Zeitschrift ffir Physik 212, 71 -- 82 (1968)

Angular Correlation Measurements in the Bt~ 7)B 11 Reaction M. N. H. COMSAN, M . A . FAROUK, A . A . EL-KAMHAWY, M. S. M. EL-TgHAWV, a n d A . N . L v o v e Atomic Energy Establishment, Cairo, Egypt, U.A.R. Received September 29, 1967 Proton gamma angular correlations through the 6.76 and 8.92 MeV excited states of B 11 are measured at deuteron bombarding energies from 1.6 to 2.4 MeV. The correlations are measured at laboratory proton scattering angles 35 ~ in both reaction and azimuthal planes. The correlation coefficient A~ and the distortion parameter 2 are calculated. A systematic shift of the symmetry axis from the recoil direction is observed. This shift tends to zero as Ea---',Q. For the 8.92 MeV excited state of BII(Q= 0.32 MeV) the distortion parameter 2 is close to the plane wave limit. The possible values of incoming channel spins as well as gamma-ray multipole mixing ratio for the 6.76 and 8.92 MeV states are obtained. 1. Introduction I t was p o i n t e d o u t first by WILKINSON 1 t h a t (d, p) reactions with low Q-values a n d at low d e u t e r o n energies exhibit g o o d a g r e e m e n t with the p l a n e wave B o r n a p p r o x i m a t i o n t h e o r y ( P W B A ) , especially at Ed,,~Q. WARBURTON a n d CHASE 2 r e s t a t e d WILKINSON'S predictions using the dispersion relation d e r i v a t i o n of the Butler stripping theory. These a u t h o r s i n t e r p r e t e d the a p p a r e n t validity of Butler t h e o r y n e a r ENQ, as due to the absence of nuclear a n d C o u l o m b d i s t o r t i o n effects. H o w e v e r , GIBBS a n d TOBOCMAN3 in their D W B A analysis of C 12 (d, p ) C ~3 p r o t o n a n g u l a r d i s t r i b u t i o n s a n d p o l a r i z a t i o n s n o t e d t h a t the g o o d a g r e e m e n t between Butler t h e o r y a n d small Q, low-energy (d, p) r e a c t i o n s m a y n o t be due to the small-Q as suggested by WILKINSON, b u t to the fact t h a t distortions m o d i f y the Butler a n g u l a r d i s t r i b u t i o n s very little. P r o t o n - g a m m a a n g u l a r c o r r e l a t i o n m e a s u r e m e n t in (d, p 7) r e a c t i o n p r o v i d e a sensitive test of the r e a c t i o n m e c h a n i s m . P W B A t h e o r y predicts a n g u l a r c o r r e l a t i o n f u n c t i o n in which the s y m m e t r y axis coincides with the recoil direction a n d no a z i m u t h a l a n i s o t r o p y occurs. A c c o r d i n g to D W B A t h e o r y c o n s i d e r a b l e shifts of s y m m e t r y axis a n d a z i m u t h a l a n i s o t r o p y m a y be o b s e r v e d as well as r e a c t i o n p l a n e a n i s o t r o p y 4. * On leave from Atomic Energy State Committee, USSR. t WmraNSON,D.H.: Phil. Mag. 3, 1185 (1958). 2 WARP~TON, E.K., and L.F. ChasE jr.: Phys. Rev. 120, 2095 (1960). 3 GraBS, W.R., and W. TOBOCMAN:Phys. Rev. 124, 1496 (1961). 4 HUBY, R., M.Y. REFAI, and G.R. SATCHLER:Nuclear Phys. 9, 94 (1958).

72

M.N.H. COMSANet al. :

Refs. 5-12 are devoted to investigation of the B ~o (d, p) BI 1 reaction, but in the majority of these refs. special problems are studied separately. The P 4 - - T 6 . 7 6 angular correlation was measured at E d = 3 . 9 M e V s, while the P 9 - 78.92 correlation have not been investigated earlier. Accordingly, an investigation of p r o t o n - g a m m a angular correlations in a wide energy range can throw some light on the reaction mechanism. Moreover, such a study m a y help to examine whether the distortions of plane wave theory are really small when E d ~ Q.

2. Experimental Apparatus and Procedure The electrostatic generator of the U.A.R. Atomic Energy Establishment provided a deuteron beam of energy up to 2.50 MeV. The designed scattering chamber allowed measurements in both reaction and azimuthal planes. A well collimated 0.3 cm diameter deuteron beam passed through the target centre to a Faraday cup, which was placed at 70 cm from the target and was shielded with lead to reduce the background of gammarays. The energy spectra of the emitted protons were measured using a silicon surface barrier detector which could be rotated in both reaction and azimuthal planes f r o m 25 ~ to 155 ~ with respect to the beam direction. The energy resolution of the detector was about 1.5 % for a-particles of Po 21~ The g a m m a - r a y detector was a 5 cm x 5 cm N a I (TI) crystal mounted on RCA-type photo-tube and has an energy resolution of 8 % for T-rays of Co 6~ The photo tube with crystal was placed outside the chamber at a distance of 11 cm f r o m the target and was shielded with a lead cylinder. The p r o t o n - g a m m a angular correlation functions were measured using the apparatus and coincidence system derived in ref. 13. The geometry was checked by measuring the C lz (d, p, T) C13 angular correlation. This correlation should be isotropic since the spin of the intermediate state is 89 This isotropy was measured within 3 %. Correlations for accidental coincidence were determined by introducing an additional delay into the circuit for the T-ray pulses. Typical true-to-accidental ratios for 5 PENMAN,W.C.: Phys. Rev. 79, 6 (1950). 6 PRATT, W.W. : Phys. Rev. 93, 816 (1954). 7 MARION, J.B., and G. WEBER: Phys. Rev. 103, 1408 (1956).

8 Cox, S.A., and R.M. WILLIAMSON:Phys. Rev. 105, 1799 (1957). 9 BILANIUK, O.M., and J.C. HENSEL:Phys. Rev. 120, 211 (1960). 10 GORODETZKY, S., M. CROISSIAUX, A. GALLMANN, P. FINTZ, J. SAMUEL and

G. BASSOMPIERRE:Nuclear Phys. 18, 286 (1960). 11 .BREUER,G. : Z. Physik 178, 268 (1964). 12 POOR, R.V., P.E. SHEARIN, D.R. TILLEY, and R.M. WILLIAMSON: Nuclear Phys. A 92, 97 (1967). 13 FAROUK,M.A., M.N.H. COMSAIq,A.Z. EL-BF~HAY11. I.I. ZALOUBOVSKY:Atomkernenergie 12--22, 133 (1967).

Angular Correlation Measurements in the Bl~

p, 7) ]]11 Reaction

73

the P4-]16.76 and 1~9 -~8.92 measurements were about 9:1 when the full energy and both escape peaks were included. The deuteron current on the target were measured by a current integrator of the type A 3 0 9 A and the results were normalised to a constant number of the studied proton group. The target used was an enriched B~~ obtained from AERE Harwell. The thickness of the target was about 15 keV for 2.0 MeV deuterons.

3. Experimental Results and Discussions 3.1. The 6.76 MeV State Angular Correlation A typical charged particle spectrum observed at 0p=35 ~ for Ea= 2.40 MeV is shown in Fig. 1. A kinematic analysis of the spectrum shows the presence of proton groups P 2 : P 9 from the reaction BlO(d,p)B t~ 8 000

BlO{d,p) B11 Op =35 ~

6 000

E d = 2.40 MeV

I

~ + P6 % 4000

z

017

:l,r

2000

1

,~

II

9~ i" 20

40 60 Channel number

80

Fig. 1. A typical pulse-height spectrum f r o m the reaction Bt~ a n d a lab angle of 35 ~

100

p)]]tl at

Ea=2.40 MeV

as well as protons from C~2(d,p)C 13 and Ot6(d,p)O t7 reactions. The P4, 5 proton groups were not separated. However, in refs. 9' 12 these groups were separated and the contribution of the P5 proton group was found to be very small (about 5 %), for wide range of bombarding energy (from 1.75 to 7.80 MeV). Thus for the P4,5 group, the measured angular correlation can be considered as the correlation between P4 proton group and corresponding y-rays. A typical pulse height spectrum of the gamma-rays observed in coincidence with protons leading to the 6.76 MeV level of B 11 is shown in Fig. 2. This spectrum was observed at 0p=35 ~ and 0~=60 ~ for Ed= 1.60 MeV. A singles spectrum of the gamma rays from the target is

74

M . N . H . COMSANet al. :

shown on the same plot for comparison. The 3.09 MeV gamma line in this spectrum corresponds to the decay of the first excited state of C 13 from the reaction C 12 (d, p) C 13. 16

8.

x 103 12 -

BlO(d,p4 y ) B ;1 E d = 1.60 MeV 197 = 60 ~

~

__

:~

3.09

8

V:,

i

o.o

9

to

8.9~

4

o "5 _o

E Z

[

0

300

I

I

I

I

I

I ~'t

/gp : 35 ~

b

e 7 = 60 ~

200

100

I

I 20

I

f

I

40

60

Channel

number

80

Fig. 2a and b. Pulse height spectra of the gamma rays from the B t ~ reaction at Eu= 1.60 MeV. a Singles spectrum, b spectrum in coincidence with the protons leading to the 6.76 MeV state of B 11 at 0 p = 3 5 ~ 0 r = 6 0 ~

The angular correlation functions were measured in the plane containing Ka and Kp (reaction plane) and plane containing Ka and n=Kd xKp, (azimuthal plane), at proton scattering angles 35 ~ and at deuteron energies 1.6, 1.8, 2.0, 2.2 and 2.40 MeV. These angles are of interest since they correspond to the stripping peak in the proton angular distributions. The results of angular correlation functions are presented in Fig. 3 and 4. The error indicated on the graphs are sum of the statistical error for each measurement and a systematic error estimated as 4 %.

Angular Correlation Measurements in the Bl~

1.6

E d = 2.40 MeV

E d = 2.20 MeV

@p=35 o I L

"~1.2 1.0 SR

i11

OI 8 0

40

80

120

p, ~,)B it Reaction

1{50

1.2~ Xp = 35 ~

I

0

I

E d = 2.00 MeV

] ~-

0p=35 o

75

(~p= 35 o

+ -+ SR /,A

[

40

I'['['[

80 07

SR

I

i

120

;

40

150

Xp = 35 ~

-

80

120 160

Xp= 35 ~ I

0"6f O.L0

L [ Z./TO

T I I l 80 120 160

0

I

I I 1 I I I 40 80 120 160

I

] 40

i

; 80

I

I

f

120

160

0,1,,

Fig. 3. Angular correlations through the 6.76 MeV state of Btt from ]~l~ P4-76.76)t~ll measured at Ed=2.40, 2.20 and 2.00 MeV. The correlations with 0p--=35~ were measured in the reaction plane, while those with Zp= 35 ~ were measured in the azimuthal plane. The B 11 recoil axis is indicated by R and the experimental symmetry axis is indicated by S. The solid curve is the least squares fits according to expression (1) and (2) F o r the correlations measured in the reaction plane the solid curves are the least squared fits of the function. W ( n / 2 , ~b)= 1 + ~ cos" (~b -~b0)

(1)

where the angle q~ must be measured f r o m the recoil direction. In this case ~bo is the angle between the recoil direction and the s y m m e t r y axis. In the figures the directions of the recoil axis and s y m m e t r y axis are indicated where qSr is the l a b o r a t o r y g a m m a angle with respect to the beam. In the azimuthal plane the solid curves represent the least squares fits of the function.

w(o, ~ , )

= 1 +/~ cos 2 0 .

(2)

The angle 0 must be measured f r o m the direction of n = K a x K v. In our g e o m e t r y 0 = 0 ; , - 9 0 where 0~ is the l a b o r a t o r y g a m m a angle. The parameters A ~ and 2 in the expression given by HuBY et al. 4 for the angular correlation were calculated using the obtained values of 7,/3 and q5o . These results are summarised in Table 1.

76

M.N.H. COMSaNet al.:

1.4. &

1.2

E d = 1.60MeV

E d = 1.80MeV

1.6

k_

1.0

Op = 35 ~

0p=35 ~

SR

-

t,I,l 0.8- r 40r Et 80

120

s R

T.. ,tT t,,r 40

160

80

120

160

1.2 Xp -- 35 ~

Xp= 35 ~ 1.0 0.8 _

0.6-

0.40 i

I ; I 40 80

I

r r 120 160

~..L.(

'

0

r

r l l [ i F 40 80 120 160

07

Fig. 4. Angular correlations through the 6.76 MeV state of B11 from Bt~ d, P4-- 76.76)Bll measured at Ea= 1.80 and 1.60 MeV Table 1 Ea

o:

fl

(MeV) 2.40 2.20 2.00 1.80 1.60

~bo

-- 2 A o (exp)

(degree) 0.31+ 0.05 0.26+0.06 0.28-+0.04 0.26+0.07 0.24-+0.03

--0.20+0.06 --0.31-+0.05 --0.23-+0.08 --0.33+0.04 --0.39+0.10

1-+4 4+5 7-+3 153-7 30__+4

0A7+ 0.20 0.59-+0.39 0.55-+0.14 0.38___0.20 0 . 4 8 - + 0 . 2 1 0.51-+0.33 0.59___0.20 0.44___0.32 0 . 6 3 - + 0 . 2 2 0.33-+0.18

F r o m this table it is seen that the coefficients ~ and fi remains constant within the experimental accuracy for deuteron energies f r o m 1.60 to 2.40 MeV. The angle ~bo is close to zero at a deuteron energy of 2.4 MeV and increases gradually as the deuteron energy decreases. Such dependence of q~0 on energy as illustrated in Fig. 5 may be related with the departure from the stripping pole and thus with the increase of the distortion effects. For the reaction Bt~ p4)B 11, the pole of the pure stripping process is located in the vicinity of E a ~ Q ( Q =2.48 MeV). Unfortunately, the large error in the distortion parameter 2 makes it impossible to do some concIusions about the dependence of this para-

Angular Correlation Measurements in the Bt~

p, 7) Btt Reaction

77

meter on energy. However, the value of 2 tends to its plane wave limit by increasing the deuteron energy f r o m 1.60 to 2.40 MeV in agreement with the polology predictions. m

~o

+

3e 20

-

t

1C

1.5

1,7

+ 1.9 Ed

2.1 (MeV)

Fig. 5. Dependence of ~bo on Ea for Bl~

.

2.5

P4-- ~6.76)Bll angular correlation

According to the D W B A theory the coefficient A ~ depends only on the angular m o m e n t u m transfer, nuclear spins and g a m m a ray multipole mixture, and is independent of the distortion. As seen from Table 2 the experimental value of - 2 A ~ is quite energy independent and its mean value is 0.55___0.08. Comparison of the experimental value of A ~ and theoretical one may throw some light on the possible combinations of channel spin S and g a m m a ray multipolarity 6. The 6.76 MeV state of B 11 is known to have spin and parity 7/2ref.9, ~4. So a mixture of E2 and M 3 multipoles is allowed for transition to the ground state (3/2-). Table 2 gives the experimental coefficients along with those predicted by the stripping theory for J = 7/2 and for the two possible combinations of channel spin. Table 2 J

S

62

-- 2 A~2 (8)

Expel Theor.

0.55 + 0.08 7/2

5/2

d2-

I(M3) I (E2)

7/2

7/2

62_ I(M3) I (E2)

0.306--0.662 5+0.357 ~2 I + 5z

--0.408+0.882 5--0.476 d2 1q- 32

The predicted coefficient for S = 5 / 2 and S = 7 / 2 are plotted as a function of the M 3 / E 2 multipole mixing parameter 6 in Fig. 6. In the S = 5 / 2 curve, the experimental value of - 2 A ~ crosses the theoretical curve at d = - 0 . 4 5 + 0 . 1 5 and at 6"= - 3 . 0 _ 3+i ~ o2 whereas in the S = 7 / 2 curve no agreement could be found for any 6. 14 GREEN, L.L., G.A. STEPHENS,and J.C. WILLMOXX:Proc. Phys. Soc. (London) 79, 1017 (1962).

78

M.N.H. COMSANet al. :

According to the shell model the 6.76 MeV state of B t i can be considered as a single particles. The Weisskopf estimation for this level predicts the intensity ratio 6 =[I(M3)/I(E2)] ~ =0.003. Therefore on the merit of this argument alone the smaller value of 5 (5 =0,45) is favoured. '

-2~

'

J I

' '

' I

,

L

' I

, E

, J = 7/2-

0.8

S =

7/2

, V 10.0

~ ,

0.6 0.4 0.2

0

6>0

-0.2 -0.4 -0.6 -0.8

-

i

0.01

i

r

,,

]

0.1

1.0

10.0

100.0

0.01

,

I 0.1

,,

~ I 1.0

,,

100.0

I~1

Fig. 6. Proton-gamma angular correlation coefficients for Bl~ P4--76.76)Bll for a level spin of J=7/2. The coefficients are calculated for the correlation function W(O)=I--2A ~ Pz(cos 0) and ~2 is the intensity ratio I(M3)/1(E2). The measured coefficients are indicated by the hetched areas

The experimental value of - 2 A ~ m a y be predicted theoretically taking into account the admixture of two channel spins S = 5 / 2 and S=7/2. However, the contribution of S = 7 / 2 can not exceed 20 %.

3.2. The 8.92 MeV State Angular Correlation Fig. 7 shows a typical P9--~8.92 coincidence spectrum, corresponding to the 8.92 MeV g a m m a - r a y in coincidence with protons leading to the 9 th excited state of B ~t. This coincidence spectrum was detected at 0p =35 ~ and 07 = 120 ~ with deuteron bombarding energy of 2.40 MeV. A single spectrum of the g a m m a rays from the target is shown in part (a) of this figure for comparison. Angular correlations were measured at deuteron energies of 1.6, 2.0 and 2.4 MeV. For each of these energies angular correlations were measured with the proton detector at Op= 35 ~ corresponding to the maxim u m of the angular distribution. The results of angular correlation measured in both reaction and azimuthal planes along with the least-squares fits to Eqs. (1) and (2) are shown in Fig. 8. The data obtained in a manner like that of P4-]26.76, are summarized in Table 3.

Angular Correlation Measurements in the Bt~ xl03

i

16

12

79

B'~~ (d,pg,T) Bn

e.--,

20

p, y) Btt Reaction

Ed = 2.40 M eV

O~ = 120~

"L-,]

ITI

""

8.

2~, ~ x

\

. 9,,.o .- ,.j.g

I

0

,

I

[

t

"4 .....

300

r

Op = 35 ~

Z

O/.,=120~ 200

I

/~ I

100

I

b

\

.... ":" ':".';:'~'~'; F

~

20

J

f

I

,

I'~:..

,

40 60 80 Channel number

r

100

Fig. 7a and b. Pulse height spectra of the gamma rays from the l~ reaction a Ed=2.40 MeV. a Singles spectrum at 0 p = 3 5 ~ b spectrum incoincidcnce with the proton leading to the 8.92 MeV state of Btt with 0 p = 35 ~ 0 r = 60 ~ E d =2.00 MeV

E d =1.60 MeV

r

/"-d = 2.Z,0MeV

1s "K 0.8 aS

SR ,

,It,

I

,

r

RS

ill,

,

,

,

,

,

,

;l l,

,

,

,

t

_

Xp=35 ~ 1.2 -&

-LT..+<,

v 1.0 0.8 ,

I

/'0

r

l

80

,

I

120

.

[

160

r

[

40

,

I

80

,

I

120

r

r

160

,

[ ..E--I

40

,

80

T

r

120

Fig. 8. Angular correlations through the 0.92 MeV state of l i b from 131O(d' P 9 - - 7,8 . 9 2 ), nD n measured at Ed= 1.60, 2.00 and 2.40 MeV

I

16007

80

M.N.H. COMSANet al. : Table 3

Ea (MeV)

:~

B

~bo (degree)

--2A~

2

1.60 2.00 2.40

--0.20_+0.02 --0.21-+0.02 --0.26_+0.03

0.11_+0.04 0.10_+0.07 0.09-+0.02

0_+4 5-+4 9_+4

--0.15_+0.08 --0.16-+0.08 --0.18_+0.08

0.75-t-0.48 0.87_+0.68 0.82_+0.37

As is seen f r o m this table the coefficients c~ and/3 do not show any remarkable dependence on energy. Moreover, the angle equals 0 at a deuteron energy of 1.6 MeV, and then increases gradually with the deuteron energy. However, its value, even at Ed =2.4 MeV, remains very close to zero (Fig. 9). Inspite of the large error in the distortion para20 10 0

I

1.7

~

I

~

I

1.9 2.1 Ea (MeV)

Fig. 9. Dependence of ~o on Ed for Bl~

,

I

2.3

,

I

2.5

Pg-- ~8.92)Bll angular correlation

meter 2, its value are close to the plane wave limit (2 = 1). The fact that the angle ~bo and the distortion parameter 2 are very close to their plane wave limits in the deuteron energy range from 1.6 to 2.4 MeV, can be explained according to the WELKINSON'Sinterpretation of the stripping theory at low Q-values and low deuteron energies. The obtained value of A ~ is independent of the deuteron energy, within the experimental accuracy in accordance with the D W B A theory. The mean value of - 2 A 0 is - 0.16 +_0.02. An attempt was made to have some information about the channel spin of captured neutrons S and the gamma-ray multipolarity mixing ratio 6. This information can be obtained by comparing the experimental value of - 2 A ~ with those predicted by the stripping theory. The 8.92 MeV state of B t~ has J ~ = 5 / 2 - , while the ground state of this nucleus has J ~ = 3 / 2 - (ref.9'x2). Thus for the 8.92--+0 transition a mixture of M1 and E 2 multipoles is allowed. Table 4 gives the experimental coefficients along with those calculated according to the stripping theory for possible combinations of channel spin and gamma-ray multipolarity. Fig. 10 shows the theoretical - 2A ~ coefficient predicted for S = 5/2 and S = 7 / 2 as a function of the E 2 / M 1 multipole mixing parameter 6. The experimental anisotropy crosses the S = 5 / 2 curve at ~ =0.64-0.04 and ] 61 > 30, and the S =7/2 curve at 5 = - 0.11 +_0.04 and 6 = - 2.4 4- 0.4.

p, 7) B v Reaction

A n g u l a r Correlation M e a s u r e m e n t s in t h e Bl~

81

Table 4

s

s

_2AO(6)

~2

Exper. Theor.

- - 0.16 • 0.02 5/2

5/2

I(E2) d2 = - I(M1)

5/2

7/2

62 =

0 . 3 2 0 - - 0 . 8 1 2 6 - - 0 . 1 6 3 62 1 + d2

I(E2) -

- - 0 . 1 0 0 + 0 . 5 0 7 6 + 0 . 0 5 1 52

-

t(M1)

1 + d2

In Weisskopf units the extreme single-particle model predicts 5 = [I(E2)/I(M1)] { =0.07. Of the possible experimental values for the values 5=0.60 and 5 = - 0 . 1 1 is favoured for S=5/2 and S=7/2 respectively. 1.0 /

0.8 l -

J = 5/2

S = 5/2

|

0,6

O04

I

r I

r I

, r

6<0 r T EI I I

, I

I

I p

I

1.0

0.8-

J=5-/z

s--7/z

0.60.4.6>0 <

0.2

C'-4

r

0

-0.2 -0.4

-

-0.~

i

0.01

i

I I

0.1

i

i

,

[

1.0

I I

T I

10

100

r, l Fig. 10, P r o t o n g a m m a a n g u l a r correlation coefficients for Bt~ Pg--78.92) B l l for a level spin of J = S]2. T h e coefficients are calculated for the correlation f u n c t i o n W ( 0 ) = 1 - - 2 A ~ P2(cos 0) a n d d 2 is the intensity ratio I(E2)/I(]~41). T h e m e a s u r e d coefficients are indicated by the cross hetched areas 6 Z. Physik, Bd. 212

82

M.N. T-[.COMSANet al. : Angular Correlation Measurements

On the other hand, the intermediate coupling model has been successfully used to predict the excitation energies and spins of some odd-parity low-lying levels of B 11. The best fit between theory and experiment was found to take place using for the intermediate coupling parameter ~ the value ~ =4.0 (ref. 9). Moreover, ~ =4.5 was found to predict the gammaray branching ratios obtained from the reaction Beg(He 3, p)B 11 (ref.15). Both of these values of ~ provide nearly an optimum fit of the calculated Bo11 magnetic dipole moment 9. The gamma ray mixing ratio was calculated according to the intermediate coupling model for several values of in ref. 16. Assuming ~ =4.5 one can obtain for the 892 MeV level the value of [fi[ =0.14. The excellent agreement between the theoretical value of t5 and that obtained for S = 7 / 2 (~ = - 0 . 1 1 +_0.4) can be interpreted as a confirmation of the validity of the intermediate coupling model to B 11, as well as of the S =7/2 neutron capture to the 8.92 MeV state of this nucleus. 4. Conclusion

The analysis of the angular correlation measurements can be interpreted as a confirmation of WILrdNSON'S predictions. G o o d agreement with the PWBA theory was obtained for the P9 proton group with low Q value (Q =0.32 MeV). For this group angular correlation parameters r and 2 are close to their plane wave limits. For the P4 proton group with Q-value=2.48 MeV the angular correlation parameters are close to the plane wave limit only at Ed"~Q. Using the experimental values of A ~ an attempt was made to get some information about the channel spin of the neutron capture S and the gamma ray multipole mixing ratio/5. For the 6.76 MeV state it was found that the S = 5 / 2 neutron capture provides major contribution in the formation of this state, while the 8.92 MeV state is formed as a result of S = 7 / 2 neutron capture. The decay of the 6.76 MeV state is an (E2, M3) gamma transition with 6 = - 0 . 4 5 _ 0.18, while the 8.92 MeV state decays by means of an (M1, E2) transition with 6 = - 0 . 1 l +0.04. The authors express their sincere gratitude to Prof. Dr. M. EL-NADIfor valuable remarks and fruitful discussions. The authors are also indepted to the stuff of the accelerator for efficient operation. lS Govr, H.E., J.A. KULHNER, A.E. LrrHERLAND, E. ALMQVIST, and D.A. BROMLEY:Phys. Rev. Letters 1, 414 (1958).

16 KURATrLD. : Phys. Rev. 106, 975 (1957). Dr. M.N.H. COMSAN Physics Department Atomic Energy Establishment Cairo, U.A.R.