Total absorption ${\hat{I}}^3$-ray spectroscopy of the ${\hat{I}}^2$ decays of ${}^{96\text{gs,m}}$Y

Abstract

The ${\hat{I}}^2$ decays of the ground state (gs) and isomeric state (m) of ${}^{96}$Y have been studied with the total absorption ${\hat{I}}^3$-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 ${\hat{I}}^2$-intensity distributions of both decays have been independently determined. In the analyses the de-excitation of the 1581.6 keV level in ${}^{96}$Zr, in which conversion electron emission competes with pair production, has been carefully considered and found to have significant impact on the ${\hat{I}}^2$-detector efficiency, influencing the ${\hat{I}}^2$-intensity distribution obtained. Our results for ${}^{96\text{gs}}$Y (0${}^{+}$) confirm the large ground state to ground state ${\hat{I}}^2$-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 ${\hat{I}}^2$ intensity. Given that the decay of ${}^{96\text{gs}}$Y 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 ${}^{96\text{m}}$Y (8${}^{+}$), previously undetected ${\hat{I}}^2$ intensity in transitions to states above 6 MeV has been observed. This shows the importance of total absorption ${\hat{I}}^3$-ray spectroscopy measurements of ${\hat{I}}^2$ decays with highly fragmented de-excitation patterns. ${}^{96\text{m}}$Y (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 ${\hat{I}}^2$ and ${\hat{I}}^3$ energies differ significantly from the previous values in evaluated databases (...)