McStas simulations of neutron beam transport for the Nab experiment

The Nab experiment aims to accurately measure the little a and little b correlation parameters in free neutron decay. For the ‘a’ parameter, the beam must be completely unpolarized as it passes through the decay volume. Two He3 cells (polarizer and analyzer) in combination with an accompanying collimation scheme will be used to measure the beam’s polarization. It must be verified that the beam polarization with the current collimation scheme can be generalized to the larger beam used in the Nab experiment. This verification was completed with McStas, a monte carlo neutron ray tracing software package. Neutron guide scattering events were simulated for all proposed collimation schemes, comparisons will be presented between the beam phase space for the polarimetry measurement and the full Nab experiment.