The photoneutron cross sections $\sigma (\gamma ,n)$ and $\sigma (\gamma ,2n)$, and total photofission cross sections $\sigma (\gamma ,F)$ have been measured for ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ from threshold to 18.3 MeV using monoenergetic photons from the annihilation in flight of fast positrons and neutron-multiplicity detection in an efficient $4\pi$ neutron detector. Use of the ring-ratio technique allowed both the average photofission neutron energy for each nucleus to be obtained as a function of photon energy and, for ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, the determination of the partial cross sections for first-chance $\sigma (\gamma ,f)$ and second-chance $\sigma (\gamma ,nf)$ photofission as well. Information extracted from the data includes integrated cross sections and their moments, giant-resonance parameters, deformation and radius parameters, and relative and absolute neutron and fission probabilities.

NUCLEAR REACTIONS ${}_{}{}^{235,236,238}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ ($\gamma$,$n$,$2n$,$F$), $E_{\gamma }=5-18.3\mathrm{}$ MeV; measured $4\pi$ neutron yield, neutron multiplicities, and average energies for monoenergetic photons; $\sigma (E_{\gamma },1n)$, $\sigma (E_{\gamma },2n)$, $\sigma (E_{\gamma },F)$, integrated cross sections and moments, GDR parameters, nuclear shape parameters, neutron and fission probabilities.

• Received 21 November 1979

DOI:https://doi.org/10.1103/PhysRevC.21.1215