Photofission of heavy nuclei from 0.2 to 3.8 GeV

Abstract

Total photofission cross sections for ${}^{237}\mathrm{Np},$ ${}^{233}\mathrm{U},$ ${}^{235}\mathrm{U},$ ${}^{238}\mathrm{U},$ ${}^{232}\mathrm{Th},$ and ${}^{\mathrm{nat}}\mathrm{Pb}$ have been measured simultaneously, using tagged photons in the energy range ${E}_{\ensuremath{\gamma}}=0.17--3.84 \mathrm{GeV}$ in Hall B at Jefferson Lab. The fission fragments were detected using parallel-plate avalanche detectors. Our results show that, of these nuclei, ${}^{237}\mathrm{Np}$ has the largest photofission cross section per nucleon, and that the photofission cross section for ${}^{238}\mathrm{U}$ relative to that for ${}^{237}\mathrm{Np}$ is about 80% over the entire energy range. The relative photofission cross sections per nucleon increase with the fissility parameter ${Z}^{2}/A,$ approaching an apparent asymptote at the ${Z}^{2}/A$ value for ${}^{237}\mathrm{Np},$ and they do not depend strongly on the incident photon energy over this entire energy range. These results, together with a comparison of the ${}^{237}\mathrm{Np}$ photofission cross section per nucleon with total photoabsorption data, indicate that the photofission probability for ${}^{237}\mathrm{Np}$ is almost unity. If we make this assumption, we observe a significant shadowing of the interior nucleons starting below 1.5 GeV, so that the photoabsorption cross section decreases by 20% near 4 GeV.