Characterization of a low-pressure high-capacityX129eflow-through polarizer

Abstract

We describe a low-pressure flow-through apparatus for generating hyperpolarized $^{129}\text{X}\text{e}$ and report its performance by examining both the output $^{129}\text{X}\text{e}$ polarization ${P}_{\text{Xe}}$ by NMR and the in situ Rb polarization profile by optically detected electron paramagnetic resonance. The polarizer is based on a previously presented design employing a long optical pumping cell, lean Xe mixture at low pressure, Rb presaturation, and counterflow of gas with respect to the direction of light propagation. The numerical model to which we compare the polarizer's performance includes the temperature dependence of the $\text{Rb-}^{129}\text{X}\text{e}$ spin-exchange rate, which has not previously been treated. The qualitative trends in the data mostly follow those in the model, although the model predicts ${P}_{\text{Xe}}$ to be up to a factor of two higher than observed. This discrepancy cannot be attributed to low Rb polarization: the model and the optically detected electron paramagnetic resonance data (acquired at six points along the length of the heated portion of the optical pumping cell) are in reasonable agreement and show typical values of 85%--95%, although measurements also reveal an anomalous region of depressed Rb polarization near the middle of the cell. The highest output $^{129}\text{X}\text{e}$ polarization ${P}_{\text{Xe}}=84\ifmmode\pm\else\textpm\fi{}16%$, was recorded using $\ensuremath{\approx}60\text{ }\text{W}$ of frequency-narrowed laser light at a Xe partial pressure (referenced to $20\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$) of $1.1\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{mbar}$, flowing at 1 sccm of Xe; typical values were ${P}_{\text{Xe}}\ensuremath{\approx}20%$ flowing at 10 sccm of Xe with $\ensuremath{\approx}30\text{ }\text{W}$ of laser light.