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

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.