Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment

The NPDGamma experiment measures the asymmetry in $\gamma$-ray emission in the capture of polarized neutrons on liquid parahydrogen. The beam polarization is measured using ${}^3$He spin analysis, but this measurement does not account for the contribution of depolarization from spin–flip scattering primarily due to orthohydrogen in the bulk liquid. This is a systematic effect that dilutes the experimental asymmetry and is modeled using Monte Carlo. Methods for tracking neutron spin in MCNPX were developed in order to calculate the average neutron polarization upon capture for use as a multiplicative correction to the measured beam polarization for the NPDGamma experiment.