Subthreshold neutron-induced fission of aligned ²³⁷Np nuclei

Abstract

The anisotropy in the angular distribution of fission fragments from neutron-induced fission of aligned ²³⁷Np has been measured as a function of neutron energy from 0.4 to about 2000 eV. The ²³⁷Np nuclei were aligned in thin layers of monocrystalline rubidium neptunyl nitrate, cooled to about 0.1 K. The interpretation of the group structure in the subthreshold fission cross section of ²³⁷Np by Strutinsky's double-humped deformation barrier implies that the spins of all resonances in one group are equal. The anisotropy values for the individual resonances in the first group around 40 eV agree with the fission channel (J^π; K) = (2⁺; 2), where K is the projection of spin J on the symmetry axis of the compound nucleus. Some resonance groups at higher energies, which could not be resolved into individual resonances, have also been studied; the information is rather inconclusive, but tends to favour fission channels (3⁺; 2) and (2⁺ ; 2).