Evidence of nuclear shell effects in Ni 58 reactions induced with 0-48 MeV alpha particles

Abstract

Excitation functions for Ni 58(α, αp)Co 57, Ni 58(α, αn)Ni 57, Ni 58(α, αpn)Co 56 and Ni 58(α, α2n) Ni 56 reactions have been measured radiochemically for alpha particle energies up to 48 MeV. The decrease in the experimental (α, αn) and (α, αp) excitation functions above the threshold of the competing (α, αpn) and (α, α2n) excitation functions is in qualitative agreement with the compound nucleus reaction model. The experimental results have been compared with statistical theory calculations that assume level densities of the form ρ(E) = ( const) E −2 exp {2(aE) 1 2 } , where the nuclear excitation energy E was corrected for pairing. The value of the parameter a was set equal to the experimental value determined by Brady and Sherr. The statistical theory calculations utilized optical model inverse cross sections calculated with independently determined optical model parameters when these parameters were available. Cross sections were calculated for all permutations of n, p, d, t, He 3 and α emission for two successive evaporations, followed by calculation of a third evaporation of a neutron, proton or alpha particle where it was energetically possible. The influence of competition between gamma de-excitation and low energy proton emission on excitation function cross sections was estimated. The nuclear level density expression used in these statistical theory calculations was not modified to consider nuclear shell structure effects. The closer the residual nucleus is to the doubly closed shell nucleus Ni 56, the greater the divergence between the calculated and the experimental cross sections. This divergence between calculated and experimental excitation functions is discussed in terms of both the Bloch and the Rosenzweig treatments of the influence of nuclear shell structure on nuclear level densities.