Complete β-decay pattern for the high-priority decay-heat isotopes I137 and Xe137 determined using total absorption spectroscopy

B. C. Rasco, K. P. Rykaczewski, A. Fijałkowska, M. Karny, M. Wolińska-Cichocka, R. K. Grzywacz, C. J. Gross, D. W. Stracener, E. F. Zganjar, J. C. Blackmon, N. T. Brewer, K. C. Goetz, J. W. Johnson, C. U. Jost, J. H. Hamilton, K. Miernik, M. Madurga, D. Miller, S. Padgett, S. V. Paulauskas, A. V. Ramayya, and E. H. Spejewski
Phys. Rev. C 95, 054328 – Published 31 May 2017

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 βγ decay heat should be restudied using the total absorption technique. The fission products I137 and Xe137 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 I137 and Xe137 and determine their consequences for reactor decay heat and ν¯e 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 I137.

Method: We measured the complete β-decay intensities of I137 and Xe137 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 Xe137β decay. We find that major changes to the current ENSDF assessment of I137β-decay intensity are required. The average γ energy per β decay for I137β decay (γ decay heat) increases by 19%, from 1050–1250 keV, which increases the average γ energy per U235 fission by 0.11%. We measure a β-delayed neutron branching fraction for I137β 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 I137 and Xe137 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.

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  • Received 19 January 2017
  • Revised 27 April 2017

DOI:https://doi.org/10.1103/PhysRevC.95.054328

©2017 American Physical Society

Physics Subject Headings (PhySH)

Nuclear Physics

Authors & Affiliations

B. C. Rasco1,2,3,4,*, K. P. Rykaczewski2, A. Fijałkowska5,3, M. Karny5,2,1, M. Wolińska-Cichocka6,2,1, R. K. Grzywacz3,2,1, C. J. Gross2, D. W. Stracener2, E. F. Zganjar4, J. C. Blackmon4, N. T. Brewer1,2,3, K. C. Goetz7,3, J. W. Johnson2, C. U. Jost2, J. H. Hamilton8, K. Miernik5, M. Madurga3, D. Miller3,9, S. Padgett3, S. V. Paulauskas3, A. V. Ramayya8, and E. H. Spejewski2

  • 1JINPA, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 2Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 3Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37966, USA
  • 4Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA
  • 5Faculty of Physics, University of Warsaw, Pasteura 5, PL-02-093 Warszawa, Poland
  • 6Heavy Ion Laboratory, University of Warsaw, PL-02-093 Warszawa, Poland
  • 7CIRE Bredesen Center, University of Tennessee, Knoxville, Tennessee 37966, USA
  • 8Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37212, USA
  • 9Chemical and Radiation Measurements Department, Idaho National Laboratory, Idaho Falls, Idaho 83415, USA

  • *brasco@utk.edu

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Issue

Vol. 95, Iss. 5 — May 2017

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