3He neutron polarising filters — theoretical comparison with supermirrors and Heusler alloy polarisers

There is currently intense interest in the development of gaseous 3He neutron polarising transmission filters. The key areas of development are increasing the currently achievable levels of nuclear spin polarisation, π, within the 3He gas and improving the reliability of the devices. Recent work has identified “quality factors” which can be used to determine the optimum thickness of such filters under various experimental conditions. The best achievable quality factor for a 3He polariser with a given π is determined. It is demonstrated that the optimum filter thickness depends only weakly on π and is always approximately given by the dimensionless expression ρσλt=3.4 where ρ is the density of 3He atoms in the filter, σ is the absorption cross section at a wavelength of 1Å, λ is the neutron wavelength in Å and t is the filter thickness. Quality factors are calculated for existing types of neutron polariser (supermirrors and Heusler alloy monochromators) as a function of instrumental parameters. These results are used to determine the “break even” point for the new polarisers — the π needed for the new polarisers to outperform existing types of polarisers. A quantitative discussion of regimes of usefulness of the different polariser types for all common instrument types is presented.