Neutron Path Length Correction of a 3He Spin Filter

3He neutron spin filters (NSF) have been widely used for polarized neutron instrumentation for worldwide neutron facilities. Here we report characterization of the two-dimensional neutron path variation of a 3He NSF when a large divergent, scattered neutron beam passes through the end windows of a cylindrical 3He cell. Path length variations of the transmission of the unpolarized neutrons through a 3He NSF and neutron polarization produced from a 3He NSF are characterized. We present a ray-tracing model to explain the path length variation and corresponding neutron transmission and neutron polarization variations, and compare the measured variations to those calculated from the model. Although the path length effect is not large, it should be corrected in the polarization efficiency correction software when a 3He NSF is used for SANS polarization analysis. The path length variation effect can be adopted to other types of neutron scattering spectrometers when using 3He NSFs.