³He has a very large 1/v absorption cross-section and only absorbs neutrons in the opposite spin state to that of the ³He nucleus. The cross-section for scattering is very small and essentially isotropic. These two features make polarised ³He NSFs a very efficient and also a very “clean” technology for neutron beam polarisation. The filters are optically neutral in contrast to other polarisation methods, which invariably modify the angular and/or wavelength distribution and can also produce spurious effects, such as off-specular or small-angle scattering (supermirrors) or multiple or higher-order Bragg scattering (polarising crystals). The beam polarisation obtained with an NSF is perfectly homogeneous over the beam profile and there is no g-background created by the absorption of the neutrons in the NSF.

The level of gas polarisation is the critical parameter for the performance of the NSF. The Tyrex central filling station typically provides ³He gas at a polarisation of 75-80% on the neutron instrument. After being polarised on Tyrex, the ³He gas will inevitably start to depolarise with a characteristic depolarisation time T1. The main relaxation contributions typically arise from the material of the NSF cells and magnetic field gradients. Relaxation times at the moment are of the order of 100-300 hours, which means that the NSF gas is typically replaced on the instrument once every 1-2 days. The polarisation decays in a regular and predictable way, allowing for accurate data correction.

Gaseous ³He NSF provide very interesting properties :

• simple transmission device of predictable characteristics
• no impact on the optical functions
• can be made in virtually any desired shape
• homogeneous polarisation over the whole section of the beam
• Broad-band enough for Time-Of-Flight instruments…