The 3He nuclear spin is carried mainly by the neutron. Neutron spin filters (NSFs) have strongly spin-dependent neutron absorption cross sections. Neutrons anti-aligned to the polarized 3He gas encounter a thick absorption target. While, neutrons aligned with the polarized 3He gas encounter a thin absorption target. The image above illustrates this point.
The plot above shows the relationship between polarized neutronic performance and 3He opacity. P and A are the polarizing efficiencies for the polarizer and analyzer, respectively. Tn and T+ are the neutron transmissions for the unpolarized beam and for the desired spin state, respectively. P2Tn is the figure of merit (FOM) for a NSF. It is important to note the inverse relationship between neutron transmission, Tn, through the cell and the neutron polarization. Here, the flipping ratio, F, is yielded from the NSF only. These parameters are plotted as a function of the opacity, which is defined as σnl; where σ is the neutron absorption cross section for 3He gas (this is linearly porpotional to the neutron wavelength), n is the 3He number density, and l (cm) is the cell path length. This plot assumes a 75% polarization of the 3He gas and is intended as a guide only.