Investigation of γ -Ray Emission Preceding Isomeric Fission of U236

Measurements were made to detect $\ensuremath{\gamma}$ rays preceding isomeric fission in $^{236}\mathrm{U}$ induced by eV-range neutrons captured in $^{235}\mathrm{U}$. A limit of \ensuremath{\le} 6 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ was placed on the ratio of the rate of isomeric fission events with prefission $\ensuremath{\gamma}$ 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 ${3}^{\ensuremath{-}}$ and ${4}^{\ensuremath{-}}$ states in $^{236}\mathrm{U}$.