Some Theory

The efficiency of ³He NSF (Neutron Spin Filter) used as a polariser or analyser is usually described with quality factor Q, Q = P_n² T_n where T_n transmission and P_n polarisation are depending in a different way from the cell's "opacity" which is the effective absorption coefficient of polarised ³He and neutrons with antiparallel oriented spin. The following figure presents neutron's polarisation, transmission and quality factor Q of 10 and 15 cm long cells filled with 3 bars of ³He polarised at 70% as a function of the neutron wavelengths accessible at HEiDi. Optimising the cell parameters for the wavelength of 0.68 Å and 0.87 Å, the first NSF cells specifications were elaborated. Cells with 13 cm length, 6 cm in diameter and working pressure up to 3 bar seems to be optimal for our application. Not the last role in the practical application play also relaxation characteristics of the polarised ³He containers. The values of 200 hours for characteristic relaxation T1 at 1 bar would be preferable, but 100 hours as a lower threshold is reasonably desirable.