A semi-empirical model to optimize continuous-flow hyperpolarized 129Xe production under practical cryogenic-accumulation conditions.

Continuous-flow spin exchange optical pumping (SEOP) with cryogenic accumulation is a powerful technique to generate multiple, large volumes of hyperpolarized (HP) 129Xe in rapid succession. It enables a range of studies, from dark matter tracking to preclinical and clinical MRI. Multiple analytical models based on first principles atomic physics and device-specific design features have been proposed for individual processes within HP 129Xe production. However, the modeling efforts have not yet integrated all the steps involved in practical, large volume HP 129Xe production process (e.g., alkali vapor generation, continuous-flow SEOP, and cryogenic accumulation). Here, we use a simplified analytical model that couples both SEOP and cryogenic accumulation, incorporating only two system-specific empirical parameters: the longitudinal relaxation time of the polycrystalline 129Xe "snow', T1snow, generated during cryogenic accumulation, and 2) the average Rb density during active, continuous-flow polarization. By fitting the model to polarization data collected from >140 L of 129Xe polarized across a range of flow and volume conditions, the estimates for Rb density and T1snow were 1.6 ± 0.1 × 1013 cm-3 and 84 ± 5 min, respectively - each notably less than expected based on previous literature. Together, these findings indicate that 1) earlier polarization predictions were hindered by miscalculated Rb densities, and 2) polarization is not optimized by maximizing SEOP efficiency with a low concentration 129Xe, but rather by using richer 129Xe-buffer gas blends that enable faster accumulation. Accordingly, modeling and experimentation revealed the optimal fraction of 129Xe, f, in the 129Xe-buffer gas blend was ~2%. Further, if coupled with modest increases in laser power, the model predicts liter volumes of HP 129Xe with polarizations exceeding 60% could be generated routinely in only tens of minutes.