Charge distributions for the photofission ofU235andU238with 12—30 MeV bremsstrahlung

A systematic study of the charge distribution for bremsstrahlung-induced photofission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with the end point energies ranging from 12 to 30 MeV was performed using direct $\gamma$-ray spectrometry of irradiated uranium samples or of fission product catcherfoils, and also employing chemical separation techniques. For both fissioning systems the width of the charge distribution was found to be practically independent of the average excitation energy and the values obtained are in very good agreement with those reported in the literature for low-energy fission. The deviation of the most probable charge $Z_p$ from the unchanged charge density value $Z_{\mathrm{UCD}}$ as a function of the fragment mass shows the influence of the 50-proton shell in the charge distributions and a higher charge-to-mass ratio of the light fragments independent of the compound nucleus excitation energy. For the necessary conversion of postneutron into preneutron masses, neutron emission curves, $\nu \left(m^{*}\right)$, were deduced from previously measured postneutron and provisional mass distributions. Calculations following the scission-point model of Wilkins et al. and the predictions of the empirical relation of Nethaway reproduce very well the experimentally determined $Z_p$ behavior, except in the mass region affected by the $Z=50\mathrm{}$ closed shell.