Search for a low lying 12+ level in B9

Volume 11, number 1

PHYSICS

LETTERb

1 J u l y 1964

Table 1 Values of ¥2 Target nucleus

Knocked-out configuration

Jmax

~2 MR J=O ~

y2 MR Jmax ~

2 YJmax

Y2=0 O 16

lp4

4

0 49

3 82

8

Ca40

(IdS) 4

8

0 018

O90

50

Zr 90

(lg~) 2 ( l f ~ ) 2

14

8 4x 10 -4

2 3x10 -2

27

Te 134

(lh~) 2 (lg~) 2

18

3 3 x 10 -4

8 2 x 10 -3

25

pb208

23 n2 (lh!~) 2 (11~)

22

1 6 × 10 -4

2 7 × 10 -3

17

l a r g e s p i n s T h e h e a v i e r the k n o c k e d - o u t c l u s t e r , the s t r o n g e r w i l l be the d o m i n a n c e of the e x c i t a tion of t h e l a r g e spin s t a t e s , and the l a r g e r w i l l be e x c i t a b l e spin v a l u e s T h e e x a m p l e of P o 212 s h o w s that t h e s e States m a y be i s o m e r i c 5)

2) 3) 4)

1) H.J. Mang, Z Physlk 146 (1957) 582, V V Balashov, V G Neudachm, Ya~ F Smlrnov and N P Yudln, J E T P (USSR) 37 (1959) 1385,

SEARCH

FOR

A LOW

5)

LYING

H J Mang, Phys Rev 119 (1960) 1069, Yu F Smlrnov and D Chlebovska, Nucl Phys 26 (1961) 306 A Harada, P r o g r Theor Phys 26 (1961) 667 G Igo, L. F Hansen and T. I Goodmg, Phys Rev 131 (1963) 337 M A Preston, Physics of the nucleus (AddisonWesley Publ Comp , 1962) N K Glendennmg, Phys Rev 127 (1962) 923, L A S h v and Yu I Charltonov, JETP {USSR) 44 (1963) 247

½+ L E V E L

IN

B9

E F F A R R O W and H J HAY Research School of Physwal Sczences, Australzan Natzonal Unzverszty, Canberra Reeelved 28 May 1964

E v i d e n c e f o r a B 9 s t a t e w h i c h c o r r e s p o n d s to the ½+ 1.75 MeV Be 9 s t a t e i s i n c o n c l u s i v e . N e u t r o n s f r o m Be9(p, n)B 9 h a v e i n d i c a t e d the o o s s l b l h t y of a w e a k b r o a d l e v e l n e a r 1 5 MeV 1, 2), but the B10(He 3, a)B 9 r e a c t i o n h a s s h o w n no p r o m m e n t l e v e l s b e t w e e n the g r o u n d and 2 34 M e V s t a t e s 3 - 5 ) D a t a f r o m C 1 2 ( p , a ) B 9 led to t h e s u g g e s t i o n of a l e v e l at 1.7 MeV 6), but an a l t e r n a t i v e e x p l a n a t i o n h a s now b e e n p r o f f e r e d 7) A p r e l i m i n a r y r e p o r t of an e x p e r i m e n t on t h e B 1 0 ( p , d ) B 9 r e a c t i o n s h o w e d a d e u t e r o n s p e c t r u m with a v e r y 8 s t r o n g l e v e l about 0.5 M e V w i d e a l s o at 1 7 MeV ) The e x p e r i m e n t s r e p o r t e d h e r e w e r e u n d e r t a k e n to c l e a r up t h i s c o n f u s e d s i t u a t i o n , and r e v o l v e d an e x t e n d e d i n v e s t i g a t i o n of t h e r e a c t i o n B10(p,d)B9 A self-supporting enriched boron t a r g e t (89°/o B 10) w a s u s e d and the r e a c t i o n p r o d u c t s w e r e a n a l y s e d by a 65 c m b r o a d r a n g e m a g n e t i c s p e c t r o g r a p h It w a s found that d e u t e r o n s 50

do o c c u r m the e n e r g y r e g i o n w i n c h would c o r r e spond to a b r o a d l e v e l m B 9 n e a r 1 7 M e V , but the r e a c t i o n d o e s not r e a l l y g i v e p o s i t i v e e v i d e n c e f o r s u c h a l e v e l I n d e e d , the p r e s e n c e of d e u t e r o n s b e t w e e n the g r o u n d and 2 34 MeV l e v e l s can be e x p l a i n e d m g e n e r a l t e r m s by p r o c e s s e s w h i c h w i l l be c o m m o n to e v e r y r e a c t i o n m v o l v m g B 9. In a f i r s t e x p e r i m e n t w i t h 11 M e V i n c i d e n t p r o t o n s , a l l t y p e s of o u t g o i n g c h a r g e d p a r t i c l e s w e r e r e c o r d e d m n u c l e a r p l a t e s at l a b o r a t o r y a n g l e s of 40 ° and 60 o. In a s e c o n d s e r i e s of e x p e r i m e n t s s i l i c o n s u r f a c e b a r r i e r d e t e c t o r s w e r e u s e d m the f o c a l s u r f a c e of the m a g n e t to obtain t h e d e u t e r o n s p e c t r u m c o r r e s p o n d i n g to e x c i t a t i o n s m B 9 up to 3 5 M e V T h e d a t a f o r 40 ° a r e s h o w n m f i g 1. T h e p l a t e and c o u n t e r d a t a a r e m a g r e e m e n t p r o v i d e d that d + and He 3++ a r e i d e n t i f i e d s e p a r a t e l y . After magnetic analysis these ions have practically the s a m e r a n g e s m the p l a t e s , but s i n c e t h e i r

Volume 11, number 1

1,2oo

PHYSICS LETTERS

t

i

i

i

=

sIO(pd) 8 9

,

~

E = II OI MeV. I,ooo[

oo[

P

g.... d store

5850t90

~

i

"'",

•

,,

2ooI

/ •

~

I i

.¢

I

I 2

Excltatlon m B 9 -

I

I 5

, I

97

8 89

I

I

I

MoV I 7148

1

i

It II II //

I I 5 MeV 0 120 MeVZl

I O--

1 July 1964

688 6 97 ,

7 56 778

'

= "=

97toZgm

Be 8

Fig 1 The deuteron yield.from 1000 ac of protons on a 50 ~g/cm 2 B 10 target The magnetic spectrograph subtended 3.0 × 10-4 steradians and was at 40 ° to the incident beam direction The detector m the focal surface accepted an energy range of 1%, and the yield has been normalised to an energy interval of 10 keV This interval includes the whole of the ground state yield At the 2 34 MeV level the instrumental width was 19 keV and observed v~dth 80-85 keV The bars below the graph show the ranges of energues available for deuteron emission following inelastic proton scattering through the indicated levels in B 10 Deuterons from the B 11 impurity m the target were not ~bserved at the expected energy corresponding to 2 96 MeV exmtation in B" e n e r g z e s a r e m the r a t i o 3 8 they a r e s e p a r a t e d m the c o u n t er s p e c t r u m . M a r k e d on f i g 1 i s the He 3 group identified, by i t s change m e n e r g y wzth changes m angle of o b s e r v a t i o n and p r o t o n e n e r g y , as B10(p,He3)Be 8 f r o m t r a n s i t i o n s to the 2 9 l e v e l of Be 8. T h i s explains the group obs e r v e d by A r m l t a g e and M e a d s 8) and i n c o r r e c t l y a t t r i b u t e d to a 1.7 MeV l e v e l m B 9 On fig. 1 a c u r v e h a s been drawn to d e m o n s t r a t e that it i s p o s s i b l e to account f o r the d e u t e r o n yie ld with 4 l e v e l s m B 9, w z . the 3 known l e v e l s 9) at 0, 2.34 a, ld 2 8 MeV, and a l e v e l at 1.6 to 2 MeV with a wudth of 1 2 to 1.5 MeV 10,11). On t h i s b a s i s , about t h r e e q u a r t e r s of the yield at 1.7 MeV c o m e s f r o m a b r o a d l e v e l m this region. Its a n g u l a r d i s t r i b u t i o n , shown m fig. 2, is lSOt r o p i c , m m a r k e d c o n t r a s t to the a n g u l a r d i s t r i butions f o r the ground and 2.34 MeV l e v e l s 12) This suggests v e r y different reaction mechanisms, such as compound n u cl e u s f o r m a t i o n and d i r e c t i n t e r a c t i o n s r e s p e c t i v e l y , whzch ~s not an u n e x p e c t e d r e s u l t c o n s i d e r i n g that the d i r e c t d e u t e r o n r e a c t i o n i s f o rb i d d en f r o m the 3 + ground state of

B I0 ff the 1.7 MeV l e v e l , s ½+ w h e r e a s the ground and 2.34 MeV s t a t e s a r e of n e g a t i v e p a r i t y and a r e expected to be 4 - and ~- by analogy wlth Be 9. ]ntegratzon o v e r the an g u l ar dlstrlbutzons g l v e s the t o t a l m t e n s l t y of the b r o ad l e v e l as ¼ to ½ of the t o t al ground state intensity. A l t e r n a t l v e l y , if the ground state yzeld is r e g a r d e d a s a dfffractlon type d l s t r l b u t l o n s u p e r p o s e d on an i s o t r o p l c p a r t , then the b r o ad l e v e l has about the s a m e s t r e n g t h as the zsotroplc f r a c t l o n of the ground state. A f t e r r e m o v i n g the co n t r l b u t l o n s f r o m the e s t a b l i s h e d ground and 2 34 MeV l e v e l s , f l t t m g the e n t i r e r e m a i n i n g d e u t e r o n yleld by l e v e l s at 2.8 and 1.5 to 2 MeV ~ p r o b a b l y u n r e a h s t l c . Using I I MeV m c l d e n t p r o t o n e n e r g y , zt w as found that the n u m b e r s of d e u t e r o n s d e c r e a s e d v e r y r a p l d l y at about 3 MeV e x c l t a t l o n in B 9. To see w h et h er thls w as a r e f l e c t l o n of the p r o p e r t i e s of s t a t e s m B 9 o r was due m e r e l y to m s u f f l c l e n t e n e r g y f o r the outgoing d e u t e r o n , s o m e m e a s u r e m e n t s w e - e m a d e f o r h l g h e r e x c l t a t l o n s using 11.5 and 12 MeV p r o t o n s The r e s u l t s a r e included m fig. I It was found m all c a s e s that the n u m b e r s of deu51

Volume 11, number 1

PHYSICS LETTERS

!

I0,000

•

ground state

•

5,000

2,000

-o

1,000

#

#

500

200

At 17 MoV IO0

50

I

30

Centre

I

I

l

60

90

,20

I

150

of Moss Angle-- Degrees

Fig 2 Angular distributions of deuterons for the ground state and 1 7 MeV excitation m B 9 from the reaction B10(p, d)B 9 at 11 0 MeV proton e n e r g y t e r o n s began to fall when t h e i r o b s e r v e d e n e r g y was l e s s than 1 5 MeV T h i s e n e r g y c o r r e s p o n d s to about 0.7 MeV in the c e n t r e - o f - m a s s s y s t e m , indicating that the l o w e r yield is due to the o p e r a tion of p e n e t r a b i l i t y f a c t o r s to lnhlblt d e u t e r o n e m i s s i o n Again c o n s i d e r i n g the d e u t e r o n s not included in the well defined l e v e l s , it is found that t h e s e could a r i s e f r o m the s e q u e n c e of r e a c t i ons B l 0 ( p , p ' ) B 10. (d) Be 8. E v e n In the abse nc e of any data c o n c e r n i n g c r o s s s e c t i o n s and angular c o r r e l a t i o n s , it is p o s s i b l e to d e t e r m i n e the upper and l o w e r l i m i t s of d e u t e r o n e n e r g i e s which can a r i s e f r o m t r a n s i t i o n s through l e v e l s In B 10 T h e s e r a n g e s of e n e r g i e s a r e m a r k e d on fig 1 for s o m e of the m o r e p r o m i n e n t l e v e l s of B 10 It is s e e n that such r e a c t i o n s could c o n t r i bute d e u t e r o n s to e v e r y p a r t of the o b s e r v e d s p e c t r u m As the incident proton e n e r g y IS changed, d i f f e r e n t B 10 s t a t e s will contribute at an e n e r g y c o r r e s p o n d i n g to a given e x c i t a t i o n in B 9, and above 2 MeV e x c i t a t i o n in B 9 the e f f e c ts of the broad f i r s t e x c i t e d s t a t e in Be 8 could b e c o m e n o t i c e a b l e H e n c e , without even c o n s i d e r i n g

52

1 July 1964

the possxblhty of t h r e e body b r e a k - u p , we cons i d e r that the a l t e r n a t i v e r e a c t i o n s e q u e n c e m ay explain m o s t of the d e u t e r o n yield between the ground and 2 34 MeV s t a t e s of B 9 and the c o n tlnued i n c r e a s e in yield as e x c i t a t i o n s above 3 MeV b e c o m e p o s s i b l e In g e n e r a l , si n ce B 9 is unstable to proton e m i s s i o n , all r e a c t i o n s in the c l a s s T (a,b)B 9 . (p)Be 8 can a l s o p r o c e e d in the o r d e r T ( a , p ) R * (b)Be 8 B e c a u s e of this, i n v e s t i g a t i o n of the l e v e l s t r u c t u r e of B 9 by ex am i n i n g the s p e c t r u m of b can only s u c c e e d if those p a r t i c l e s or photons of type b which c o m e f r o m the a l t e r n a t i v e s e q u e n c e can be identified Most i n v e s t i g a t i o n s have been of 10% or p o o r e r a c c u r a c y in the r e g i o n between the ground and 2 34 MeV s t a t e s and give no d e finite Indication of a l e v e l analogous to the 1 75 MeV l e v e l in Be 9 The Be9(p,n) w o r k done to 1% a c c u r a c y shows a broad r i s e and fall amounting to s o m e 10% of the total slow n e u t r o n y l e l d , but w h et h er this c o m e s f r o m a l e v e l in B g m u s t be c o n s i d e r e d unproven B e c a u s e of the above cons l d e r a t l o n s , the m o s t p r o m i s i n g r e a c t i o n s to inv e s t i g a t e will be t h o se giving a p a r t i c l e s f r o m the proton b o m b a r d m e n t of C 12, the wide spacing of l e v e l s below 12 MeV in C 12 m e a n s that a good quantitative e s t i m a t e of the r e a c t i o n s p r o c e e d i n g by i n e l a s t i c proton s c a t t e r i n g should be p o s s i b l e , but g r e a t c a r e will be needed to el u ci d at e the a p a r t i c l e s p e c t r u m b e c a u s e of the d i s i n t e g r a t i o n of Be 8

References 1) J B Marion and J.S Levm, Phys Rev 115 (1959) 144 2) S Takayanagl, N H Gale, J . B . G a r g and J.M. Calvert, Nucl Phys 28 (1961) 494 3) B Povh, Phys Rev 114 (1959) 1114 4) R R.Spencer, G.C.Phxlhps and T.B.Young, Nucl. Phys 21 (1960) 310 5) L G Earwaker, J G.Jenkms and E.W Titterton, Nucl Phys 46 (1963) 540 6) G D Symons and P B. Treacy, Physics Letters 2 (1962) 175 7) G.D Symons, Physics Letters 10 (1964) 89. 8) B H Armxtage andR.E Meads, A.E R.E -PR/NP 4 (1963) 28. 9) T Lauritsen and F.A3zenberg-Selove, Nuel. Data Sheets Sets 5-6 (1962) 10) F.C. Barker and P.B. Treacy, Nucl Phys 38 (1962) 33 11) E Teranlshi and B Furubayasln, Physics Letters 8 (1964) 157 12) J B.Re3molds and K.G Standing, Phys. Rev. 101 (1956) 158.