Nuclear Reactions of Intermediate Energy Heavy Particles

Abstract

Heavy-particle reactions at intermediate energies have been an impor­ tant part of nuclear physics since its early days, but lately this sub-field has taken on special interest. The impetus for recent developments derives largely from experiment, with theoretical interpretation still in the process of following after the fact. Since activity in this field appears to be scarcely, if at all, past its peak, the present review can be only a sort of interim report. Enough progress has already been made, however, so that a survey of the present position seems worth while. The general tenor of recent studies has been that the statistical model (1, 2, 3) of nuclear reactions is of less u niversal validity than has usually been assumed. Whethe� this also reflects a failure of the compound nucleus model (3) seems largely a matter of semantics. The original formulation of the concept (3) contained two distinct parts: the notion that the incident particle amalgamated into a compound state by relatively rapid (10-20 sec.) sharing of the incident energy among all the nucleons in the nucleus; and that at lower energies were to be found individual, isolated resonances or quasi-stationary (eigen) states of the compound system. The first picture of rapid diffusion of excitation energy through the nucelus is essen­ tially a classical one; according to the correspondence principle, this can be represented quantum mechanically only as a sum over many eigenstates. For an individual resonance level, no such classical picture is permissible: it is simply an eigenstate of energy E, with ti!lle dependence given by a mul­ tiplicative factor exp( -iEt/iI). It is not possible to make any measurement of time-dependent processes, like the diffusion of energy, on this state alone. The classical, energy-sharing picture of the compound nucleus applies only at high enough excitation that the compound nucleus is represented mathe­ matically as a sum over many individual eigenstates. In the original formulation (3) this sum was not performed explicitly, but intuitive arguments were used to predict the expected results of such a summation: the incident energy would be rapidly divided among all the nucleons, and