Mass yield distributions and fission modes in photofission ofU238below 20 MeV

Abstract

Photon activation technique is used to obtain photofission mass distributions of $^{238}\mathrm{U}$ after irradiation in a bremsstrahlung beam with end-point energies 19.5, 29.1, 48.3, and 67.7 MeV. The weights of the symmetric (SL) and asymmetric (STI and STII) modes in the mass distributions of photofission of $^{238}\mathrm{U}$ and neutron-induced fission of ${}^{235,238}\text{U}$ at the average excitation energies of up to 20 MeV are calculated on the basis of the obtained results and other available data. The weight of the SL mode increases by about an order of magnitude as the average excitation energy increases from 8.5 to 17 MeV, while the weight of the STI mode decreases by two times, and the weight of the STII mode slightly increases. It is shown that the peak-to-valley ratios in photofission of $^{238}\mathrm{U}$ and neutron-induced fission of $^{235}\mathrm{U}$ are very close across almost the whole studied energy range. A similar conclusion is made concerning the behavior of asymmetric and symmetric fission modes in photofission of $^{238}\mathrm{U}$ and neutron-induced fission of $^{235}\mathrm{U}$ and $^{238}\mathrm{U}$.