Post-Neutron Mass Yield Distribution in the Thermal Neutron–Induced Fission of ²³⁵U

The cumulative and independent yields of various fission products within the mass ranges of 78 to 108 and 123 to 155 have been measured in the thermal neutron–induced fission of ²³⁵U by using an off-line gamma-ray spectrometric technique. The post-neutron mass yield distribution was obtained from the cumulative yields after applying the charge distribution correction. The data from present and earlier work of our laboratory in the ²³⁵U(n_th,f) reaction were compared with similar data of ²²⁹Th(n_th,f), ²⁴⁵Cm(n_th,f), and ²⁵²Cf(SF) reactions to examine the fine structure in the mass yield distribution for four different even-even fissioning systems with charge of 90 to 98. The comparison shows that the fine structure in the mass yield distribution depends on spherical and deformed neutron shell combinations. The shell combination favors the standard I asymmetric mode of fission in the ²³⁵U(n_th,f) and ²⁴⁵Cm(n_th,f) reactions, whereas it favors the standard II asymmetric mode of fission in the ²²⁹Th(n_th,f) and ²⁵²Cf(SF) reactions.