Product yields for the photofission ofU235with 12-, 15-, 20-, 30-, and 70-MeV bremsstrahlung

The mass distributions for the photofission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ using bremsstrahlung with end-point energy of 12, 15, 20, 30, and 70 MeV were determined by $\gamma$ spectrometry of fission product catcher foils. Smoothly varying curves without fine structure were obtained. A third hump in the symmetric region is not present. Several fractional independent chain yields in the mass region 126-140 were measured and the corresponding most probable charges $Z_p\left(E_e\right)$ were deduced. A comparison with the charge expected from the unchanged charge density hypothesis is made and an influence of the 50-proton shell on the behavior of the $Z_p$ function is observed. Except in this mass region the determined $Z_p\left(E_e\right)$ values are very well described by the empirical relation of Nethaway. From the isomeric ratios of ${}_{}{}^{126}{}_{}{}^{}\mathrm{Sb}_{}^g{}_{}{}^{}-{}_{}{}^{126}{}_{}{}^{}\mathrm{Sb}_{}^m{}_{}{}^{}$, ${}_{}{}^{128}{}_{}{}^{}\mathrm{Sb}_{}^g{}_{}{}^{}-{}_{}{}^{128}{}_{}{}^{}\mathrm{Sb}_{}^m{}_{}{}^{}$, and ${}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^g{}_{}{}^{}-{}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^m{}_{}{}^{}$ average initial fragment spins are calculated using a statistical model analysis. The values increase with increasing end-point energy of the bremsstrahlung.