn~ and I s a b e l l e [5], the t r a n s v e r s e contribution is not n e g l i g i b l e and has to be c o r r e c t e d f o r . The q - d e p e n d e n c e of the c r o s s s e c t i o n was m e a s u r e d f o r the 6.92 MeV l e v e l at c o n s t a n t s c a t t e r i n g angle, 117o, by v a r y ing the incident e l e c t r o n e n e r g y . D r e c h s e l [6] has c a l c u l a t e d the q - d e p e n d e n c e of longitudinal E2 c r o s s s e c t i o n s by p h a s e shift a n a l y s i s f o r d i f f e r ent i nit i al e n e r g i e s . With t h e s e r e s u l t s we have c o r r e c t e d the m e a s u r e d c r o s s s e c t i o n s to infinite

Counts pC

/

E o = 6 0 . 5 MeV

80-

0

=117 o

''

°

60 .=, =7

t °

=

40

20

&6

&8

50

5;' E [Me¥] 54

Fig. 1. Energy distribution of 60.5 MeV electrons scattered from 180 mgfcm 2 BeO through 117 °. e n e r g y , and, then, u s e d the B o r n a p p r o x i m a t i o n f o r m u l a e in o r d e r to e v a l u a t e the r e d u c e d t r a n s i tion p r o b a b i l i t i e s . F i g . 2 shows the s q u a r e r o o t of the r e d u c e d t r a n s i t i o n p r o b a b i l i t y B(C2, q) -= -- B(C2, q, fi --. Kf) as a function of q2. The e x t r a polation to q = k = Ex/Pic ~ i v e s a B ( C 2 , k) = = B ( E 2 , k) = 3 9 . 4 + 5 . 9 f m ~ and a t r a n s i t i o n r a d i u s R t r = 3.82 + 0.46 f m . The v al u e of R t r a g r e e s w e l l with t r a n s i t i o n r a d i i f o r o t h e r n u c l e i as quoted by

485

Volume 22, n u m b e r 4

PHYSICS

LETTERS

1 September 1966

Table 1 Reduced t r a n s i t i o n p r o b a b i l i t i e s B(E2, 1) which differ f r o m the B(E2, k) used in the text by the s t a t i s t i c a l f a c t o r I/(21+ I) =-~, with B(E2, k) = 6.2 x 106 rO(eV) Ex5 (MeV) fm4. B(E2, I) =+B(E2, k) fm 4

Experiment

6.92 --* g,s.

(~,~) [10]

4.3 _ 0.9

(~,T [11]

5.2 _ 1.2

+ 1.4

+ 2.3

6.92 --~ 6,05 40 *

48 *

(or,y) [12]

5.5 • 1.2

(ot, T) [13]

4.0 ± 0.6

(e,e')[5] this work

q2 If m- 2] 0.05

0.10

0.15

0.20

0.25

11.52 --~g.s.

Theory[l]

5.2 10,6 7.9 + 1.2 5.3

73 * 103

3.15 ± 0.8 3.3

* Values calculated With a branching ratio of 2.8 × 10-4 (average from [8] and [9] ).

Fig. 2. Square root of the reduced t r a n s i t i o n p r o b a b i l i t y of the 6.92 MeV (2 +) level in I 6 0 . S p a m e r [7]. T h e r e f o r e , we a s s u m e d t h e s a m e t r a n s i t i o n r a d i u s f o r t h e 2 + l e v e l a t 11.5 M e V to e x t r a p o l a t e t h e B ( C 2 , q ) m e a s u r e d a t q = 0.475 a n d 0.338 f m -1 to k. The extrapolated transition probabilities are listed in table 1 together with results from other experiments. For comparison with theoretical estimates, the transition probability from the 6.92 M e V l e v e l to t h e f i r s t e x c i t e d 0 + s t a t e i s also given. The overall agreement with calcul a t e d B ( E 2 ) ' s of B r o w n a n d G r e e n [1] i s i m p r o v e d . At p r e s e n t we c a n m a k e o n l y q u a l i t a t i v e r e m a r k s a b o u t t h e o t h e r p e a k s s e e n in f i g . 1. T h e p e a k l a b e l e d 6.1 M e V i s d u e to t h e e x c i t a t i o n of t h e 0 + l e v e l a t 6.05 M e V a n d t h e 3 - l e v e l a t 6.13 M e V w h i c h a r e n o t r e s o l v e d in t h i s e x p e r i m e n t . H o w e v e r , t h e w i d t h of t h i s p e a k i s l a r g e r t h a n t h e e x p e r i m e n t a l o n e , a n d we h o p e to b e a b l e to s e p a r a t e t h e two c o n t r i b u t i o n s . T h e p e a k l a b e l e d 12.0 M e V i s c e r t a i n l y d u e to t h e 2 + l e v e l a t 12.02 M e V w h i c h h a s a l r e a d y b e e n i n v e s t i g a t e d in e l e c t r o n s c a t t e r i n g w o r k of B i s h o p e t a l . [5] a n d V a n p r a e t a n d B a r b e r [14]. T h e a n a l y s i s of t h e p e a k a t 13 M e V w i l l b e v e r y d i f f i c u l t b e c a u s e a t least four levels with different spins and parities c o n t r i b u t e to t h e c r o s s s e c t i o n . O u r s p e c t r a do n o t e x h i b i t a p e a k a t 1 1 . 0 M e V excitation energy, although with the cross section g i v e n b y V a n p r a e t a n d B a r b e r [14] t h e p e a k s h o u l d clearly be visible. The ground state radiation w i d t h of a p o s s i b l e E 2 - t r a n s i t i o n a t 11.0 M e V m u s t b e l e s s t h a n 0.1 e V . T h e s m a l l p e a k a t 12.5 486

MeV in fig. 1 stems mainly from the 2- level at 12.53 M e V . F o r t h i s M 2 - t r a n s i t i o n B i s h o p e t a l . [5] h a v e e x t r a p o l a t e d a B ( M 2 , k) = 3.0 ~ 0.4 f m 4. T h e c r o s s s e c t i o n m e a s u r e d a t v e r y low q i n o u r experiment indicates, however, that the transit i o n p r o b a b i l i t y m u s t b e l e s s t h a n 1 f m 4. A l s o , t h e M l - t r a n s i t i o n to t h e 1 + l e v e l a t 13.65 M e V s h o u l d c l e a r l y b e v i s i b l e if w e a s s u m e a r e d u c e d t r a n s i t i o n p r o b a b i l i t y of B ( M 1 , k) = 0.15 + 0.02 f m 2 a s g i v e n b y B i s h o p e t a l . [5]. W e f i n d t h a t t h e B ( M 1 , k) of t h i s l e v e l m u s t b e l e s s t h a n 0.005 fm 2 . We thank Professor P. Brix for many stimulati n g d i s c u s s i o n s a n d O. T i t z e f o r h i s h e l p w i t h t h e c o m p u t e r p r o g r a m s . W e a r e v e r y g r a t e f u l to D r . D. D r e c h s e l f o r c o m m u n i c a t i n g h i s r e s u l t s p r i o r to p u b l i c a t i o n . T h i s w o r k w a s s p o n s o r e d b y t h e B u n d e s m i n i s t e r i u m fiir w i s s e n s c h a f t l i c h e F o r schung.

References 1. G . E . B r o w n a n d A . M . Green, Nucl. P h y s . 7 5 (1966) 401. 2. F.Gudden, G. F r i c k e , H . - G . C l e r e and P. Brix, Z. Phys. 181 (1964) 453. 3. H. -G. Clerc, E. Spamer and K. J. Wetzel, private communication. 4. H. F. E h r e n b e r g , R.Hofstadter, U. M e y e r - B e r k h o u t , D. G. Ravenhall and S. E. Sobottka, Phys. Rev. 113 (1959) 666.

Vohune 22, number 4

P H Y SI C S L E T T E R S

5. G.R.Bishop, C.B~tourn~ and D.B.Isabelle, Nuc]_. Phys. 53 (1964) 366. 6. D.Drechsel, Z . P h y s . , to be published. 7. E. Spamer, Z. Phys. 191 (1966) 24. 8. H.Fuchs, K.Hagemann and C.Gaarde, Nucl.Phys. 66 (1965) 638. 9. S.Gorodetzky, P.Mennrath, W.Benneson, P. Chevallier and F. Scheibling, J.de Phys. 24 (1963) 887. 10. C.P. Swann and F.R.Metzger, Phys.Rev. 108 (1957) 982.

HELIUM-8

PRODUCTION BY CARBON

1 September 1966

11. K. Reibel and A.K.Mann, P h y s . R e v . l l 8 (1960) 701 .. 12. R. E. Meads and J. E. G. McIldowie, Proc. Phys. Soc. 75 (1960) 257. 13. J.D. Larson and R.H.Spear, Nucl.Phys. 56 (1964) 497. 14. G.J.Vanpraet and W.C.Barber, Nucl.Phys. 79 (1966) 550.

IN N E G A T I V E PION AND OXYGEN NUCLEI

CAPTURE

Yu. A. BATUSOV, S.A. BUNYATOV, V.M. S]I)OROV and V. A. YARBA Laboratory of Nuclear Problems, Joint Institute f o r Nuclear Research, Dubna, USSR Received 12 July 1966

The events of 8He production and decay in negative pion capture by carbon and OXlcgenhave been detected in photoemulsion. The probability of Vile production is (2~3) × 10-6. The mass of ~He has been measured MSHe = 7481.3 + 0.9 MeV.

F o r the f i r s t t i m e the p o s s i b i l i t y of 8He e x i s t e n c e was c o n s i d e r e d by Zeldovich [1] and Goldansky [2] in 1960. Using the data on the e n e r g y of n e u t r o n p a i r i n g the above a u t h o r s conclude that v e r y p r o b a b l y the 8He isotope is s t a b l e to n e u t r o n e m i s s i o n . However, t h e r e was no s t r i c t e x p e r i m e n t a l e v i d e n c e on 8He e x i s t e n c e u n t i l r e c e n t l y . All the p a p e r s p u b l i s h e d b e f o r e 1965 have b e e n c o n s i d e r e d in a s u r v e y [3]. Two p u b l i c a t i o n s on the 8He e x i s t e n c e a p p e a r e d in the l i t e r a t u r e in 1966. P o s k a n z e r et al. [4] studied the decay s c h e m e of the 8He n u c l e u s p r o d u c e d by exposing p l a s t i c o r cotton t a r g e t s to 2.2 GeV p r o t o n s . C e r n y et al. [ 5 ] m e a s u r e d the 8He m a s s in the r e a c t i o n 4He + 2 6 M g ~ 8He + 22Mg. The p r e s e n t p a p e r r e p o r t s on the d i r e c t detection of the p r o duction and decay of 8He n u c l e i p r o d u c e d in the c a p t u r e of stopped n e g a t i v e pions by c a r b o n and oxygen n u c l e i in p h o t o e m u l s i o n . The m a s s of 8He is d e t e r m i n e d . The v a l u e s of the p r o b a b i l i t i e s of 8He p r o d u c t i o n by v a r i o u s nuclei in p h o t o e m u l s i o n a r e given. If the 8He n u c l e u s is stable to n u c l e o n e m i s s i o n , it should decay a c c o r d i n g to the following schemes: I. 8He ~-,-8Li* (0.978; 1+) ~L 8Li~_~ 8Be, (2.9; 2 +) --~ 2 4He;

II. 8He ~ - 8 L i * (3.22;1 +) ~ 7Li + n . The f i r s t of the two r e a c t i o n s i s e s p e c i a l l y conv e n i e n t to be detected i n p h o t o e m u l s i o n , s i n c e in this c a s e one o b s e r v e s at the end of the 8He t r a c k two e l e c t r o n t r a c k s and two a l p h a - p a r t i c l e t r a c k s going in opposite d i r e c t i o n s and equally long (a " h a m m e r " t r a c k with two e l e c t r o n s i n c o n t r a s t to the h a m m e r t r a c k s of 8Li and 8B which a r e a c c o m p a n i e d by an e l e c t r o n o r p o s i t r o n t r a c k , r e s pectively). The p r e s e n t e x p e r i m e n t a i m s at s e a r c h i n g f o r 8He which might be p r o d u c e d in negative pion capt u r e by light n u c l e i C, O and N in photoemulsion. In this case ~He c a n be o b s e r v e d in the following reactions $ : 1. ~ - + 1 6 0 -~ 8He+ 8B

2-prongstars

2. ~- +12C -~ 8 H e + 3 H e + I H 3. ~- + 14N ~ 8He + 3He + 3He

3-prong stars

4. ~- +160 -~ 8He + 7 B e + I H

Reactions with a larger number of charged particles are not given.

487