Post-Neutron Mass Yield Distribution in the Thermal Neutron–Induced Fission of ²³⁹Pu

Mass chain yield distribution has been done in the thermal neutron–induced fission of ²³⁹Pu by measuring the cumulative yields of various fission products within the mass range of 78 to 159 and the independent yields of a few products. An off-line gamma-ray spectrometric technique was used to measure the gamma-ray activities of the fission products. From the measured values of the cumulative yields, the post-neutron mass chain yield distribution was obtained after applying the charge distribution correction. Data from the present and earlier work of our laboratory in the ²³⁹Pu(n_th,f) reaction were compared with similar data of ^238,241Pu(n_th,f) and ²⁴⁰Pu(n,f) reactions, and it was found that the fine structures of the mass yield distributions are similar. The mass yield distribution in the ²³⁹Pu(n_th,f) reaction was also compared with those of ²²⁹Th(n_th,f) and ²⁵²Cf(SF) reactions to examine the effect of charge and mass difference of the fissioning systems on the mass yield distribution. It was found that the asymmetric standard I mode of fission is favorable in the ^238,239,241Pu(n_th,f) and ²⁴⁰Pu(n,f) reactions whereas the standard II mode is favorable in the ²²⁹Th(n_th,f) and ²⁵²Cf(SF) reactions.