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