Precision Measurements of Correlated Energies and Velocities ofCf252Fission Fragments

Correlated energies and velocities of single fission fragments from the spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ have been measured. Absolute energies and times of flight are obtained with estimated accuracies of ±0.5% from direct comparison measurements with 30- to 120-MeV ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}$, ${}_{}{}^{81}{}_{}{}^{}\mathrm{Br}$, and ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}$ ions from the tandem Van de Graaff accelerator. The post-neutron-emission kinetic parameters obtained from this experiment for ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ fragments are compared with pre-neutron-emission quantities obtained from double-time-of-flight experiments of Whetstone and of Milton and Fraser. Of special interest is the fine structure which appears in the post-neutron-emission mass distribution. It is shown that this structure reflects the fine structure observed in the pre-neutron-emission mass distribution, which in turn is known to reflect certain energetically preferred even-even fragment configurations in ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ spontaneous fission. Within the ≈2.5% mass resolution of the present experiment, no additional fine structure appears as a result of neutron boil-off to specific fragment masses. The number of neutrons $\nu \mathrm{}\left(M\right)\mathrm{}$ as a function of fragment mass has been calculated from the pre- and post-neutron-emission mass distributions. In an appendix, a "universal" energy calibration procedure for solid-state detectors for fission fragments is given; this procedure is based on the present absolute fragment energy determinations and takes into account the mass dependence of the pulse-height-versus-energy relation.