Mass Yield Distributions in the 13.75-MeV Quasi-Mono-Energetic Neutron-Induced Fission of ²³²Th and ²³⁸U

The cumulative yields of various fission products in the 13.75-MeV quasi-mono-energetic neutron-induced fission of ²³²Th and ²³⁸U have been measured using an off-line γ-ray spectrometric technique. The mass chain yields were obtained from the cumulative yields by applying charge distribution correction. From the mass yield data, the peak-to-valley (P/V) ratio, the average value of light mass (< A_L>), heavy mass (< A_H>), and the average number of neutrons (<ν>) were obtained.

The data from the present work and literature at various neutron energies were compared between the ²³²Th(n, f) and ²³⁸U(n, f) reactions, and the following facts were observed. (1) The mass yield distribution in the ²³⁸U(n, f) reaction is double-humped unlike in the ²³²Th(n, f) reaction, where it is triple-humped. This is due to the different type of potential energy surface for the fissioning system ²³³Th* than in ²³⁹U*. (2) In both reactions, the yields of fission products for A = 133 to 134, A = 138 to 139, and A = 143 to 144 and their complementary products are higher than those of other fission products, which has been explained based on the standard I and standard II asymmetric modes of fission besides the even-odd effect. (3) The yields for A = 133 to 134 are higher in the ²³⁸U(n, f) reaction, whereas it is higher for A = 143 to 144 in the ²³²Th(n, f) reaction, which has been explained based on the presence of shells in the complementary fragments. (4) In both reactions, the yield of symmetric product increases with excitation energy, which causes a decreasing trend in the P/V ratio. (5) At the same excitation energy, the yield of symmetric products is lower in the ²³⁸U(n, f) reaction than in the ²³²Th(n, f) reaction, which causes a higher P/V ratio in the former than in the later.