We describe a low-pressure flow-through apparatus for generating hyperpolarized 129 Xe and report its performance by examining both the output 129 Xe polarization P 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 Rb-129 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 P 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 cell͒ 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 Xe polarization P Xe =84Ϯ 16%, was recorded using Ϸ60 W of frequency-narrowed laser light at a Xe partial pressure ͑referenced to 20°C͒ of 1.1Ϯ 0.2 mbar, flowing at 1 sccm of Xe; typical values were P Xe Ϸ 20% flowing at 10 sccm of Xe with Ϸ30 W of laser light.

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