Prompt Neutron Emission from Spontaneous-Fission Modes ofCf252

Abstract

The number of prompt neutrons emitted and the velocities of the fragment pairs have been measured for individual spontaneous fissions of ${\mathrm{Cf}}^{252}$. The time-of-flight measurements of the fragment velocities have sufficient resolution to provide a good determination of the mode of fission as characterized by the total kinetic energy $E_K$ and the mass ratio $R_A$ of the fragments. The neutrons are detected with high efficiency in a large cadmium-loaded liquid scintillator. It is found that the dependence of the average number of neutrons per fission, $\overline{\nu }$, on the parameters $E_K$ and $R_A$ may be approximated by a plane $\overline{\nu }(E_K, R_A)$ over the region that includes the majority of the fission events. The slopes that specify the orientation of this plane are determined to be $\frac{\partial \overline{\nu }(E_K, R_A)}{\partial E_K}=-0.143\mathrm{}\pm 0.020$ (neutrons/fission)/Mev and $\frac{\partial \overline{\nu }(E_K, R_A)}{\partial R_A}=-6.3\mathrm{}\pm 1.1$ (neutrons/fission)/unit mass ratio. The value of the first slope indicates that the average total excitation energy of the fragments, required for the emission of one more neutron on the average, is 7.0±1.0 Mev. From this number and the measured dependence of $\overline{\nu }$ on mass ratio, the average excitation energy ${\overline{E}}_X$ of the fragments is determined as a function of the mass ratio. This function ${\overline{E}}_X\left(R_A\right)$ and the measured dependence ${\overline{E}}_K\left(R_A\right)$ determine the average prompt energy of fission as a function of mass ratio. The widths of the neutron-number distributions have been obtained as functions of $E_K$ and $R_A$. The data do not support the statistical theory of fission proposed by Fong.