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Experimental Characterization of the Advanced Liquid Hydrogen Cold Neutron Source Spectrum of the NBSR Reactor at the NIST Center for Neutron Research

Published

Author(s)

Jeremy C. Cook, John G. Barker, J. Michael Rowe, Robert E. Williams, Cedric V. Gagnon, Richard M. Lindstrom, Richard Michael Ibberson, Dan A. Neumann

Abstract

The recent expansion of the NIST Center for Neutron Research facility has offered a rare opportunity to perform an accurate measurement of the cold neutron spectrum at the exit of a newly-installed neutron guide. Using a combination of a neutron time-of-flight, a gold foil activation measurement, and Monte Carlo simulation of the neutron guide transmission, we obtain our most reliable experimental characterization of the Advanced Liquid Hydrogen Cold Neutron Source brightness to date. Measurements were performed at three different stages of the reactor cycle, including one at reactor startup. Prior to the latter measurement the hydrogen was maintained in a liquefied state for an extended period in an attempt to observe the anticipated growth of the ortho (0)-hydrogen concentration following startup. Since papra (p)-hydrogen has a small scattering cross-section for neutron energies below 15 meV (neutron wavelengths, λ. greater than 2.3 A), changes in the o:p ratio and in the void distribution in the boiling hydrogen impats the spectral distribution. The nature of such changes is investigated using the MCNP6 transport code[1] using the Los Alamos o and p T=20 K hydrogen scattering kernels shipped with the code and a best estimate of the hydrogen density distribution based on the heating distribution. A comparison of the simulations with the means of the measurements suggest a best overall fit o:p ratio of about 17.5% o: 82.5: p for E~
Citation
Nuclear Instruments & Methods in Physics Research A
Volume
792

Keywords

Liquid hydrogen, cold neutron sources, research reactors, neutron guides, neutron time-of-flight, neutron guide simulations
Created August 1, 2015, Updated February 19, 2017