NSF FIGURE OF MERIT

Neutron Spin Filter

Diagram of Polarized ³He as Neutron Spin Filter

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.

Polarized Neutronic Performance for NSF

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. T_n and T⁺ are the neutron transmissions for the unpolarized beam and for the desired spin state, respectively. P²T_n  is the figure of merit (FOM) for a NSF. It is important to note the inverse relationship between neutron transmission, T_n, 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 proportional 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.