Near threshold photofission and photoneutron emission in 238U and 232Th

Bremsstrahlung photons produced by electrons from the University of Illinois superconducting microtron have been used to induce photofission and photoneutron emission near the fission and neutron emission thresholds in the even-even nuclei 238U and 232Th. Coincident detection of the reaction products and momentum analysis of the post bremsstrahlung electrons in the bremsstrahlung monochromator defined the photon energy to within 1.0-1.7 percent (full width) from 5 to 8 MeV. The time of flight of photoneutrons and fission neutrons was measured and the two components of the neutron yield were separated on the basis of their characteristic energy spectra. The photofission cross sections were obtained from the observed fission neutrons using values of anti ν, the fission neutron multiplicity, from the literature. (auth)