Mass-yield distributions of fission products in bremsstrahlung-induced fission of Th232

Abstract

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 \ensuremath{\gamma}-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 ($\ensuremath{\langle}{A}_{L}\ensuremath{\rangle}$) and heavy mass ($\ensuremath{\langle}{A}_{H}\ensuremath{\rangle}$), and the average postfission number of neutrons (${\ensuremath{\langle}v\ensuremath{\rangle}}_{\mathrm{expt}}$) were obtained from the mass yield of the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction. The present and literature data in the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction were compared with the similar data in the $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ 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 $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction is triple humped, unlike $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ 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 $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction is always lower than that of the $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reaction due to the presence of a third peak in the former. (iii) In both the $^{232}\mathrm{Th}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ and $^{238}\mathrm{U}(\ensuremath{\gamma},\phantom{\rule{0.16em}{0ex}}f)$ reactions, the nuclear structure effect almost vanishes at the bremsstrahlung end-point energies of 2.5--3.5 GeV.