Charge distribution for the photofission ofU238with 20-MeV bremsstrahlung

Fractional independent or cumulative chain yields of 35 fission products were determined for the photofission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with 20-MeV bremsstrahlung, using direct $\gamma$ spectrometry of irradiated uranium samples, $\gamma$-ray spectrometry of fission product catcher foils, and chemical sparation techniques. For the mass chains 131-136 the width parameter $c$ of the charge distribution was obtained. An average value of 0.93 ± 0.06 was deduced for $c$. A comparison of this $c$ value with the results for other low-energy fissioning systems indicates that the width of the charge distribution is practically independent of the excitation energy of the compound nucleus. Using the average $c$ value, the most probable charges $Z_p$ (20 MeV) were calculated for 8 light and 12 heavy mass chains. A comparison of the deduced $Z_p$ (20 MeV) behavior with the expectations of the unchanged charge density hypothesis shows the higher charge-to-mass ratio of the light fragments compared to the heavy ones and the influence of the closed 50-proton shell on the charge distribution. The $Z_p$ (20 MeV) values determined are very well reproduced by the empirical relation of Nethaway except in the vicinity of the $Z=50\mathrm{}$ line.