Speaker
Description
Measurements of photofission cross sections utilizing quasimonoenergetic $\gamma$-ray beams historically rely on neutron detection, producing photoneutron data that must be sorted into the relevant reaction channels: ($\gamma$,n), ($\gamma$,2n), ($\gamma$,3n), and ($\gamma$,f). This deconvolution is a model-dependent process that has led to longstanding systematic discrepancies in photonuclear measurements from LLNL (USA) and Saclay (France), the two main laboratories performing these measurements. In order to resolve these discrepancies for a new photonuclear data evaluation, we have performed new photofission measurements
utilizing fission ionization chambers – an approach that does not depend on neutron detection. An experimental campaign was carried out at the High Intensity Gamma-ray Source (HIGS) facility at Triangle Universities Nuclear Laboratory in spring 2025. Fission ionization chambers loaded with highly enriched spectroscopic samples of $^{235}$U, $^{238}$U, $^{239}$Pu, and $^{240}$Pu were simultaneously irradiated with the quasimonoenergetic HIGS $\gamma$-ray beam for 45 incident energies from 7 to 19 MeV. The absolute $\gamma$-ray beam flux was measured with a novel mirror-paddle system, in addition to $^{197}$Au($\gamma$,n) activation measurements at select energies. The $^{238}$U($\gamma$,n) reaction cross section was also measured at select energies via activation and post-irradiation $\gamma$-ray spectroscopy. Preliminary photofission cross sections will be presented and compared with literature data.
