Measurement of prompt neutron polarization asymmetries in photofission of 235 , 238 U, 239 Pu, and 232 Th

A photofission experiment was performed on targets of U-235, U-238, Pu-239, and Th-232 using nearly 100% linearly polarized, high-intensity (similar to 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 U-235 and Pu-239 while a significant asymmetry (similar to 0.5) was found for U-238 and Th-232. 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.