Neutron multiplicity measurements of Cf and Fm isotopes

Prompt neutrons in coincidence with the fission fragments from the spontaneous fission of ${}_{}{}^{250,252,254}{}_{}{}^{}\mathrm{Cf}$ and ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$ were measured inside a 75-cm-diameter, Gd-loaded liquid scintillation counter having a neutron-detection efficiency of about 78%. Measurements for ${}_{}{}^{256}{}_{}{}^{}\mathrm{Fm}$ were done just outside the counter with an efficiency of 31%. The kinetic energies of both fission fragments and the number of neutrons for each fission event were recorded. From these data, the fragment kinetic energies and masses and the neutron multiplicity distributions were determined for ${}_{}{}^{250,252,254}{}_{}{}^{}\mathrm{Cf}$ and ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$. Variances of neutron multiplicity distributions as a function of total fragment kinetic energy and the ratio of fragment masses have been calculated and are presented for all the nuclides studied.