Complete β-decay pattern for the high-priority decay-heat isotopes 137I and 137Xe determined using total absorption spectroscopy

Background: An assessment done under the auspices of the Nuclear Energy Agency in 2007 suggested that the beta decays of abundant fission products in nuclear reactors may be incomplete. Many of the nuclei are potentially affected by the so called pandemonium effect and their beta-gamma decay heat should be restudied using the total absorption technique. The fission products I-137 and Xe-137 were assigned highest priority for restudy due to their large cumulative fission branching fractions. In addition, measuring beta-delayed neutron emission probabilities is challenging and any new technique for measuring the beta-neutron spectrum and the beta-delayed neutron emission probabilities is an important addition to nuclear physics experimental techniques. Purpose: To obtain the complete beta-decay pattern of I-137 and Xe-137 and determine their consequences for reactor decay heat and (nu) over bar (e) emission. Complete beta-decay feeding includes ground state to ground state beta feeding with no associated gamma rays, ground state to excited states beta transitions followed by gamma transitions to the daughter nucleus ground state, and beta-delayed neutron emission from the daughter nucleus in the case of I-137. Method: We measured the complete beta-decay intensities of I-137 and Xe-137 with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a technique for measuring the beta-delayed neutron energy spectrum, which also provides a measurement of the beta-neutron branching ratio, P-n. Results: We validate the current Evaluated Nuclear Structure Data File (ENSDF) evaluation of Xe-137 beta decay. We find that major changes to the current ENSDF assessment of I-137 beta-decay intensity are required. The average. energy per beta decay for I-137 beta decay (gamma decay heat) increases by 19%, from 1050-1250 keV, which increases the average. energy per U-235 fission by 0.11%. We measure a beta-delayed neutron branching fraction for I-137 beta decay of 7.9 +/- 0.2(fit) +/- 0.4(sys)% and we provide a beta-neutron energy spectrum. Conclusions: The Modular Total Absorption Spectrometer measurements of I-137 and Xe-137 demonstrate the importance of revisiting and remeasuring complex beta-decaying fission products with total absorption spectroscopy. We demonstrate the ability of the Modular Total Absorption Spectrometer to measure beta-delayed neutron energy spectra.