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 ${}^{232}$Th by using a recoil catcher and an off-line γ-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 ${}^{232}$Th(γ,$f$) reaction at various energies are compared with those from the ${}^{232}$Th($n$,$f$), the ${}^{238}$U($n$,$f$), and the ${}^{238}$U(γ,$f$) 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 ${}^{232}$Th(γ,$f$) reaction compared to those of the ${}^{232}$Th($n$,$f$), ${}^{238}$U($n$,$f$), and ${}^{238}$U(γ,$f$) 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 ${}^{232}$Th(γ,$f$) and ${}^{232}$Th($n$,$f$) reactions; however, those trends are reversed in the ${}^{238}$U(γ,$f$) and ${}^{238}$U($n$,$f$) 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 ${}^{232}$Th(γ,$f$), ${}^{232}$Th($n$,$f$), ${}^{238}$U($n$,$f$), and ${}^{238}$U(γ,$f$) 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 ${}^{232}$Th(γ,$f$) and ${}^{238}$U(γ,$f$) reactions at different energies from the present data and the existing literature data are interpreted to examine the role of excitation energy.

• Received 27 December 2011

DOI:https://doi.org/10.1103/PhysRevC.85.024623