Speaker
Description
Isomeric yield ratios (IYRs) are key observables in nuclear fission, as they provide insight into the angular-momentum distribution of primary fission fragments. Accurate IYR data are also essential for modeling the r-process in stellar nucleosynthesis and for determining the antineutrino mixing angle from reactor spectra. In addition, IYRs play a role in nuclear-energy applications, including β-delayed neutron emission and decay-heat calculations. Nevertheless, experimental data remain scarce, and evaluations therefore rely heavily on model estimates, such as those from the Madland–England approach.
In a recent study, a large set of IYRs was measured in proton-induced fission of ^{238}U. The experiment was performed at the IGISOL facility using the Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique, which enables separation of ground-state and excited-state isomers at mass differences down to a few tens of keV. The analysis included newly developed procedures to correct for detector efficiency and decay losses.
The measured IYRs, together with 12 ratios from an earlier experiment, were compared with predictions from the Madland–England model and the GEF fission code. The results show that the Madland–England model is too crude to describe the population of long-lived states, and that GEF also fails to reproduce many of the ratios, partly due to limited proton-induced fission data and simplified de-excitation modeling. To mitigate the latter, a hybrid approach combining GEF with the reaction code TALYS was evaluated. Overall, the comparisons indicate that while spin configurations of the populated states account for part of the trends, additional effects—such as the state of the fissioning system and the configuration of the complementary fragment—are also important.
To estimate fragment angular momenta from the measured IYRs, a surrogate model based on GEF was developed to generate average properties of primary fragments. By varying the four model parameters, including the fragment angular momentum, and using TALYS to compute the relative isomer populations, average angular momenta with uncertainties were extracted from 24 of the measured ratios. The results reveal a mass-dependent trend consistent with earlier observations in other fissioning systems.