Investigation ofγ-Ray Emission Preceding Isomeric Fission ofU236

Abstract

Measurements were made to detect $\gamma$ rays preceding isomeric fission in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ induced by eV-range neutrons captured in ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$. A limit of ≤ 6 × ${10}^{-5\mathrm{}}$ was placed on the ratio of the rate of isomeric fission events with prefission $\gamma$ rays to the rate for prompt fission events. This experiment provides direct evidence that the penetration of the outer barrier is much greater than that for the inner barrier for $3^{-}$ and $4^{-}$ states in ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$.