UNC-CH PHYSICS AND ASTRONOMY PhD DEFENSE
Jack A. Silano, UNC-CH
“Sub-barrier Photofission Measurements in 238U and 232Th”
A study of photofission of 232Th and 238U was performed using quasi-monoenergetic, linearly-polarized γ-ray beams from the High Intensity γ-ray source at TUNL. The prompt photofission neutron polarization asymmetries, neutron multiplicities and the photofission cross sections were measured in the sub-barrier energy range of 4.3-6.3 MeV. This data set constitutes the lowest energy measurements of those observables to date. Large polarization asymmetries are observed in both nuclei, consistent with the only other measurement of this kind made at a higher energy range. Over the newly probed energy range the asymmetries, multiplicities and cross sections all vary smoothly, revealing no new resonant fission processes. The present cross sections are consistent with a triple-humped 232Th fission barrier and a double-humped 238U fission barrier. Previous experimental evidence of a third minimum in the 238U fission barrier has been identified as an accelerator-induced background. Applications of the present data will also be discussed.