New approach to precisely measure γ-ray intensities for long-lived fission products, with results for the decay of 95Zr

For many fission products, the gamma rays emitted following beta decay provide an easily-detectable signature that can be used to identify their quantities and distributions in a sample. As a result,. gamma-ray spectroscopy is often exploited to study fission-product yields, provided sufficiently accurate information on the gamma-ray intensity is available. However, in many cases, the uncertainties in the existing nuclear data are large enough that they compromise the precision achievable for modern experiments and applications. To address this need, we have developed a new experimental method that is well suited to precisely measure absolute gamma-ray intensities in the beta decay of long-lived fission products. The approach involves the production of a radiopure sample by implantation of a mass-separated ion beam from the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility on a thin carbon foil. The emitted beta-decay radiation is detected with a 4 pi s proportional counter and a meticulously efficiency-calibrated high-purity germanium (HPGe) detector. As a first measurement to demonstrate the approach, we studied the absolute gamma-ray intensities of the strongest transitions following the beta decay of Zr-95 and its decay-daughter Nb-95, and determined them to fractional precisions of better than 1-2%. In addition, with a larger sample of activity produced through neutron irradiation of an isotopically-enriched Zr foil, we performed a high-precision measurement of the relative gamma-ray intensities following the decay of Zr-95 with just the HPGe detector. The sample-production method at CARIBU and the coincidence detection approach demonstrated here can be applied to study fission products with half-lives longer than a day, which includes isotopes important not only for nuclear-energy and national-security applications, but also for medical-isotope research and environmental monitoring.