Abstract: The beta decay of Zr-101,Zr-102 and Tc-109 was studied using the technique of total absorption spectroscopy. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) (SuN) detector in the first-ever application of total absorption spectroscopy with a fast beam produced via projectile fragmentation. The beta-decay feeding intensity and Gamow-Teller transition strength distributions were extracted for these three decays. The extracted distributions were compared to three different quasiparticle random-phase approximation (QRPA) models based on different mean-field potentials. A comparison with calculations from one of the QRPA models was performed to learn about the ground-state shape of the parent nucleus. For Zr-101 and Zr-102, calculations assuming a pure shape configuration (oblate or prolate) were not able to reproduce the extracted distributions. These results may indicate that some type of mixture between oblate and prolate shapes is necessary to reproduce the extracted distributions. For Tc-109, a comparison of the extracted distributions with QRPA calculations suggests a dominant oblate configuration. The other two QRPA models are commonly used to provide beta-decay properties in r-process network calculations. This work shows the importance of making comparisons between the experimental and theoretical beta-decay distributions, rather than just half-lives and beta-delayed neutron emission probabilities, as close to the r-process path as possible.

Abstract: The ss decays of the ground state (gs) and isomeric state (m) of Y-96 have been studied with the total absorption gamma-ray spectroscopy technique at the Ion Guide Isotope Separator On-Line facility. The separation of the 8(+) isomeric state from the 0(-) ground state was achieved thanks to the purification capabilities of the JYFLTRAP double Penning trap system. The ss-intensity distributions of both decays have been independently determined. In the analyses the deexcitation of the 1581.6 keV level in Zr-96, in which conversion electron emission competes with pair production, has been carefully considered and found to have significant impact on the ss-detector efficiency, influencing the ss-intensity distribution obtained. Our results for Y-96gs (0(-)) confirm the large ground state to ground state ss-intensity probability, although a slightly larger value than reported in previous studies was obtained, amounting to 96.6(-2.1)(+0.3) % of the total ss intensity. Given that the decay of Y-96gs is the second most important contributor to the reactor antineutrino spectrum between 5 and 7 MeV, the impact of the present results on reactor antineutrino summation calculations has been evaluated. In the decay of Y-96m (8(+)), previously undetected ss intensity in transitions to states above 6 MeV has been observed. This shows the importance of total absorption gamma-ray spectroscopy measurements of ss decays with highly fragmented deexcitation patterns. Y-96m (8(+)) is a major contributor to reactor decay heat in uranium-plutonium and thorium-uranium fuels around 10 s after fission pulses, and the newly measured average ss and gamma energies differ significantly from the previous values in evaluated databases. The discrepancy is far above the previously quoted uncertainties. Finally, we also report on the successful implementation of an innovative total absorption gamma-ray spectroscopy analysis of the module-multiplicity gated spectra, as a first proof of principle to distinguish between decaying states with very different spin-parity values.