Precise β-Decay Measurements of Long-Lived Fission Products*

Our understanding of the fragment mass distributions following fission impacts many applications, including reactor-neutrino studies, nuclear forensics, and stockpile stewardship science. The γ-ray emissions following the β-decay of fission products provide a reliable signature that is often used to identify relative yields and distributions from fission. However, the uncertainties in the existing nuclear data for γ-ray intensities and β-decay branching ratios for some of the isotopes used in these applications range from 5-30%, which propagates to significant error contributions in fission-chain yield calculations. To address this issue, we have developed an experimental approach to precisely measure γ-ray branching ratios following the decay of long-lived fission products. We have identified ¹⁶¹Tb and ^115mCd as two important cases where the data can be significantly improved, and we prepared reactor-irradiated radioactive samples for measurement with our newly established on-site detection system at LLNL. We will present results from recent measurements on these isotopes.