Measurement of pre- and post-fission neutron emission at moderate excitation energies

Abstract

Fission fragment mass-energy distributions and the coincident neutron velocity distributions have been measured at 0° and 90° with respect to the fragment direction. The fissioning systems studied were ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$, ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, and ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ bombarded with 45 MeV $\alpha$ particles and ${}_{}{}^{226}{}_{}{}^{}\mathrm{Ra}$ bombarded with 16 MeV protons. From the above measured quantities the number of pre-fission neutrons and the number of post-fission neutrons were obtained as a function of fragment mass and total fragment kinetic energy. The average number of pre-fission neutrons emitted is 2.9±0.9 for the ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ target, 3.3±1.5 for ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$, 3.6±1.6 for ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, and 2.7±0.8 for ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$. The average number of post-fission neutrons emitted by these systems is 4.4±0.3, 4.2±0.7, 4.6±0.7, and 5.1±0.3, respectively. (The values reported for pre-fission neutrons include also contributions from scission neutrons.) The number of pre-fission neutrons from the ${}_{}{}^{226}{}_{}{}^{}\mathrm{Ra}$ + 16 MeV proton system is 0.5±0.3. The number of post-fission neutrons is 3.6±0.4. The number of neutrons emitted from the ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$+45 MeV $\alpha$ particle system is 3.6±0.2. The number of post-fission neutrons as a function of fragment mass and total kinetic energy which were obtained by the direct neutron measurements of this work are lower and are more consistent with calculated total energy balances than the values obtained by the indirect method based on the simultaneous measurement of both the kinetic energy and the velocity of the fission fragments. Our results for the average number of pre-fission neutrons are compared with calculations based on the conventional expression for $\frac{{\Gamma }_f}{{\Gamma }_n}$. The agreement between the calculated results and our experimental results is satisfactory. However, the calculations predict spallation cross sections which are much larger than those obtained experimentally by radiochemical methods.