Fission fragment properties in fast-neutron-induced fission of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$

We report on a four-parameter measurement of the kinetic energies E and the velocities v of correlated fragments in fast-neutron-induced fission of ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$. The influence of excitation energy on the important fragment properties such as mass, kinetic energy, and prompt neutron emission have been investigated experimentally at 0.80 and 5.55 MeV neutron energy. Our results include mean values of fragment properties before and after neutron evaporation, e.g., of fragment velocities and masses, total kinetic energies, and the respective variances. Also given are the distributions of fragment mass, of $E_k$,tot, the variance of $E_k$,tot, as well as the number of prompt fission neutrons as a function of fragment mass. These results show that shell effects are strong near threshold at $E_{\mathrm{n}=0.80\mathrm{}}$ MeV, but decrease significantly at the higher excitation energy, in qualitative agreement with the model of Wilkins, Chasman, and Steinberg. However, the observed increase in the number of prompt fission neutrons, which appears only in the heavy fragment, cannot be explained by this model.