Mass-yield distributions of fission products from photofission of Th-232 induced by 45-and 80-MeV bremsstrahlung

Abstract

The mass-yield distributions of various fission products in the 45- and 80-MeV bremsstrahlung-induced fission of Th-232 have been determined by using a recoil catcher and an offline 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 peak-to-valley (P/V) ratio, the average value of light mass (< AL >) and heavy mass (< AH >), and the average number of neutrons (<nu >) in the bremsstrahlung-induced fission of Th-232 at different excitation energies were obtained from the mass-yield data. From the present measurements and the existing data from the Th-232(gamma, f) reaction and those from the Th-232(n, f) reaction at various energies, the following observations were obtained: (i) The mass-yield distributions in the (232)(gamma, f) reaction at various energies are triple humped, similar to those of the Th-232(n, f) reaction. (ii) The yields of fission products for A = 133-134, A = 138-139, and A = 143-144 and their complementary products in the Th-232(gamma, f) reaction are higher than those of other fission products due to the nuclear structure effect. (iii) The yields of symmetric fission products for A = 133-134 and their complementary products in the Th-232(gamma, f) reaction are lower than those in the Th-232(n, f) reaction, whereas those for A = 143-144 and their complementary products are reversed. (iv) The result of increasing of the symmetric product yield causes the decreasing of the peak-to-valley ratio with increasing the excitation energy. However, it is surprising to see that the increasing trends for the symmetric products yields and the decreasing trends for the P/V ratio in the Th-232(gamma, f) and Th-232(n, f) reactions are not similar but those in the U-238(gamma, f) and U-238(n, f) reactions are similar to each other. (v) The average values of < AL >, < AH >, and <nu > at different excitation energies in the Th-232(gamma, f) and Th-232(n, f) reactions are similar but those in the U-238(gamma, f) and U-238(n, f) reactions are different.