Abstract
A study of photofission of $^{232}\mathrm{Th}$ and $^{238}\mathrm{U}$ was performed by using quasimonoenergetic, linearly polarized $\ensuremath{\gamma}$-ray beams from the High Intensity $\ensuremath{\gamma}$-ray Source at Triangle Universities Nuclear Laboratory. The prompt-photofission neutron polarization asymmetries, neutron multiplicities, and the photofission cross sections were measured in the near-barrier energy range of 4.3 to 6.0 MeV. This data set constitutes the lowest energy measurements of those observables to date using quasimonoenergetic photons. Large polarization asymmetries are observed in both nuclei, consistent with the $E1$ excitation as observed by another measurement of this kind made in a higher-energy range. Previous experimental evidence of a deep third minimum in the $^{238}\mathrm{U}$ fission barrier has been identified as an accelerator-induced background.
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