Some notes on He polarimetry using alkali EPR

It is sometimes profitable to measure the longitudinal nuclear spin polarization of He. The method described here takes advantage of the vaporized alkali metal already present in a glass target cell. Long range and short range interactions with polarized He gas result in an “effective” magnetic field felt by the alkali metal atoms. A precisely detectable Zeeman shift in the alkali EPR frequency that is proportional to the He polarization and density is produced. The EPR frequency is located by exciting the EPR transition and observing the resulting change in the D2 fluorescence. A PI feedback loop is used to lock to the EPR frequency while the direction of the He spins is reversed via AFP with respect to any baseline magnetic fields. This isolates the He contribution to the EPR frequency and consequently the He polarization can be extracted.