Deciphering 98Nb ß decay with the Modular Total Absorption Spectrometer at ORNL

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

Automated summary

This study aims to obtain the complete ss-decay feeding pattern of Nb-98 and assess its impact on the average energy per Nb-98 ss decay and (upsilon) over bare emission calculations. The results indicate the need for changes to the current assessment of Nb-98 ss-decay intensity and offer a more accurate and precise Nb-98 ss-feeding pattern.