Stochastic Nuclear Reaction Theory: Breit-Wigner Nuclear Noise

Abstract

Our present understanding of neutron-nucleus interaction is largely based on Bohr’s statistical compound nucleus model.¹ However, theoretical developments based on quantum corrections² to Bohr’s model, together with considerable improvements in neutron spectroscopy have predicted and revealed the presence of marked fluctuations in the neutron cross sections of both fissile and fertile nuclei. These fluctuations are wider than the sharp resonances associated with the compound nucleus levels and narrower than the broad structure due to the energy dependence of the neutron penetration coefficients. They, in fact, represent departures from the statistical compound nucleus model in localized energy regions, leading to an intermediate structure which is not predicted by Bohr’s model. The observed enhancement of the neutron cross section is due to the presence of doorway states^3,4 in the neutron channel or in the fission channels in fissile nuclei. The understanding and detection of this intermediate structure is of great relevance both in nuclear reaction theory and for the calculation of nuclear reactor parameters.