Prompt neutron polarization asymmetries in photofission ofTh232,U233,235,238,Np237, andPu239,240

Abstract

Photofission experiments were performed on targets of $^{232}\mathrm{Th}$, $^{233,235,238}\mathrm{U},$ $^{237}\mathrm{Np}$, and $^{239,240}\mathrm{Pu}$ using nearly 100% linearly polarized, high intensity ($\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{6}\ensuremath{\gamma}/$s), and nearly-monoenergetic $\ensuremath{\gamma}$-ray beams having energies between 5.3 and 7.6 MeV at the High Intensity $\ensuremath{\gamma}$-ray Source (HI$\ensuremath{\gamma}$S) located at Duke University and Triangle Universities Nuclear Laboratory. An array of 12--18 liquid scintillator detectors was used to measure prompt fission neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, the differences between the in-plane and out-of-plane yields divided by their sums, were measured. Asymmetries close to zero were found for ${}^{233,235}$U, $^{237}\mathrm{Np}$, and ${}^{239}$Pu while significant asymmetries ($\ensuremath{\sim}$0.2--0.5) were found for $^{232}\mathrm{Th}$, ${}^{238}$U, and ${}^{240}$Pu. Predictions based on previously measured photofission fragment angular distributions combined with a model of prompt neutron emission agree well with the experimental results.