Mass distribution in the 50-, 60-, and 70-MeV bremsstrahlung-induced fission of Th-232

Abstract

The postneutron yields of various fission products in the mass regions of 77-153 have been determined in the 50-, 60-, and 70-MeV bremsstrahlung-induced fission of Th-232 by using a recoil catcher and an off-line gamma-ray spectrometric technique in the electron linac at the Pohang Accelerator Laboratory, Korea. The mass-yield distributions were obtained from the fission-product yield data using charge-distribution corrections. The fission yields of the present paper and the existing data from the Th-232(gamma, f) reaction at various energies are comparedwith those from the Th-232(n, f), the U-238(n, f), and the U-238(gamma, f) reactions. We observe that the yields of fission products for A = 133-134, A = 138-139, A = 143-144, and their complementary products in the above fissioning systems are higher than those of other fission products, which is explained based on the nuclear-structure effect. However, we observed that the yields of fission products for A = 133-134 were lower than those for A = 143-144 in the Th-232(gamma, f) reaction compared to those of the Th-232(n, f), U-238(n, f), and U-238(gamma, f) reactions. The yields of fission products for A = 133-134 increase, but those for A = 143-144 decrease with an increase in the excitation energy in the Th-232(gamma, f) and Th-232(n, f) reactions; however, those trends are reversed in the U-238(gamma, f) and U-238(n, f) reactions. The increasing or decreasing trends for the yields of fission products for A = 133-134 and A = 143-144 with the excitation energy in the Th-232(gamma, f), Th-232(n, f), U-238(n, f), and U-238(gamma, f) reactions are explained from the shell effect of the complementary products based on the static scission-point model and the standard I and II channels of bimodal fission. The peak-to-valley (P/V) ratio for the above fissioning systems also was obtained from the mass-yield distribution. The P/V ratio for the Th-232(gamma, f) and U-238(gamma, f) reactions at different energies from the present data and the existing literature data are interpreted to examine the role of excitation energy.