Characterization of a Low Pressure, High Capacity $^{129}$Xe Flow-Through Polarizer*

Hyperpolarized $^{129}$Xe produced {\it via} spin-exchange optical pumping continues to be an interesting physical system to study and is useful in many NMR and MRI applications. The generation of large quantities of highly polarized $^{129}$Xe is complicated by xenon's large cross section for spin destruction of the alkali-metal electron. This problem has been addressed in recent years by the development of flow-through xenon polarizers, which operate with a gas mixture that is lean in xenon flowing continuously through the optical pumping cell. We describe here our own flow-through xenon polarizer that is based on the University of New Hampshire design: it operates at low pressure, employs counterpropagating laser beam and gas flow, and has a long narrow optical pumping region. In our version, the systems for heating and Rb vapor generation have been simplified. We examine both the output $^{129}$Xe polarization by NMR and the {\it in situ} $^{85}$Rb polarization by optically detected EPR as a function of position in the meter-long cell. Under near-optimal conditions with 28 W of frequency-narrowed laser light, we achieve $^{129}$Xe polarizations $>30$\% with a flow of 5 bar$\cdot$cm$^3$/min of natural xenon. We compare our results with a numerical model.