Experimental Determination of Photofission Neutron Multiplicities for ²³⁵U, ²³⁶U, ²³⁸U, and ²³²Th Using Monoenergetic Photons

The prompt neutron multiplicities for photofission of the four isotopes ²³⁵U, ²³⁶U, ²³⁸U, and ²³²Th have been measured with monoenergetic photons over the energy range from 5.5 to 18 MeV using the annihilation in flight of fast positrons. The delayed neutron yield has been measured for all four isotopes at 10.9- and 16.8-MeV photon energies. The ratio of first- to second-chance fission has been measured as a function of energy up to 17-MeV excitation energy for ²³⁶U and ²³⁸U photofission.