Analysis of isotopic yield distributions inB−11andNe22-induced fission ofU238

First moments and variances of measured rubidium and cesium isotopic distributions in the fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ by ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$($E_{\mathrm{lab}}=85\mathrm{}$ MeV) and ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$ ($E_{\mathrm{lab}}=132\mathrm{} \mathrm{and} 162$ MeV) are analyzed in the framework of the statistical model for nuclear fission and of evaporation theory. Excellent agreement with the data is obtained for ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$-induced fission, assuming a compound-nuclear mechanism. The comparatively large isotopic variance observed in ${}_{}{}^{22}{}_{}{}^{}\mathrm{Ne}$-induced fission can be explained if moderate contributions from fission following direct reactions are taken into account.