Complete β -decay pattern for the high-priority decay-heat isotopes I137 and Xe

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 137 I and 137 Xe 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 137 I and 137 Xe and determine their consequences for reactor decay heat andνe 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 137 I. MethodWe measured the complete β-decay intensities of 137 I and 137 Xe with the Modular Total Absorption Spectrometer at Oak Ridge National Laboratory. We describe a novel 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 ENSDF evaluation of 137 Xe β decay. We find that major changes to the current ENSDF assessment of 137 I β-decay intensity are required. The average γ energy per β decay for 137 I β decay (γ decay heat) increases by 19%, from 1050 keV to 1250 keV, which increases the average γ energy per 235 U fission by 0.11%. We measure a β delayed neutron branching fraction for 137 I β decay of 7.9 ± 0.2(f it) ± 0.4(sys)% and we provide a β-neutron energy spectrum. ConclusionsThe Modular Total Absorption Spectrometer measurements of 137 I and 137 Xe 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.

Characterization and performance of the DTAS detector

“…We found that in the range 300-500 ns the results were stable within 3%. It should be noted that the value we obtain is 14% lower than the value of 7.9(4)% obtained by a similar procedure with MTAS [29]. In view of this discrepancy and the fact that for 95 Rb we also obtain a large difference but of opposite sign, we conclude that further investigations are needed before deciding on the reliability of P n extraction from TAGS spectra [29].…”

“…It should be noted that the smaller efficiency of DTAS does not affect its performance as a total absorption spectrometer. A nice example is provided by the decay of 137 I which has been measured by DTAS [4] (see also subsection 4.2) and MTAS [29]. Figure 4 in [29] compares the spectrum measured with the full MTAS with the spectrum measured with the sub-detector consisting of the 7 most central modules.…”

Total absorption γ -ray spectroscopy of the β -delayed neutron emitters I137 and Guadilla1, Taín2, Algora3 et al. 2019Phys. Rev. C114110View full textAdd to dashboardCite

“…For that, the information on this decay available in ENSDF [30] was used as input for the DECAYGEN event generator [51]. This information is reliable according to our TAGS analysis of the β-gated 137 Xe spectrum and is in agreement with the recent TAGS result of B. C. Rasco et al [59]. This procedure avoids the impact of the large statistical fluctuations of the experimental 137 Xe singles spectrum after background subtraction.…”

“…These two values are larger than the quoted number in ENSDF: 45.2(5)%. It is also worth mentioning that B. Fogelberg et al [58] reported a β intensity of 47% to the g.s., while the recent TAGS measurement of B. C. Rasco et al determined a value of 49(1)% [59].…”

β− decay study of the Mn66−Fe66−CoStryjczyk1, Tsunoda2, Darby3 et al. 2018Phys. Rev. C171150View full textAdd to dashboardCite

“…The low-efficiency experimental setups are known to be burdened with a systematic error related to the inability to detect high-energy and low-intensity γ -ray transitions. The total absorption spectroscopy (TAS) measurements, which are not affected by this issue, can significantly reduced the β feedings of the low-lying states [92][93][94][95][96][97][98]. Consequently, the β feedings presented in our work have to be treated as the upper limits while the associated log(f t) values are the lower limits.…”