The cumulative yields of various fission products within the 77–153 mass regions in the 2.5-GeV bremsstrahlung-induced fission of ${}^{232}\mathrm{Th}$ have been determined by using the recoil catcher and an off-line γ-ray spectrometric technique at the Pohang Accelerator Laboratory, Korea. The mass-yield distributions were obtained from the cumulative yields after charge-distribution corrections. The peak-to-valley ($\mathrm{P}/\mathrm{V}$) ratio, the average value of light mass ($\langle A_L\rangle$) and heavy mass ($\langle A_H\rangle$), and the average postfission number of neutrons (${\langle v\rangle }_{\mathrm{expt}}$) were obtained from the mass yield of the ${}^{232}\mathrm{Th}(\gamma ,f)$ reaction. The present and literature data in the ${}^{232}\mathrm{Th}(\gamma ,f)$ reaction were compared with the similar data in the ${}^{238}\mathrm{U}(\gamma ,f)$ reaction at various excitation energies to examine the role of potential energy surface and the effect of standard I and standard II asymmetric modes of fission. It was found that (i) even at the bremsstrahlung end-point energy of 2.5 GeV, the mass-yield distribution in the ${}^{232}\mathrm{Th}(\gamma ,f)$ reaction is triple humped, unlike ${}^{238}\mathrm{U}(\gamma ,f)$ reaction, where it is double humped. (ii) The peak-to-valley ($\mathrm{P}/\mathrm{V}$) ratio decreases with the increase of excitation energies. However, the $\mathrm{P}/\mathrm{V}$ ratio of the ${}^{232}\mathrm{Th}(\gamma ,f)$ reaction is always lower than that of the ${}^{238}\mathrm{U}(\gamma ,f)$ reaction due to the presence of a third peak in the former. (iii) In both the ${}^{232}\mathrm{Th}(\gamma ,f)$ and ${}^{238}\mathrm{U}(\gamma ,f)$ reactions, the nuclear structure effect almost vanishes at the bremsstrahlung end-point energies of 2.5–3.5 GeV.

• Received 13 September 2017
• Revised 11 December 2017

DOI:https://doi.org/10.1103/PhysRevC.97.014614