There is currently intense interest in the development of gaseous He-3 neut
ron polarising transmission filters. The key areas of development are incre
asing the currently achievable levels of nuclear spin polarisation, pi, wit
hin the He-3 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 He-3 polariser with a given pi is determine
d. It is demonstrated that the optimum filter thickness depends only weakly
on pi and is always approximately given by the dimensionless expression rh
o sigma lambda t = 3.4 where rho is the density of He-3 atoms in the filter
, sigma is the absorption cross section at a wavelength of 1 Angstrom, lamb
da is the neutron wavelength in Angstrom and t is the filter thickness. Qua
lity factors are calculated for existing types of neutron polariser (superm
irrors and Heusler alloy monochromators) as a function of instrumental para
meters. These results are used to determine the "break even" point for the
new polarisers - the pi needed for the new polarisers to outperform existin
g types of polarisers. A quantitative discussion of regimes of usefulness o
f the different polariser types for all common instrument types is presente
d. (C) 2000 Elsevier Science B.V. All rights reserved.