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

Abstract

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', T₁^snow, 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 T₁^snow 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.

Keywords: (129)Xe-buffer gas blend; Continuous-flow spin-exchange optical pumping; Hyperpolarized(129)Xe; Laser power; Parameter optimization; Polarization model; Rb density; Solid T(1) relaxation.