To increase the 129Xe polarization for emerging biomedical applications, the Rb polarization on which the 129Xe directly depends must be first optimized. Radiation trapping is a form of Rb depolarization thought to be quenched by the N2 added to SEOP gas mixtures. Here we use absorption spectroscopy to measure the Rb density, optical spectroscopy to monitor the fraction of Rb atoms involved in radiation trapping, and field cycling to measure the Rb polarization. We find that radiation trapping is still present during spin-exchange optical pumping and appears to limit the achievable Rb polarization.
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