Mass distribution in the quasi-mono-energetic neutron-induced fission of 238U

The yields of various fission products in the 3.72, 5.42, 7.75 and 10.09 MeV quasi-mono-energetic neutron-induced fission of 238U fission have been determined using off-line γ-ray spectrometric technique. The mass-chain yields were obtained from their fission product yields using charge distribution correction. The peak-to-valley (P/V) ratio, the average value of light mass (image), heavy mass (image) and the average number of neutrons (image) at four different neutron energies of present work and at other energies from literature in the 238U(n, f) were obtained from the mass yield data. The present and the existing literature data in 238U(n, f) at various energies were compared with the similar data in the 232Th(n, f). The following observations were obtained: (i) The mass yield distribution in 238U(n, f) is double humped unlike in the 232Th(n, f), where it is triple humped. (ii) The yields of fission products for image, image, and image and their complementary products in the 238U(n, f) at four neutron energies are higher than those of other fission products as in the 232Th(n, f). The fine structure in the mass yield distribution has been explained from the point of view of standard I and standard II asymmetric mode of fission besides even–odd effect. (iii) The yields of fission products for image are higher in the 238U(n, f) than in the 232Th(n, f), whereas it is reversed for image. This has been explained from the point of shell combination of complementary fragments. (iv) In both the 238U(n, f) and 232Th(n, f), the yields of symmetric products increase with excitation energy, which causes the decrease of peak-to-valley (P/V) ratio with excitation energy. (v) At the same excitation energy, the yields of symmetric products are lower in the 238U(n, f) than in the 232Th(n, f). This causes higher value of P/V in 238U(n, f) than in 232Th(n, f) for same excitation energy. (vi) The approach of symmetric split with excitation energy is slower in the 238U(n, f) than in the 232Th(n, f). This is due to the different type of potential energy surface for the fissioning system 233Th⁎ than in the fissioning system 239U⁎.