The photonuclear cross sections for ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{234}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{237}{}_{}{}^{}\mathrm{Np}$, and ${}_{}{}^{239}{}_{}{}^{}\mathrm{Pu}$ have been measured from threshold up to 18 MeV. The source of radiation was the monoenergetic photon beam from the annihilation in flight of fast positrons. The branching among the neutron-producing reaction channels was determined by measuring the photofission prompt neutron multiplicities ν${¯}_p$. One interesting result is the complete absence of any (γ,2n) cross section for ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{234}{}_{}{}^{}\mathrm{U}$. The values of ν${¯}_p$(E) for ${}_{}{}^{234}{}_{}{}^{}\mathrm{U}$ agree with those measured with neutrons incident on ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$. The parameters of the giant dipole resonance deduced from the total photonuclear cross sections show that these nuclei have large static deformations, as expected. The integrated photofission cross sections are large (as are the absolute fission probabilities), and account for 60% to 80% of the total photonuclear absorption strength.

• Received 18 August 1986

DOI:https://doi.org/10.1103/PhysRevC.34.2201