Alpha particles from the interaction of 14 MeV neutrons with calcium

Nuclear Physics 55 (1964) 127--129; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

ALPHA PARTICLES F R O M T H E I N T E R A C T I O N O F 14 MeV N E U T R O N S WITH CALCIUM o. N. KOUL Government Engineering College, Jabalpur, India

Received 27 January 1964

Abstract: Energy and angular distribution of alpha particles emitted as a result of 14 MeV neutron interaction with calcium was undertaken by the emulsion technique making use of 200/~m IIford CI nuclear emulsions. The energy distribution seems to be completely Maxwellian with a peak value ranging from 4-5 MeV alpha energy with an excessiveemission of alpha particles on the tow-energy side. The angular distribution in the c.m. system is anisotropic, but approximately symmetrical about 90°. A semilog plot of relative level densities of the residual nuclei calculated from the alpha energy fits a straight line except on the low-energy side, where it becomes concave upwards. EI

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NUCLEAR REACTION CaSe(n,~), E~"-~ 14 MeV; measured or(0), ~-spectrum. A nTdeduced level density. Natural target. 1. Introduction

A study of 14 MeV neutron activation cross-section of elements made by Paul and Clarke 1) has shown that (n, p) and (n, u) cross-sections for large mass numbers are larger than those predicted by the evaporation theory. They suggested the existence of non-compound processes for such interactions. K u m a b e and others 2) have studied the (n, ~) cross sections of 14 MeV neutrons by emulsion technique in case of A1, Co, Mn, S and V. Their findings are more or less in keeping with the compound nucleus theory. Brolly et aL 3) have reported lower values of cross-sections than those reported by Paul and Clarke in the case of Zr. We studied previously the angular and energy distribution of alpha particles emitted as a result of 14 MeV neutron interaction in the case of As, Na, and Mn, (refs. 4- 6), respectively). Our energy distribution results follow more or less a statistical pattern showing a Maxwellian distribution. Our angular distribution results and the behaviour of relative level densities of the residual nuclei also favour the compound nuclear processes for such interactions. In the present work we extended our investigations to Ca by loading Ilford C2 nuclear emulsions with calcium acetate.

2. Experimental Arrangement and Results The experimental arrangement has been discussed elsewhere4-6). Scanning was accomplished by a Zeiss opton microscope having a magnification o f (8 × 100), as usual; processing was done by the usual temperature method using low P H 6.6 amidol 127

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developer. For the hot stage 20 rain and 17°C were selected as the suitable time and terapcrature. The energy distribution shows a peak value in the neighbourhood of 5 MeV energy

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Fig. 1. Energy distribution of alpha particles from Ca4°(n, co)A" reaction.

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Fig. 2. Relative level density n/~ 6c for A 8~.

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Fig. 3. Angular distribution of alpha particles from Ca4°(n, ~)A 8~ reaction,

ALPHA PARTICLES

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and the distribution of particles extends from 0.5 to 14 MeV. The complete energy distribution is represented in fig. 1. The angular distribution in the centre-of-mass system is shown in fig. 2. The angular distribution data were found similar for the energy regions above and below 7 MeV. Further, the angular distribution appears to be approximately symmetrical about 90 °, and concave upwards. Symmetry of the angular distribution about an angle close to 90 ° suggests a compound nuclear formation mechanism for most of the reactions, although a fairly large deviation from isotropy is not understood on the basis of the statistical theory of nuclear reactions. It is assumed that the relation between the measured alpha particle energy distribution, and the level densities of the residual nucleus is governed by the Weisskopf formula 7) n(e)de = const ~ao(8)o~(sr)de. In this expression n(8)de represents the number of alpha particles emitted between the energies e and e + ~ and the quantity crc(8) represents the cross-section (including the effect of the barrier penetration) for the formation of the compound nucleus in the same state of excitation by the reverse reaction in which the particle of energy e strikes the excited residual nucleus, while o~(~r) represents the energy level density of the residual nucleus at the excitation er = 8m~~ - ~, where em,x represents the maximum energy which the alpha may have. Fig. 3 represents the results obtained by plotting, on a logarithmic scale, the measured energy spectra, divided by sac. The data fit exactly a straight line except on the low-energy side. A nuclear radius of 1.5A t fm was used for the calculations. References 1) E. B. Paul and R. L. Clarke, Can. J. Phys. 31 (1953) 267 2) I. Kumabe, E. Takekoshi, H. Ogata, Y. Tsuneoka and S. Oki, Phys. Rev. 106 (1957) 155, J. Phys. Soc. Japan 13 (1958) 129; I. Kumabe. J. Phys. Soc. Japan 13 (1958) 325 3) J. F. Brolley, Jr. M. E. Bunker, D. R. I. Cochran, R. L. Henkel, J. P. Mize and J. W. Starner, Phys. Rev. 99 (1955) 330 4) O. N. Koul, Nuclear Physics 29 (1962) 522 5) O. N. Koul, Nuclear Physics 33 (1962) 177 6) O. N. Koul, Nuclear Physics 39 (1962) 325 7) J. M. Blatt and V. F. Weisskopf, Theoretical nuclear physics (John Wiley and Sons, New York, 1952) p. 353