Optimised adiabatic fast passage spin flipping for He neutron spin filters

We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized ³He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the ³He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500 ms we can spin flip a polarized ³He neutron spin filter with only 1.8×10⁻⁵ loss in ³He polarization. The small magnetic fields (10-15 G) used to house neutron spin filters mean the ³He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using ³He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.