Energy and angular distribution of alpha particles from the Na23(n, α)F20 reaction induced by 14 MeV neutrons

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Nuclear Physics 33 (1962) 177--181 ; ¢~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

ENERGY AND ANGULAR DISTRIBUTION OF ALPHA PARTICLES FROM THE Na23(n, ot)F:° REACTION INDUCED BY 14 MeV NEUTRONS O. N. K A U L

Saha Institute of Nuclear Physics, Calcutta-9, India Received 4 December 1961 Abstract: Alpha particles produced by the interaction of 14 MeV neutrons with sodium have been studied with the help of Ilford 200 txm nuclear emulsion plates. The energy distributions of the alpha particles agree well with the predictions of the evaporation theory. The angular distribiations are consistent with compound nucleus formation in the sense that it shows a symmetry about 90 ° as in the case of the As~5(n, a)Ga 7z reaction reported earlier. The relative level density calculated from the energy distribution of the alpha particles fits exactly a straight line corresponding to a Maxwellian distribution with a temperature of 1.55 MeV. The total (n, =) cross section is estimated to be 185 mb:kl5 %.

1. Introduction

The extensive survey of (n, p), (n, ct) and (n, 2n) reactions at 14 MeV neutron energy carried out by Paul and Clarke t) has shown agreement with the evaporation theory in the case of (n, 2n) interactions, whereas for (n, ~t) and (n, p) reactions in the case of heavy elements these investigations suggested the direct interaction processes. In the past few years, several experiments have been performed to measure the energy level density of nuclei as a function of nuclear excitation energy. In these experiments the energy distributions of neutrons and protons were measured from (p, n) (refs. 2.3)), (p, p) (refs. ,--6)), (n, n) (refs. ~' 8)) and (ct, p) (refs. 9. to)) interactions. The presence of secondary nuclear reactions and non-compound nuclear processes throws doubt on the interpretation of the results of these experiments. On the other hand the presence of direct interaction processes in the case of (n, ct) interactions is rather improbable, and such interactions are, consequently, useful for giving reliable information about energy level densities. The energy distribution was measured by Ribe and Davis tt) in the case of zirconium for which the angular distribution above 120° and the energy distribution were not reported in detail. Kumabe et al. 12-14) studied the.alpha particles emitted in the 14.8 MeV neutron bombardment of A1, Co, Mn, V and S by the emulsion technique, taking either a very thin foil of the element or vacuum evaporating a sample of the element onto a platinum foiL The sample was sandwiched between two emulsion plates and exposed to the neutron beam at 45 °. The results reported by these authors are more or less in conformity with the compound nucleus formation. We studied arsenic by loading 200 gm Ilford c2 emulsion plates with ammonium arsenate and then exposing such plates to 14 MeV neutrons at grazing incidence ts). May 1962

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The results thus obtained have been explained on the basis of the compound nucleus theory. As arsenic is a medium weight odd-mass nucleus, it was thought useful to study the 14 MeV neutron induced (n, ~) reactions in low weight elements. Sodium was selected as a suitable mono-isotopic low-weight element having an odd-mass nucleus.

2. Experimental Arrangement The experimental arrangement was the same as used for the study of arsenic ~5). 200/zm c2 Ilford nuclear plates were loaded with sodium acetate. Sodium acetate fulfilled the conditions needed for a substance to be used for this purpose f. The plates were bombarded with mono-energetic neutrons of 14 MeV energy resulting from the (D + T) reaction by the accelerated deuteron ions of 200 keV energy from a radio frequency Cockcroft Walton type accelerator, falling on a direct refrigerant, cooled zirconium target. The target cooling was arranged so as to make it capable of handling a beam current of the order of 1 mA without causing any appreciable rise in the temperature of the target. The neutron yield as monitored by calibrated long counters and also by standard foil activity showed a value in the region of 10 l° to 1011 neturons per second from the target during the course of irradiation. T ' e neutron flux was also determined from recoil proton track analysis. 200 #m Ilfo~ ~ c2 plates enclosed in cadmium boxes were exposed to the neutron beam at grazing incidence. Unloaded plates exposed under exactly identical conditions allowed a fair assessment of the background. In order to discriminate between alpha particles and protons in the emulsion, the plates were processed by the temperature method, using low PM 6.6 amidol developers. For the hot stage, 20 min and 17°C were selected as the suitable time and temperature after several trials. Alpha tracks were further distinguished from proton tracks by grain density criteria. The scanning was done with a Zeiss opton microscope using an oil immersion eyepiece of (8 x 100) magnification.

3. R ~ The energy distribution represented by fig. 1 shows a peak value in the neighbouro hood of 5 MeV alpha energy, and the distribution of particles extends from 0.5 to 14 MeV, showing the emission of more alpha particles on the low energy side (below the Coulomb barrier) than is expected. The angular distribution was divided into two energy regions of greater and smaller than 7 MeV. Since the angular distributions for these regions were identical, it was assumed that the angular distributions for all the energy regions were identical. The angular distribution appears to be approximately symmetrical about 90 ° and concave upwards as represented in fig. 2. It is assumed that the relation between the measured alpha particle energy distribution and the level density of the residual nucleus is governed by the Weisskopf formula 16) n(e)de = const.~a=(~)w(e,)d& i" Details to be published elsewhere.

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In this expression, n(Ode represents the number of alpha particles emitted in the energy interval (s, ~+ de) and ~c(e) represents the cross-section which also includes the effect of barrier penetration for a nuclear radius of 1.5.4~ fm for the formation of the compound nucleus in the same state of excitation by the reverse reaction, in which the particles of energy e strike the excited residual nucleus; finally, de,) represents the energy level density of the residual nucleus at the excitation e, = era,-e--* ema~ representing the maximum energy which the emitted particle may have.

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Fig. 3 represents the results obtained by plotting the measured energy spectra divided by e~c(e) in a logarithmic scale. The data fit exactly a straight line, in contrast to the results reported by the author is) in the case of the As~S(n, ~)Ga 72 interaction, where the curve becomes concave on the low energy side. The graph observed in the present case shows a close correspondence to the Maxwenian distribution with a

ENERGY AND ANGULAR DISTRIBUTION OF ALPHA PARTICLES

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temperature of 1.55 MeV for F 2°. The total (n, =) cross section is found to be 185 m b + 1 5 %. The temperature at the low energy side is not included in the present discussion, because of the contribution of alpha particles from (n, n=) events. The approximate 90 ° symmetry of the angular distribution of alpha particles indicates that most of the reactions occur through the formation of a compound nucleus. The angular distribution however is inconsistent with the isotropic pattern suggested by the statistical theory, whereas the energy distribution agrees very well with the predictions of the evaporation theory. The author is deeply indebted to Dr. B. D. Nag-Chaudhuri for his guidance and complete association with this work. Thanks are also due to Mr. B. Chatterjee of the neutron generator team for his help in the operation of the C.W. machine and the exposure of the plates; and also to Mr. M. K. Chakravorty for his help in the scanning and calculations. References I) E. B. Paul and R. L. Clarke, Can. J. Phys. 31 (1953) 267 2) P. C. (3ugelot, Phys. Rev. 81 (1951) 51 3) D. M. Thomson, Proc. Phys. Soc. 69 (1956) 447 4) P. C. (3ugelot, Phys. Rev. 93 (1954) 425 5) R. M. Eisberg and (3. Igo, Phys. Rev. 93 (1954) 1039 6) K. Britten, Phys. Key. 88 (1952) 283 7) E. R. Graves and L, Rosen, Phys. Rev. 89 (1953) 343 8) B. (3. Whitmore and (3. E. Dennis, Phys. Rev. 84 (1951) 296 9) Eisberg, Igo and Wegner, Phys. Rev. 100 (1955) 1309 10) Brookhaven National Laboratory Report BNL-331 1955 (unpublished) 11) F. L. Rib¢ and W. Davis, Phys. Rev. 99 (1955) 331 12) Isao K u m a b e et al., Phys. Rev. 106 (1957) 155 13) Isao K u m a b e et al., Proc. Phys. Soc. Japan 13 (1958) 129 14) Isao Kumab¢, Proc, Phys. So¢. Japan 13 (1958) 325 15) O. N. Kaul, Nuclear Physics 29 (1962) 522 16) J. M. Blatt and V. F. Weisskopf, Theoretical nuclear physics (John Wiley and Sons, New York, 1952) p. 353