Investigating Rubidium Density and Temperature Distributions in a High-Throughput 129Xe-Rb Spin-Exchange Optical Pumping Polarizer

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of Xe-129-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput Xe-129-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S(1/2) -> 5(2)P(1/2) (D-1)/5(2)P(3/2) (D-2)) and violet (5(2)S(1/2) -> 6(2)P(1/2)/6(2)P(3/2)) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D-2 and 6(2)P(3/2) absorption by comparison with measurements of the D-1 and 6(2)P(1/2) absorption lines, respectively. D-2 absorption deviates from the Beer-Lambert law at [Rb](D2 )> 4 x 10(17) m(-3) whilst 5(2)S(1/2) -> 6(2)P(3/2) absorption deviates from the Beer-Lambert law at [Rb](6P3/2 )>(4.16 +/- 0.01) x 10(19) m(-3). The measured [Rb] was used to estimate a Xe-129-Rb spin exchange cross section of gamma ' =(1.2 +/- 0.1) x10(-21) m(3) s(-1), consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.