A Rapid and Precise Probe for Measurement of Liquid Xenon Polarization

The relaxation time of liquid 129Xe is very long (>15 min) and the signal at thermal equilibrium is weak. Therefore, determination of the absolute polarization enhancement of hyperpolarized 129Xe by direct measurement is tedious. We demonstrate a fast and precise alternative, based on the dipolar field created by liquid hyperpolarized 129Xe contained in a cylindrical sample tube. The dipolar field is homogeneous in the bulk of the tube and adds to the external field, causing a shift in the Larmor frequencies of all nuclear spins. We show that the frequency shift of the proton in CHCl3 (chloroform), which dissolves homogeneously in xenon over a fairly broad temperature range, is an excellent probe for 129Xe polarization. Frequency measurements are precise and the experiment is much faster than by direct measurement. Furthermore the 129Xe polarization is minimally disturbed since no rf pulses are applied directly to 129Xe and since chloroform is a fairly weak source of 129Xe relaxation. The experiments are reproducible and require only standard NMR instrumentation.