Measurement of prompt neutron polarization asymmetries in photofission of235,238U,239Pu, and232Th

Abstract

A photofission experiment was performed on targets of ${}^{235}$U, ${}^{238}$U, ${}^{239}$Pu, and ${}^{232}$Th using nearly 100$%$ linearly polarized, high-intensity ($\ensuremath{\sim}$10${}^{7}$ $\ensuremath{\gamma}/$s), and nearly monoenergetic $\ensuremath{\gamma}$-ray beams having energies between 5.6 and 7.3 MeV at the High Intensity $\ensuremath{\gamma}$-ray Source (HI$\ensuremath{\gamma}$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 ($\ensuremath{\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.