Mass Yield Distribution in the Quasi-Mono-Energetic Neutron-Induced Fission of ²³⁸U

Fission product cumulative yields within the mass ranges of 83 to 117 and 123 to 153 have been measured in the 4.93- and 8.31-MeV quasi-mono-energetic neutron-induced fission of ²³⁸U by using an off-line gamma-ray spectrometric technique. From the cumulative yields, the mass chain yields were obtained by applying charge distribution correction. From the mass yield data, the peak-to-valley (P/V) ratio, the average values of light mass <A_L> and heavy mass <A_H>, and the average number of neutrons <υ> were obtained. The data from the present work and literature in the ²³⁸U(n,f) reaction at various energies were compared with similar data in the ²³⁸U(γ,f) reaction. In both the reactions, the mass yield distributions are asymmetric and double humped. The yields of fission products for A = 133 to 134, A = 138 to 140, and A = 143 to 144 and their complementary products are higher than those of other fission products because of the nuclear structure effect. With the increase of excitation energy, the yield of the symmetric product increases, which causes a decrease of the P/V ratio. The <υ> value also increases with excitation energy in a similar way in both the reactions. However, it is surprising to see a different changing trend of <A_L> and <A_H> values with an increase of excitation energy between the ²³⁸U(n,f) and ²³⁸U(γ,f) reactions.