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, ML, of a fissile sample. It is shown that albedo neutrons contribute, in the case of small plutonium bearing samples, to a significant part of ML, and that their effect has to be taken into account in the relationship between the measured coincidence count rates and the 240Pu 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 MCNPTM 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 MCNPTM calculations showed that for typical safeguard samples the newly developed code estimates (ML−1) to within 1% of the MCNPTM results. The precision of these results along with the rapidity of the proposed calculation method therefore make the use of a “known ML” approach for solving the Böhnel equations very attractive when measuring density controlled gram size PuO2 or Mixed Oxide (MOX) samples.