Deciphering Nb98 β decay with the Modular Total Absorption Spectrometer at ORNL

Abstract

Background: An assessment done under the auspices of the Organization for Economic Co-operation and Developement Nuclear Energy Agency (OECD-NEA) in 2007 suggested that the $\ensuremath{\beta}$ decays of many abundantly produced fission products in nuclear reactors may be incomplete. In this assessment, the fission product $^{98}\mathrm{Nb}$ was assigned the highest priority for study by total absorption spectroscopy due to its large cumulative fission branching fraction and because the $\ensuremath{\beta}$-decay data from several experiments are discrepant.Purpose: To obtain the complete $\ensuremath{\beta}$-decay feeding pattern of $^{98}\mathrm{Nb}$ and determine the impact on the average $\ensuremath{\gamma}$ energy per $^{98}\mathrm{Nb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay and ${\overline{\ensuremath{\nu}}}_{e}$ emission calculations. The complete ${}^{98}\mathrm{Nb}$ $\ensuremath{\beta}$-decay feeding pattern includes ground-state to ground-state $\ensuremath{\beta}$ feeding and direct $\ensuremath{\beta}$ feeding to the ${0}^{+}$ first-excited state (both have no associated $\ensuremath{\gamma}$ rays), and the ground-state to excited-state $\ensuremath{\beta}$ transitions followed by $\ensuremath{\gamma}$ transitions to the ground state of the daughter nucleus, $^{98}\mathrm{Mo}$.Method: The complete $\ensuremath{\beta}$-decay intensities of $^{98}\mathrm{Nb}$ were measured with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory (ORNL). The $^{98}\mathrm{Nb}$ was produced by the $\ensuremath{\beta}$ decay of mass 98 fission fragments at ORNL's On-Line Test Facility (OLTF) using proton-induced fission of $^{238}\mathrm{U}$.Results: We find that changes to the current ENSDF assessment of $^{98}\mathrm{Nb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay intensity are required. We report improved uncertainties for the $\ensuremath{\beta}$-decay feeding values and report new $\ensuremath{\beta}$ feedings to high-energy levels in $^{98}\mathrm{Mo}$.Conclusions: A more complete $^{98}\mathrm{Nb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-feeding pattern with improved accuracy and precision is offered. The impacts of the measured changes to the $^{98}\mathrm{Nb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-feeding pattern on both reactor decay heat calculations and predicted detection rates of reactor ${\overline{\ensuremath{\nu}}}_{e}$ are presented. The Modular Total Absorption Spectrometer measurements of $^{98}\mathrm{Nb}$ demonstrate the importance of reexamining and remeasuring complex $\ensuremath{\beta}$-decaying fission products with total absorption spectroscopy, including nuclei very near $\ensuremath{\beta}$ stability.