Inelastic Scattering of Neutrons and Surface Direct Processes

Abstract

In the case of inelastic scattering of neutrons in which the residual nucleus is left in a definite low-lying excited state, the surface direct processes through single particle excitation and through ume sort of many particle excitations are formulated in a similar manner as in deuteron-stripping reaction. The distorted wave method is employed and its validity for the mentioned processes is discussed in some detail. The cross sections obtained are compared with that of another surface direct process accompanied by collective excitation and with that of the process through compound nucleus formation. It is shown that for intermediate and heavy nuclei (i) the collective excitation is much larger than the single particle excitation and (ii) the compound nucleus process is more important than the surface direct process in low energy region in which there are few open channels, while the latter is much larger than the former in higher energy region where many channels are open. § I. IntroductionIn recent experiments 1 > many evidences have been accumulated which clearly indicate the inadequacy of the concept of compound nucleus in explaining some of the nuclear reactions at moderate energies. One of the typical examples is the forward peaks of angular distribution appearing· in the case of inelastic scattering of particles in which the residual nucleus is left in a definite low-lying excited state. Such phenomena are evidently contradictory to the result of the statistical compound nucleus hypothesis.It, therefore, seems necessary to modify the concept of the compound nucleus, for instance, by removing some of the underlying assumptions, i. e., statistical assumption and continuum assumption, etc. Another possible and more realistic way of modification is to take into account such processes that take place without formation of the compound nucleus. These are called direct processes and may be divided into two classes : volume direct and surface direct processes. The former takes place inside of the nucleus, while the latter occurs at the diffuse layer at the nuclear boundary. The surface direct process may further be classified into various processes according to the mechanisms by which the target nucleus is excited. In the case of inelastic scattering of neutrons, Austern, Butler and McManus 2 > proposed the process by single particle excitation, whereas the direct process by collective excitation has been worked out by Hayakawa and Y oshida 3 >. As these mechanisms are of the extreme natures, there may be intermediate mechanisms of nuclear excitation, which may be called " group excitation ". a-particle excitation may be considered as one of the examples, in which a quasi a-particle in a nucleus is excited to its higher level to form residual nucleus. The mechanisms which work in actual nuclear reactions will be governed by the natures of target and residual nuclei.Downloaded from * Strictly speaking, the factor N depends on the detailed structure of the ground and excited states; under consideration. Its explicit ...