Effect of Pu240 Compound-Nucleus State on the Fission-Fragment Mass and Kinetic Energy Distributions
Fission-fragment mass and kinetic energy distributions have been obtained for fission of Pu239 induced by neutrons filtered through beryllium and by neutrons filtered through samarium. The beryllium filter enhances the contribution of the negative-energy resonance level to the fission cross section, and the samarium filter enhances the contribution of the 0.297-eV level. Surface-barrier detectors were used for the simultaneous measurement of both the fragment energies. Absolute fragment energies were calculated by using mass-dependent pulse-height energy relations. The average total kinetic energy of the fragments produced in the fission induced by samarium-filtered neutrons was observed to be 0.75 +/- 0.05 MeV greater than in the case of fission induced by beryllium-filtered neutrons. This result, when combined with the results of other experiments, implies J=0+ for the negative-energy level and J=1+ for the 0.297-eV level of Pu239. The two mass distributions are similar except for a difference in the symmetric fission yield. This difference again implies the same spin assignments as above. The absolute average total kinetic energies were determined with somewhat less accuracy and are found to be 173.0 +/- 1.5 and 173.7 +/- 1.5 MeV for fissions induced by beryllium- and samarium-filtered neutrons, respectively, as directly measured, and 175.8 +/- 1.5 and 176.5 +/- 1.5 MeV, respectively, after correction for neutron emission.