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Physical Review A

Xe 129 -Cs ( D 1 , D 2 ) versus Xe 129 -Rb ( D 1 ) spin-exchange optical pumping at high xenon densities using high-power laser diode arrays

Nicholas Whiting, Neil A. Eschmann, Boyd M. Goodson, Michael J. Barlow

Published May 26, 2011• 21 citations
Publisher

Abstract

We investigate $^{129}\mathrm{Xe}$-Cs (${D}_{1},{D}_{2}$) spin exchange optical pumping (SEOP) at high Xe densities ($~$0.12--2.44 amagat) using newly available high-power ($>$40 W) laser diode arrays and compare with $^{129}\mathrm{Xe}$-Rb ${D}_{1}$ SEOP under similar conditions. At elevated Xe densities, the spin-exchange rate (per alkali-metal atom, ${\ensuremath{\gamma}}^{\ensuremath{'}}$) for Cs-$^{129}\mathrm{Xe}$ is $~$1.5-fold greater than that for Rb-$^{129}\mathrm{Xe}$. Higher spin-exchange rates and lower $^{129}\mathrm{Xe}$ spin-destruction rates for Cs-$^{129}\mathrm{Xe}$ versus Rb-$^{129}\mathrm{Xe}$ contribute to $~$twofold improvement in $^{129}\mathrm{Xe}$ nuclear spin polarization measured at 9.4 T---with the largest gains observed at the highest Xe densities.

DOI
10.1103/physreva.83.053428

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Abstract Efficient production of hyperpolarized 129 Xe at clinically relevant output levels remains an important challenge for spin-exchange optical pumping (SEOP). In this work, we report a comparative experimental study of Cs– 129 Xe and Rb– 129 Xe SEOP. The spin-exchange and spin-relaxation rates are measured and separated into binary-collision and van der Waals (vdW) molecular contributions. The resulting parameter set shows that long-lived vdW interactions enhance the intrinsic spin-exchange efficiency within a narrow pressure window, with Cs exhibiting a modest advantage over Rb under comparable conditions. Using these experimentally constrained quantities in a device-level model, we predict that clinically relevant 129 Xe polarization levels and throughput could be achievable at pump powers near 5 W.
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We present a design for a spin-exchange optical pumping system to produce large quantities of highly polarized 129Xe and 131Xe. Low xenon concentrations in the flowing gas mixture which allow the laser to maintain high Cs polarization. The large spin-exchange rate between Cs and Xe through the long-lived van der Waals molecules at low pressure, combined with a high flow rate, results in large production rates of hyperpolarized xenon. The fast rates make it possible to obtain large nuclear polarizations after several minutes of optical pumping with a laser.At high Xe pressures. According to the theory, the longitudinal spin-elaxation rate 1T1 of Xe in a high-pressure sample containing only Xe and Cs vapor has the simple form is the velocity averaged binary spin-exchange cross section, It is the relaxation rate due to wall collisions and perhaps magnetic field inhomogeneities. Our results complement earlier studies performed at 129Xe pressures of about 20 Torr and 131Xe pressures of about 20 Torr and N2 pressures of 600 Torr . This work is useful for predicting spin-exchange rates between polarized Cs atoms and Xe nuclei.
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We have studied the efficiency of spin-exchange optical pumping (SEOP) in cells that contain either pure rubidium, mixtures of rubidium and potassium, or pure potassium. Optical pumping was performed with spectrally narrowed diode array lasers operating at either the wavelength of the Rb $5{s}_{1∕2}\ensuremath{-}5{p}_{1∕2}$ transition (795nm)(795\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(795nm) or the K $4{s}_{1∕2}\ensuremath{-}4{p}_{1∕2}$ transition (770nm)(770\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(770nm). Calculations of the expected efficiency are shown for practical conditions and compared to measurements. We have measured the vapor density ratio in Rb-K mixture cells using absorption of white light. Our focus is on the conditions that are relevant for neutron spin filters, but the results are relevant to all applications of SEOP. We find that both Rb-K and pure K cells yield improvements in the spin-exchange efficiency, and each approach has its own advantages. Overall, the best results for high polarizing rate with high polarization were obtained with mixture cells with K-Rb vapor density ratios between 2 and 6.