In Search of Non-statistical Effects in Neutron Capture

Abstract

The reaction mechanism in radiative slow neutron capture has been dominated by the statistical compound nucleus model in which the mode of decay of the compound state is independent of its mode of formation. Because of the complexity of the initial state, the partial radiative amplitudes Γ ^1/2_γij , where i and j designate the initial and final states respectively, are normally distributed with zero mean. In most cases, such a notion is born out by experiments carried out on heavy weight nuclei⁽¹⁾.However, in recent years extensive theoretical and experimental investigations have been devoted for a search of simple reaction mechanisms. In order of increasing complexity, these are (1) The direct potential or hard sphere capture in the off resonance region in which the incident S-wave neutron is scattered by the boundary of the nuclear surface into a low-lying P state, and in the process electric dipole radiation is emitted. (2) The channel capture of Lane and Lynn⁽²⁾ or valence neutron model of Lynn⁽³⁾ in which the S or P wave neutron is scattered via the resonant state into a low-lying orbit, radiating γ rays. (3) The semi-direct process of Brown⁽⁴⁾ or doorway states formations of Estrada and Feshbach⁽⁵⁾, whereby an incoming neutron scatters in the target nucleus creating a 2P - 1H state. Subsequently a particle and hole combine to yield enhanced γ radiation. Lane⁽⁶⁾ has shown that Γ_γij’s are correlated with the reduced neutron widths of the initial state. In this category, the giant dipole resonance, which is considered as a coherent superposition of 1P - 1H state, is included.