Characterization of a low-pressure high-capacity Xe flow-through polarizer

We describe a low-pressure flow-through apparatus for generating hyperpolarized Xe and report its performance by examining both the output Xe polarization PXe 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 Rb-Xe 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 PXe 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 Xe polarization PXe=84 16%, was recorded using 60 W of frequency-narrowed laser light at a Xe partial pressure referenced to 20 °C of 1.1 0.2 mbar, flowing at 1 sccm of Xe; typical values were PXe 20% flowing at 10 sccm of Xe with 30 W of laser light.