THE Li7(p, d) Li6 REACTION AT 100 MeV PROTON ENERGY

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THE Li7(p, d) Li6 REACTION AT 100 MeV PROTON

ENERGY

J. Lee, S. Mark, R. Moore

To cite this version:

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THE Li7 (p, d) Li6 REACTION AT 100 MeV PROTON ENERGY C l - 5 5

M. le Pr. Chemla qui a bien voulu mesurer la compo- sition isotopique des bores enrichis utilisés. Nous sommes très reconnaissants à M. le Pr. D. Kurath de nous avoir communiqué les facteurs spectrosco- piques calculés.

Bibliographie

[l] BACHELIER (D.), BERNAS (M.), BRISSAUD (I.), DÉTRAZ (C.), GANGULY (N. K.) et RADVANYI (P.), Comptes Rendus Congrès Intern. Phys. Nucl., Paris 1964, 11, p. 429.

DÉTRAZ (C.), Thèse de Doctorat, Paris, 1964.

BACHELIER (D.), BERNAS (M.), BRISSAUD (I.), DÉTRAZ (C.) et RADVANYI (P.), J . Physique, 1963,24, 1055. Dans cette dernière publication, nous indiquions un niveau à 9,7 MeV dans le spectre de 'B. Les mesures de contrôle et l'analyse radiocliimique nous ont montré que ce niveau provenait essen-

tiellement d'une impureté de carbone introduite dans les cibles pendant le frittage.

[2] CHEW (G. F.) et GOLDBERGER (M. L.), Phys. Rev., 1950,

77, 470.

GLENDENNING (N. K.), Ann. Rev. Nucl. Sci., 1963, 13, 191.

133 KURATH (D.) et PIEMAN (L.), Nucl. Phys., 1959,10,313. KURATH (D.), Rapport Argonne National Laboratory,

6312, 1961.

[4] LAURITSEN (T.) et AJZENBERG-SELOVE (F.), Energy Levels of Light Nuclei A = 5

-

10, Nucl. Phys. (sous presse).

[5] BALASHOV (V. V.), BOYARKINA (A. N.) et ROTTER (I.), Nucl. Phys., 1964, 59, 417.

[6] LEGG (J.), Technical report, Princeton University P. U. C., 1961, 45.

SHORT (T.), Thèse, Univ. Minnesota, 1965 (non publiée). [7] COHEN (S.) et KURATH (D.), Nucl. Phys., 1965, 73, 1 .

KURATH (D.), Communication privée.

[8] BACHELIER (D.), BERNAS (M.) BRISSAUD (I.), RADVANYI (P.) et ROY (M.), Phys. Letters, 1965, 16, 304.

THE ~ i ~ ( p ,

d) ~i~ REACTION AT 100

MeV

PROTON ENERGY

J. K. P. LEE*, S. K.

MARK^

and R. B. MOORE

Foster Radiation Laboratory, Mc Gill University, Montreal (Canada)

Résumé.

-

Les distributions angulaires des groupes de deutons conduisant au niveau fonda- mental et aux niveaux de 2,2 MeV, 3,6 MeV et 5,4 MeV du Li6 sont présentées. Les résultats sont discutés pour le modèle en couches avec couplage intermédiaire et le modèle rotationnel.

Abstract.

-

Angular distributions of deuteron groups leading to the ground state and the 2,2 MeV, 3,6 MeV and 5,4 MeV excited states of Li6 are reported. Results are discussed in the intermediate coupling shell model and the rotational model.

We have studied the Li7(p, d) Li6 reaction by using the external beam of the 100 MeV Mc Gill Synchro- cyclotron. The target used was of natural lithium (92.6

%

Li7 and 7.4

%

Li6), 69 mg/cm2 thick.

The particle separation was achieved by a detecting system consisting of a 14O deflecting magnet with an adjustable slit a t the entrance side and a AE - E

counter telescope a t the exit. By adjusting the magnetic field and the slit width, only particles within a certain range of magnetic rigidity entered the telescope, and a AE gating was then used to select the desired particles. I n actual experimental conditions, when allowing a range of 10 MeV of high energy deuterons to enter the telescope, the elastically scattered protons were completely screened. A clean particle separation was obtained with a negligible feed through of tritons (< 1

%).

The final energy resolution thus obtained was 1.3 MeV.

Figure 1 shows a typical energy spectrum of the high energy deuterons. Four peaks are clearly distin- guishible, corresponding t o deuterons leaving the residual nuclei a t the ground state (1+, T = 0) and the 2,2 MeV (3+, T = O), 3,6 MeV (O+, T = 1)

and 5,4 MeV (2+, T = 1) excited states. The evaluation of the differential cross-section corresponding to the 5,4 MeV state presents some difficulty due to the presence of two neighbouring broad levels cen- tered at 4,6 MeV (2+, T = 0) and 6,O MeV

(1

+,

T = O). The values reported here correspond to the area above the dotted line as indicated in the diagram.

(*) N. R. C . of Canada Oversea Postdoctorate Fellow, now at Lab. Joliot-Curie de Phys. Nucl., Orsay (France).

(t)

N. R. C . of Canada Postdoctorate Fellow, now at Physics Dept. University of Manitoba, Winnipeg, Manitoba (Canada).

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C l - 5 6 J. K. P. LEE ET COLL.

FIG. 1. - Energy spectrum of deuterons from Li7 (p, d) Li6 reaction at 100 MeV proton energy. Error bar corresponds to statistical error.

The angular distributions of these four groups of deuterons are shown in figure 2. They al1 exhibit similar diffraction pattern and are believed to correspond to 1 = 1 pick-up reactions. On the same

l i t l r l i l r t t l t l i l g

O IO 20 30 40 50 60 70 80 90

8 'cm

FIG. 2. - Angular distributions of deuteron groups from Li7 (p, d) Li6 reaction. Error bars include al1 relative uncertainties.

diagram, a theoretical angular distribution based on Born approximation with an 1 p harmonic oscil- lator wave function for the picked-up nucleon (v = 0.562 fm-l) is also shown. It can be seen that the

distortion effect is very strong. For (p, d) pick-up reactions, the differential cross section can be written as :

Here the value of

1

F

1'

depends on the particular nuclear model used, while

1

B,,

l2

determines the angular distribution and is dependent on the reaction mechanism. For the evaluation of

Bi,,

Butler approximation is not valid here since, for 100 MeV protons, this reaction is not restricted to the surface of the nucleus, On the other hand, distorted wave analysis is obscured by the fact that we still do not know the distorted wave functions of the incoming protons and the outgoing deuterons. However, since the values of

Bi,

for the same 1 transfer is not expected to change appreciably, the relative magnitude of the experimental

1

F

1'

may still be meaningful. A com- parison with the theoretically calculated relative

1

F

l2

based on different models will then give some indications about the validity of the particular model used.

In Table 1, the experimental values of relative

1

F

1'

are tabulated together with the theoretical values based o n LS and j-j coupling shell model. The experimental results correspond to the intermediate coupling scheme with an a / K value of about 3. This is consistent with many other studies which indicate a weak coupling with a/K values of about 1 to 4 [-31.

EXPERIMENTAL AND THEORETICAL VALUES

OF

1

F

1'

x / \ F

1'

g FOR L i 7 ( ~ , d) ~i~ REACTION L. S. j-j Rota-

J. T. cou- cou- tional Experimental energy _{pling pling model} value

- - - - - OMeV 1.0 1 1 1 1 2.2 3.0 0.56 2.33 1.81 1 .O6 3.6 0.1 0.33 0.93 0.24 0.58 4.6 2.0 0.18 O 0.22

-

5.4 2.1 0.27 0.18 0.33 0.25 (0.52)"" 6.0 1.0 0.03 O 0.09

-

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CHOCS QUASI ÉLASTIQUES SUR GROUPEMENT cc DANS 12C C l - 5 7

tional band. As for Li6, a mixing between the K = O

and K = 1 bands is necessary for the (J = 1, T = 0 )

and (J = 3, T = 0 ) levels. By using these wave functions, the relative ( F

1'

values have been calculated and are shown in the same table. The general agree- ment with the experimental results is not as good as that with the intermediate coupling scheme. However, the discrepancy could perhaps be reduced by adjusting the mixing in the wave function and the defor- mation parameter.

We would like to thank Our Colleagues at Orsay for helpful discussions. One of us (J. K. P. Lee) wishes to express his gratitude t o the National Research

Council of Canada for scholarship held during the course of this work.

Bibliographie

[l] CHEW (G. F.) and GOLDBERGER (M. L.), Phys. Rev., 1950, 77, 470.

[2] GLENDENNING (N. K.), Ann. Rev. Nucl. Sei., 1963, 13, 191.

[3] For references, see LAURIT~EN (T.) at AJZENBERG- SELOVE, Nuclear Data Sheets, National Research Council.

[4] CHESTERFIELD (C. M.) and SPICER (L.), Nucl. Phys., 1963, 41, 675.

[5] GOGNY (D.), Thèse de Doctorat, Orsay, 1965.

CHOCS QUASI ÉLASTIQUES

SUR GROUPEMENT

a

DANS

12C

POUR DES PROTONS DE

60 ET

120 M e V

C. JACQUOT, H. BRAUN, J.-P. GERBER et P. CÜER

Physique corpusculaire, Centre de Recherches Nucléaires de Strasbourg

Résumé.

-

La section efficace de la réaction 12C(p, p') a

+

8Be* a été établie au moyen des émulsions : 7

+

2,s mb à 120 MeV et 45

+

15 mb à 60 MeV. L'étude du 8Be spectateur a fait apparaître une relation angulaire en liaison avec l'état d'excitation de ce 8Be. Ceci nous a permis de donner un ordre de grandeur de la durée de vie du 8Be* dans ses différents états et d'obtenir certains résultats sur la structure du 12C excité dans cette réaction.

Abstract.

-

The cross section of the reaction W(p, p') a

+

SBe* has been established by means of nuclear emulsions and was found to be 7

i

2.8 mb at 120 MeV and 45

+

15 mb at 60 MeV. The study of the spectator 8Be gives an angular relation in connection with the excitation state of the Be8. This fact permits us to give an order of magnitude of the life time of 8Be* in its different levels and to obtain results as to the structure of the excited 12C in this reaction. Ce travail se place dans le cadre plus général d'une

étude sur la structure du 12C. NOUS avons étudié

au moyen des empilements de pellicules d'émulsion nucléaire, la réaction p

+

12C -, p'

+

4He

+

8Be*. Nous avons retenu les seuls événements où le proton incident de 120 MeV choque un groupement alpha dans des conditions très voisines de celles d'un choc élastique, le 8Be" spectateur se désexcitant en deux particules a d'énergie très faible.

Par rapport aux travaux précédents [ l , 2, 31, nous

avons pu mesurer chaque branche sur toute sa lon- gueur, ce qui n'avait encore jamais été effectué ; de plus, le traitement des mesures par calculatrice élec- tronique, en envisageant toutes les réactions possibles, nous a permis d'obtenir une certitude absolue dans l'identification des étoiles.

Nous avons obtenu les sections efficaces 7

+

2,8 m b et 45

+

15 mb aux énergies incidentes respectives de 120 et 60 MeV pour cette réaction.

Dans un choc quasi élastique, la somme des mo- ments du proton incident p, du proton sortant p' et de la particule a, est quasiment nulle ; nous l'appe- lons ( p , , ) , ; cette quantité peut être considérée comme le moment de la particule a, avant la rupture des liaisons a,, 8Be [2, 41. Dans le système du laboratoire, nous voyons (Fig. l a ) que ( p z , ) , est également déter- miné par la somme (p,, + p u , ) donc le moment cinétique du 'Be spectateur.