Noble Gas Polarimetry Using Rb EPR Frequency Shifts*

EPR frequency shifts of optically polarized alkali-metal atoms can be exploited for polarimetry of noble-gas nuclei polarized by spin-exchange optical pumping. Our group recently measured the enhancement factor $\kappa_0 = 493$ for Rb-$^{129}$Xe [1], which characterizes the electron wave-function overlap during collisions and is crucial to the calibration of the frequency-shift for $^{129}$Xe polarimetry. This type of polarimetry is useful in several applications involving optically polarized $^{129}$Xe; our particular motivation is an ${\it in\ situ}$ measurement of absolute $^{129}$Xe polarization within the optical pumping cell of a flow-through $^{129}$Xe polarizer [2]. This application has some particular challenges, and we have initially observed some unexpected shifts in the $^{87}$Rb EPR frequency measurement on board the polarizer. In effort to disentangle these apparent systematic effects, we have constructed a separate experiment to characterize Rb EPR shifts for both $^{3}$He and $^{129}$Xe in sealed cells. We present results and analysis of these experiments and discuss implications for using this method in flow-through polarizers.\\[4pt] [1] Z. L. Ma, et al., Phys. Rev. Lett., 106, 193005 (2011).\\[0pt] [2] I. Ruset et. al., Phys. Rev. Lett., 96. 053002 (2006).