Characterizing and modeling the efficiency limits in large-scale production of hyperpolarized 129Xe

The ability to produce liter volumes of highly-spin-polarized Xe-129 enables a wide range of investigations, most notably in the fields of materials science and biomedical magnetic resonance imaging. However, for nearly all polarizers built to date, both peak Xe-129 polarization and the rate at which it is produced fall far below those predicted by the standard model of Rb metal vapor, spin-exchange optical pumping (SEOP). In this work we comprehensively characterized a high-volume flow-through Xe-129 polarizer using three different SEOP cells with internal volumes of 100, 200, and 300 cm(3) and two types of optical sources: a broad-spectrum 111-W laser [full width at half maximum (FWHM) equal to 1.92 nm] and a line-narrowed 71-W laser(FWHM equal to 0.39 nm). By measuring Xe-129 polarization as a function of gas flow rate, we extracted the peak polarization and polarization production rate across a wide range of laser absorption levels. Peak polarization for all cells consistently remained a factor of 2-3 times lower than predicted at all absorption levels. Moreover, although production rates increased with laser absorption, they did so much more slowly than predicted by the standard theoretical model and basic spin-exchange efficiency arguments. Underperformance was most notable in the smallest optical cells. We propose that all these systematic deviations from theory can be explained by invoking the presence of paramagnetic Rb clusters within the vapor. Cluster formation within saturated alkali-metal vapors is well established and their interaction with resonant laser light was recently shown to create plasmalike conditions. Such cluster systems cause both Rb and Xe-129 depolarization, as well as excess photon scattering. These effects were incorporated into the SEOP model by assuming that clusters are activated in proportion to excited-state Rb number density and by further estimating physically reasonable values for the nanocluster-induced velocity-averaged spin-destruction cross section for Rb (<sigma(cluster-Rb)v > approximate to 4 x 10(-7) cm(3) s(-1)), the Xe-129 relaxation cross section (<sigma(cluster-Xe)v > approximate to 4 x 10(-13) cm(3) s(-1)), and a non-wavelength-specific photon-scattering cross section (sigma(cluster) approximate to 1 x 10(-12) cm(2)). The resulting modified SEOP model now closely matches experimental observations.