Effects of isospin on statistical nuclear decay

Abstract

Measurements of the cross sections and energy spectra of protons and α-particles emitted from the reactions 62Ni+p and 59Co + α were made to determine the extent to which isobaric spin is a good quantum number in the intermediate composite system 63Cu at an excitation energy of 18.9 MeV. The composite system 60Ni, formed by the reactions 59Co+p and 56Fe+α, was investigated also in a similar study at an excitation energy of 22.3 MeV. The experimental ratio, σ(α, α')σ(p, p') σ(α, p)σ(p, α) , was found to be 2.1 for the 63Cu system and 1.6 for the 60Ni system. Both values of this ratio exceed considerably the theoretical values of approximately unity predicted by the Bohr independence hypothesis and the statistical model of compound nuclear decay including angular momentum. The deviations in the above experimental ratios from the compound nuclear predictions are explained by an isobaric spin selection rule which causes the T > composite systems to decay preferentially by proton emission. At least 80 % of the T > channels of the composite system 63Cu decay before T > ceases to be a good quantum number. Although the T > composite systems retain isospin preference by enhanced proton decay, these protons have evaporation type spectra and angular distributions characteristic of compound nuclear reactions.