Measurement of fission product yields in the quasi-mono-energetic neutron-induced fission of 238U

The cumulative yields of various fission products in the 6.35, 8.53, 9.35 and 12.52 MeV quasi-mono-energetic neutron induced fission of 238U have been determined by using the off-line γ-ray spectrometric technique. The mass chain yields were obtained from the fission product yields by using charge distribution correction. From the mass yield data, the peak-to-valley (P/V) ratio, the average value of light mass (〈AL〉), heavy mass (〈AH〉) and thus the average number of neutrons (〈υ〉expt) were obtained in the 238U(n,f) reaction for the above mentioned four neutron energies. The present and literature data in the 238U(n,f) and 238U (γ,f) reactions at various energies were compared to arrive at the following conclusions. (i) The yields of fission products for A=133-134, A=138-140, and A=143-144 and their complementary products in the 238U(n,f) and 238U(γ,f) reactions are higher than other fission products, which has been explained from the point of even-odd effect and standard I and standard II asymmetric mode of fission. (ii) The yields of symmetric products increase and thus the peak-to-valley (P/V) ratios decrease with excitation energy, whereas the 〈ν〉expt values increase with excitation energy. (iii) The variation of 〈AL〉,〈AH〉 and 〈υ〉expt values with excitation energy behave differently in between 238U(n,f) and 238U(γ,f) reactions, which may be due to the different types of reaction mechanism for the neutron and photon with 238U.