Competition between neutron emission and fission for234U,236U,240Pu and242Pu compound nuclei

Abstract

In recent years the insufficiency of the s tandard level density expression has been demonstrated (1) and it was found necessary to derive the compound level densities directly from the spectrum of the single-particle levels (~). This approach automatically accounts for the effect of nuclear shells and in addition it allows to take into account the effects of pairing in a na tura l way (3). The problem of obtaining reliable leveI densities and thereby decay widths can therefore be solved if realistic single-particle spectra for the cases of interest are available. This approach seems part icular ly useful in the actinide nuclei, where the existence of a two-peaked barrier is a result of the variation with deformation in the spacings of single-particle levels near the Fermi surfaces (a). I t is now possible, if one knows the single-particle spectra at the relevant deformations (first minimum, fist maximum, second minimum, second maximum) to calculate by a self-consistent statistical model neutron, gamma and fission decay widths (s.s). One of the results of the previous calculations is the existence of a bumplike structure in the ratio Fn/F I of the neutron emission to fission widths at about 5 MeV above the neutron threshold. This structure is due to the different singlc-particle densities relevant for the neutron and fission channels and depends on the height of the fission barrier B relative to the neutron binding energy S; however the calculated structure persists as long as 0 < S B < 2 MeV. Moreover a similar s t ructure has been deduced for 241Cm from the excitation functions of the reactions 23spu(a, 2n) and 241Am(p, 2n) and should show up for a large number of nuclei (s).