The effect of albedo neutrons on the neutron multiplication of small plutonium oxide samples in a PNCC chamber

LCA Bourva, S Croft, David Weaver

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2 Citations (Scopus)


This paper describes how to evaluate the effect of neutrons reflected from parts of a passive neutron coincidence chamber on the neutron leakage self-multiplication, M-L, of a fissile sample. It is shown that albedo neutrons contribute, in the case of small plutonium bearing samples, to a significant part of M-L, and that their effect has to be taken into account in the relationship between the measured coincidence count rates and the Pu-240 effective mass of the sample. A simple one-interaction model has been used to write the balance of neutron gains and losses in the material when exposed to the re-entrant neutron flux. The energy and intensity profiles of the re-entrant flux have been parameterised using Monte Carlo MCNP(TM) calculations. This technique has been implemented for the On Site Laboratory neutron/gamma counter within the existing MEPL 1.0 code for the determination of the neutron leakage self-multiplication. Benchmark tests of the resulting MEPL 2.0 code with MCNP(TM) calculations showed that for typical safeguard samples the newly developed code estimates (M-L - 1) to within 1% of the MCNP(TM) results. The precision of these results along with the rapidity of the proposed calculation method therefore make the use of a "known M-L" approach for solving the Bohnel equations very attractive when measuring density controlled gram size PuO2 or Mixed Oxide (MOX) samples. (C) 2002 Elsevier Science B.V. All rights reserved.
Original languageEnglish
Pages (from-to)640-655
Number of pages16
JournalNuclear Instruments & Methods in Physics Research. Section A. Accelerators, Spectrometers, Detectors
Publication statusPublished - 1 Mar 2002


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