Very low energy photofission ofU238

Abstract

The photofission cross section of $^{238}\mathrm{U}$ was measured in the 2.75- to 5.75-MeV range. Cross sections as small as 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ b were detected. A shelf was observed which had been predicted to occur in the cross section owing to the dominance of radiative decay of levels in the second well. Barrier penetrabilities for both inner and outer barriers were determined from these measurements and together with penetrabilities from other measurements were used to infer a potential shape for $^{238}\mathrm{U}$. The total $\ensuremath{\gamma}$-ray strength function in the second well at 4.25 MeV was found to be 1.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$. From comparison of expected and measured cross sections at the lowest energies, the density of levels effective in photofission at 3 MeV was found to be greater than 4 per MeV.NUCLEAR REACTIONS, FISSION Measured photofission cross section for $^{238}\mathrm{U}$ in the 2.75-6-MeV interval. Deduced details of fission barrier shape and $\ensuremath{\gamma}$-ray decay properties in both inner and outer wells.