Precision Beta-Decay Measurements of Long-Lived Fission Products

A precise database of observables following fission events is critical for accurate attribution of nuclear events. Gamma-ray spectroscopy of characteristic emissions from cumulative fission products provides reliable signatures to identify relative product yields in fission-chain reactions. These gamma rays are emitted in only a fraction of beta decays, and this fraction (the gamma-ray intensity) has uncertainties in existing nuclear data ranging from 5–35%, which propagates into fission yield calculations for long-lived, high-priority radioisotopes. This work examines the A=111 and 115 mass decay chains, spanning the “valley” in thermal fission product yield curves, and presents high-precision results for the absolute gamma-ray emission intensities following the beta decay of 111Ag, measured to less than 1% uncertainty.