Technique for measuring magnetic filling factors with applications to cryogenic magnetic resonance experiments

We describe a new cavity perturbation technique for accurate measurement of rf magnetic filling factors, which is especially relevant to cryogenic magnetic resonance experiments. The sample volume is filled with liquid O2 which perturbs both the magnetic and electric rf fields within the electromagnetic resonator or cavity. The electric perturbation is subsequently accounted for by replacing the liquid O2 with liquid Ar which has nearly identical dielectric properties. The procedure may be refined using the dielectric properties of liquid N2 whereby the depolarizing factor associated with the sample volume may be estimated. In contrast to conventional techniques, the method usually requires little or no modification of the magnetic resonance apparatus. An ancillary result of our work has been the determination of the paramagnetic loss tangent for liquid oxygen near 80 K at 1.4 and 5.3 GHz.