MCNPX - PoliMi simulation capacity using thermal neutron cross - sections to assess the reliability of the neutron multiplicity mass analysis where shielding is unknown

In the context of warhead authentication in a future arms control verification regime, neutron multiplicity measurements are useful to determine plutonium mass which may be used as a warhead indicator. The inspecting party might have little knowledge on possible shielding between plutonium componen t and detector , as this may be sensitive information which however has an influence on the mass output of the multiplicity analysis. To investigate the confidence an inspector can have from neutron multiplicity measurements of unknown configurations, a sy s tematic analysis of its possibilities and limitations must be performed. MCNPX - PoliMi simulations are helpful in this regard. As a first step the code's functionality was investigated comparing simulation results to experimental data of plutonium neutron m ultiplicity measurements using a He - 3 detector. Thermal neutron cross - sections for the moderating material in the detector are required for a successful simulation, as scattering with the lattice structure becomes relevant. Scattering can be described by t he creation and annihilation of phonons, the cross - sections are obtained from the phonon excitation spectrum. Taking into account statistical and systematic simulation uncertainties, the deviation to the measurement data generally lies within the simulatio n uncertainty.