Reaction mechanism and shell effects from the interaction of 14.6 MeV neutrons with nuclei

Abstract

The total cross sections for (n, 2n), (n, p) and (n, α) reactions have been measured for several nuclei around Z = 40 by the activation method, and the predictions of compound nucleus theory compared with the experiment. Pairing and shell corrections have been used in the calculations. While the cross sections for (n, 2n) reactions agree fairly well with the statistical model, the (n, p) and (n, α) cross sections were mostly below the calculated values, the discrepancy being stronger for α emission. The impossibility of fitting the data with the statistical model has been interpreted by the presence of direct processes, whose influence was intensified by the Coulomb barrier. Direct evidence for strong correlation between shell effects and total reaction cross section has been found for nuclei with neutron number N = 50. The cross sections for (n, 2n) reactions show a minimum at N = 50, while the competing (n, p) reactions show a strong maximum. The ratio σm/σg has been measured for several isometric states. The cross sections for (n, p) and (n, α) reactions for different isotopes of the same element have been found to obey the general trend of decreasing with increasing neutron number.