A photofission experiment was performed on targets of ${}^{235}$U, ${}^{238}$U, ${}^{239}$Pu, and ${}^{232}$Th using nearly 100$\%$ linearly polarized, high-intensity ($\sim$10${}^7$ $\gamma /$s), and nearly monoenergetic $\gamma$-ray beams having energies between 5.6 and 7.3 MeV at the High Intensity $\gamma$-ray Source (HI$\gamma$S). An array of 18 liquid scintillator detectors was used to measure prompt fission neutron polarization asymmetries. An asymmetry close to zero was found for ${}^{235}$U and ${}^{239}$Pu while a significant asymmetry ($\sim$0.5) was found for ${}^{238}$U and ${}^{232}$Th. A simplified model of near-threshold photofission has been developed in an attempt to explain the systematic difference between the even-even and even-odd targets. The results of a simulation, based on this model and using previous measurements of fission fragment angular distributions, are shown to accurately reproduce the essential features of the data.

• Received 12 November 2011

DOI:https://doi.org/10.1103/PhysRevC.85.014605