Product yields for the photofission ofU238with 12-, 15-, 20-, 30-, and 70-MeV bremsstrahlung

Cumulative yields for about 40 mass chains and fractional independent yields of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{128}{}_{}{}^{}\mathrm{Sb}_{}^g{}_{}{}^{}$, ${}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^g{}_{}{}^{}$, ${}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^m{}_{}{}^{}$, ${}_{}{}^{132}{}_{}{}^{}\mathrm{I}_{}^g{}_{}{}^{}$, ${}_{}{}^{132}{}_{}{}^{}\mathrm{I}_{}^m{}_{}{}^{}$, ${}_{}{}^{134}{}_{}{}^{}\mathrm{I}$, ${}_{}{}^{134}{}_{}{}^{}\mathrm{Cs}$, ${}_{}{}^{135}{}_{}{}^{}\mathrm{Xe}$, and ${}_{}{}^{136}{}_{}{}^{}\mathrm{Cs}$ were determined for the photofission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with 12-, 15-, 20-, 30-, and 70-MeV bremsstrahlung. Changes in the characteristics of the mass distribution are studied. An enhanced yield in the mass region 133-134, decreasing with increasing end-point energy of the bremsstrahlung ($E_e$), is observed. From the determined independent yields the most probable charges $Z_p\left(E_e\right)$ are calculated using the value 0.85 for the width parameter $c$ of the charge distribution as deduced from our results for mass chain 134. The determined $Z_p\left(E_e\right)$ values are very well described by the empirical relation of Nethaway except for the mass chain 136. This discrepancy for mass chain 136, which decreases with increasing end-point energy of the bremsstrahlung, is attributed to an abnormal low yield of ${}_{}{}^{136}{}_{}{}^{}\mathrm{Cs}$. Fragment shell effects are discussed. From the isomeric ratios for ${}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^g{}_{}{}^{}-{}_{}{}^{131}{}_{}{}^{}\mathrm{Te}_{}^m{}_{}{}^{}$ and