22Ne(p, p1γ) angular correlations

I

2.L

I

i I

Nuclear Physws A123 (1969) 541--545; (~) North-HollandPubhshing Co., Amsterdam Not to be reproduced by photoprmt or microfilm without written permtsslon from the publisher

22Ne(p, PI?) A N G U L A R C O R R E L A T I O N S H. H U L U B E I , I. N E A M U , C. M. T E O D O R E S C U , N. S C [ N T E I , N. M A R T A L O G U a n d A. B E R I N D E

Institute for Atomw Physics, Bucharest, Romania Recel,,ed 14 October 1968 Abstract: T h e Pl-V a n g u l a r correlations m the reaction Z~Ne(p, PtV) were m e a s u r e d at Ep = 4 80, 5.05 a n d 5 50 MeV. T h e a n g u l a r correlation curves s h o w a ½~ periodicity with the s y m m e t r y axis in the vicinity o f the nucleus recoil direction and indicate the presence o f a direct interaction mechanism.

E

NUCLEAR

REACTION:

2-'Ne(p, PLY), E : 4.8-5 5 MeV; m e a s u r e d cr(E, 0o, 07). Enriched target.

I

I

I. Introduction Angular distributions of protons inelastically scattered from the first excited state of 22Ne have been reported previously 1). The averaged differential cross section in the Ep = 4.82-6.20 MeV range was explained by including a D I contribution. It is expected that Pl-7 angular correlation measurements are more sensitive to this D I contribution. The 0 + ~ 2 + ~ 0* coplanar angular correlations show a n periodicity as a function of 0~. according to the statistical model 2) and resonance theory 3), whereas the D W B A predicts 4) a ½n periodicity. The object of this experiment is to investigate the D I contribution at low energies in the 22Ne(p, PlY) reaction.

2. Experimental procedure The measurements were carried out at the cyclotron of the Institute for Atomic Physics, Bucharest. The experimental arrangement is shown in fig. 1. An enriched target of 22Ne(80.7 ~o) at a pressure of 150 Torr was used. The correlation chamber allows the simultaneous measurement of six angular correlation curves, i.e. coplanar correlations corresponding to protons detector angles 0p = 45 °, 90 ° and 135 ° and azimuthal angles ~o = 0 and n, respectively. The 7-ray counters were moved along the reaction plane in the 25°-155 ~ angular range. The protons were detected by 0.8 m m thick CsI(T1) crystals and the ~,-rays by a 3.8 cm × 2.5 cm NaI(T1) crystal. The coincidences were measured with six fast-slow coincidence circuits and two 400-channel pulse-height analysers. The resolution of the fast coincidence circuits is 25 ns. Their outputs are introduced in the slow coincidence circuits together with the g a m m a pulses from a single-channel pulse-height analyser. A proton pulse is accepted only if the 541

542

H. HULUBEIet al

control gate of the multi-channel analyser is opened by a fast coincidence (p-y simultaneous) and a slow one (y-quantum of the desired energy). The elastic scattering proton yield at 0 o = 90 ~ was measured in the monitor. The number of random coincidences could be determined from the elastic peak in

8z:

Fig. 1. Angular correlation chamber and fast-slow coincidence circuit. the coincidence spectrum which is due entirely to random coincidences. The experimental curves were corrected for the absorption of the y-rays passing through the proton detectors in the ¢p = 0 plane. 3. Results and discussion The (p, PlY) angular correlations obtained at 4.80, 5.05 and 5.50 MeV shown in figs. 2-4 are very similar to those obtained on the 2°Ne(p, PlY) reaction in the same energy range 5). The experimental angular correlation curves are compared with the DWBA prediction for the coplanar angular correlations in the ~p = 0 or ~p = ~ plane W(O~) = A + B

sin 2 2(0~+0o) ,

(1)

u~

u~

I

I

]

]

I~'~

I I

I I

i

~b

0

lOO

I

gO °

I

66 ~

30*

I

er

0"

30 °

I I

] 6"0"

] 90"

] 120"

/50 °

~'11I

] f80"

Fig. 2. The 2-~Ne(p, PlY) angular correlatzons -~t Ep = 4.8 M e V . Sohd curves are of form (l) provided by DWBA; OR is the nuclear recod direction.

/20'

I ~'01

180 ° /50 °

300t 20600-

Op~/35*

I

~

I

I

O" g~

#p= 135°

30"

~

P

30"

i

I

f:o I

I

I

GO" 90" /20" /5#" /8#"

~IaR

Ep= 5 05 MeV -/,,4 o

Fig. 3. Same as fig. 2 at Ep ~ 5.05 MeV.

60"

~-o

0/80" /50" I20" 90"

/00

0

I00

200

I

% % % %

200

3(]0

z,O0 ~ ~ 2~, ~( p, p,r )

20O

I

I

300

.

Ep =~8MeV

200

.

"'/¢e (P,~')

H HULUBEIet al.

544

400

_

~2~ cp,p ~'J

£p=55o

MeV

300 20O

100 0

8R,

f=O

m

I

~

~

t

I

I

I

I

I

I

7=g

tm

=

m

l

I

I

I

I

•

I

c~ z,OO 300

20~

, C

2OOr

~

I

-0. IT

°P "/sS°

x ~,

.

I80" 150°

'20 ° 90"

GO*

yt

30 °

O*

30"

60"

gd °

z20°

Fig 4. Same as fig. 2 at Eo = 5.50 MeV,

TABLE 1

Experimental values o f angular correlatton parameters

Ep

Op

(MeV)

(deg)

4.80

45 90 135 45 90 135 45 90 135

5.05

5 50

B/A

Oo (deg.)

2.00 1.01 1.00 1 70 | .67 1 13 1.13 1 84 1 36

80.0 41 6 84 0 71.6 54.2 33 6 66.5 46 0 10.1

/5#*

/80"

22Ne (p, pl~,)

545

where the symmetry axis 0o is expected to be near the recoil direction OR of the nucleus. The + and - sign stand for the ~p = 0 and n, respectively. The parameters A, B and 0o were fitted by a least-squares procedure and are shown in tame I. This function does not include the proton spin-flip. When the spin-flip process is included in the • 2 I D W B A calculation, a supplementary term of Csln (0y_+0o) form should be added to expression (1). This term displays a n periodicity similar to the compound nucleus prediction. To avoid confusion in the data interpretation, the spin-flip term was neglected. The experimental data displaying a ½n periodicity are in good agreement with the D W B A prediction (1). This suggests an important contribution of the direct interaction. The 0 o symmetry axis is near the nuclear recoil direction, and the B/A latio exceeds unity for all measured angular correlations• For a pure D I mechanism, this ratio would be larger than unity. In the present case, the actual values of B/A ratios are probably due to the CN mechanism contribution. An alteration of the ½n periodicity for the angular correlations at Ep -- 4.8 MeV (especially at 0p = 135°) can be noticed• This energy corresponds to a strong resonance in the Pl excitation function. At this energy, the compound nucleus contribution is larger, introducing thus, some perturbations in the angular correlation pattern, as was expected from the D W B A prediction. The experimental results reported here are consistent with a substantial D I contribution in proton inelastic scattering by 22Ne at about 5 MeV.

References

1) H. Hulubei, I. Neamu, G. Vl~.duc~t, N. Scintei, N. Martalogu, C. M. Teodorescu, M. Iva~cu and A. Berinde, Nucl. Phys. 123 (1969) 531 2) E. Sheldon, Revs. Mod. Phys. 35 (1963) 795 3) A. A. Kraus, J. P. Schiffer, F. W. Prosser and L. C. Bledenharn, Phys. Rev. 104 (1956) 1667 4) G. A. Levmson and M. K. Banerjee, Ann. of Phys. 2 (1957) 499 5) H. Hulubel, N. Martalogu, M. Iva~cu,N. Scintei, A. Berinde, I. Neamu and J. Franz, Phys. Rev. 132 (1963) 196