Near-barrier Photofission in $^{232}$Th and $^{238}$U

TL;DR

This study measures photofission properties of $^{232}$Th and $^{238}$U using polarized gamma-ray beams near the fission barrier, providing new low-energy data on neutron asymmetries, multiplicities, and cross sections.

Contribution

It presents the first low-energy measurements of photofission observables with quasi-monoenergetic photons for these nuclei, revealing large polarization asymmetries and clarifying background effects.

Findings

Large polarization asymmetries observed in both nuclei.

Data consistent with E1 excitation mode.

Deep third minimum in $^{238}$U fission barrier identified as background.

Abstract

A study of photofission of $^{232}$Th and $^{238}$U was performed using quasi-monoenergetic, linearly-polarized $\gamma$-ray beams from the High Intensity $\gamma$-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 4.3-6.0 MeV. This data set constitutes the lowest energy measurements of those observables to date using quasi-monoenergetic 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 $^{238}$U fission barrier has been identified as an accelerator-induced background.