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Physical Review A

Spin-exchange optical pumping of He3 with Rb-K mixtures and pure K

Wangchun Chen, T. Gentile, Thad G Walker, Earl Babcock

Published Jan 23, 2007• 107 citations
Publisher

Abstract

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 $(795\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ or the K $4{s}_{1∕2}\ensuremath{-}4{p}_{1∕2}$ transition $(770\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. 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.

DOI
10.1103/physreva.75.013416

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Physical Review A•May 26, 2011•21 citations
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 5s1∕2\ensuremath−5p1∕25{s}_{1∕2}\ensuremath{-}5{p}_{1∕2}5s1∕2​\ensuremath−5p1∕2​ transition (795nm)(795\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(795nm) or the K 4s1∕2\ensuremath−4p1∕24{s}_{1∕2}\ensuremath{-}4{p}_{1∕2}4s1∕2​\ensuremath−4p1∕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.
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The applicability of commonly used 129Xe nuclear polarization (PXe) buildup models to large optical cell polarizers, which operate at lower gas pressures when compared to small cell polarizers for which spin-exchange optical pumping (SEOP) models were originally developed, has yet to be systematically evaluated. In addition, spatial heterogeneity of Rb vapor density ([Rb]) and its impact on SEOP in low pressure, large optical cell polarizers remains poorly understood. In this theoretical study, it was shown that 1D analytical and numerical PXe buildup models yield diverging PXe and critical flow rate values at low flow rates and high laser absorption, contributing to the discrepancy between theoretical and experimentally measured production rates. Additionally, it was found that [Rb] heterogeneity leads to lower PXe buildup and an undesirable increase in optimal cell temperature, while the optimal laser absorption remains relatively unchanged. The updated simulation framework presented here can be utilized to model other SEOP systems.
While the method of Spin-exchange optical pumping, SEOP, of noble gases has existed since 1960 [Bouchiat60], many fundamental aspects of spin exchange optical pumping have gone largely unexplained. These questions included the values of the rate coefficients for spin-exchange, many aspects of the alkali-metal polarization and density inside the SEOP cells, the mechanisms of He wall relaxation, non-ideal values of the observed He polarization, and efficiency to produce it. This thesis describes the results of our efforts to quantity all parameters in SEOP and to begin to determine some of these unknowns. This thesis presents measurements of spin-exchange rate coefficients and efficiencies. A method to extend SEOP to multiple alkali species is presented. In addition, limits to the efficiency of optical pumping, the efficiency of spin-exchange, and the observed He polarization are explored.
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