Angular momentum effects in nuclear evaporation processes: Charged-particle emission

Abstract

The effect of angular momentum upon the evaporation of neutrons, protons and alpha particles from excited nuclei has been calculated from the statistical model. The level density assumed was of the Fermi gas type with a Gaussian J-dependence for values of J corresponding to a classical rotation energy less than the total excitation energy; it was assumed that there were no levels with J-values higher than this. Transmission coefficients for the emitted particles were computed from the optical model. Quantities computed as a function of the compound nucleus angular momentum included the average energy of the first emitted particle, the average change in angular momentum, the particle emission widths and the ratios of emission widths. The effect of angular momentum upon these quantities can be quite different for different nuclei at different excitation energies and for different choices of the parameters, especially the nuclear moment of inertia. When the classical rotational energy is considerably less than the excitation energy, these variations can be readily understood with the aid of approximate formulae derived from a very simple model. These formulae also give a rough guide as to the order of magnitude of the effects to be expected in any particular case. The results can be greatly altered when the compound nucleus spin is greater than the angular momentum of any available levels in the residual nucleus. Particle emission widths are sharply reduced, sometimes drastically, and the effect can be quite different particles, thus altering their emission ratios. Gamma de-excitation, which is not considered, will be important in some such cases.