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.

Automated summary

In this study, we measured the variation of rubidium vapor density in Xe-129-Rb spin-exchange optical pumping using near-infrared and violet atomic absorption spectroscopy. We found that the relationship between spectral absorption and rubidium density does not follow the Beer-Lambert law. The measured rubidium density was used to estimate the spin exchange cross section of Xe-129-Rb, which was consistent with literature values. Additionally, we observed heterogeneity in rubidium density in different reactors with varying distributions and amounts of rubidium metal.