Analysis of fissionability data at high excitation energies

Abstract

A level density formula that takes into account the smoothed volume, surface and curvature dependence of the single particle level density at the Fermi surface using the results of Balian and Bloch, is shown to be compatible with the level spacings found in neutron resonance data if complemented by a simple Ansatz for shell effects (due to Ignatyuk) and pairing effects. The three parameters involved, a scaling parameter, a shell damping energy and a pairing energy shift are compatible, respectively, with known nuclear radii, microscopic level density calculations and odd-even mass fluctuations. At excitation energies on the order of the neutron binding energy no evidence for an absolute level density problem or a different behaviour of level densities (collective contributions) for deformed nuclei as opposed to spherical nuclei is found. The proposed level density formula allows to calculate a priori macroscopic ratios of level densities, e.g. at the groundstate and at the saddle point, removing this important parameter from the analysis of fissionability data. As a first application, the fissionability of a number of actinide nuclei at excitation energies a few MeV above the fission barrier is analysed.