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Physical Review Letters

Hybrid Spin-Exchange Optical Pumping of H e 3

Earl Babcock, Ian A. Nelson, Steve Kadlecek, Bastiaan Driehuys, L. W. Anderson, F. William Hersman, Thad G Walker

Published Sep 16, 2003• 233 citations

Abstract

We demonstrate spin-exchange optical pumping of $^{3}\mathrm{H}\mathrm{e}$ using a ``hybrid'' K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and $^{3}\mathrm{H}\mathrm{e}$ atoms then transfer the angular momentum to the $^{3}\mathrm{H}\mathrm{e}$ with much greater efficiency than $\mathrm{R}\mathrm{b}\mathrm{\text{\ensuremath{-}}}^{3}\mathrm{H}\mathrm{e}$. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches $1/4$, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.

DOI: 10.1103/physrevlett.91.123003

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Thad G Walker

The standard model of Spin-Exchange Optical Pumping of 3 He has come under close scrutiny as researchers try to maximize the quantity and polarization of 3 He. This paper will review four important effects that have come to light: 1) pumping with frequency narrowed diode lasers significantly improves the polarization and production rate as compared to off-the-shelf broadband sources 2) Rb-K mixtures again improve the production rate due to reduced collisional loss in K-He collisions, 3) a unexplained 3 He wall relaxation mechanism that scales closely with alkali vapor pressure limits the attainable 3 He polarization to about 80%, 4) a small collisionally-induced breakdown in the selection rules for optical pumping explains why narrowband lasers are so advantageous, and may explain some observed limitations for Rb-K pumping as well.

Journal of Physics Conference Series•Jun 1, 2011•67 citations

We demonstrate spin-exchange optical pumping of

^{3}\mathrm{H}\mathrm{e}

using a ``hybrid'' K-Rb vapor mixture. The Rb atoms absorb light from a standard laser at 795 nm, then collisionally polarize the potassium atoms. Spin-exchange collisions of K and

^{3}\mathrm{H}\mathrm{e}

atoms then transfer the angular momentum to the

^{3}\mathrm{H}\mathrm{e}

with much greater efficiency than

\mathrm{R}\mathrm{b}\mathrm{\text{\ensuremath{-}}}^{3}\mathrm{H}\mathrm{e}

. For a K-rich vapor, the efficiency of the hybrid spin-exchange collisions approaches

1/4

, an order of magnitude greater than achieved by pure Rb pumping. We present the first measurements of actual photon efficiencies (polarized nuclei produced per absorbed photon), and show that a new parasitic absorption process limits the total efficiencies for both hybrid and pure Rb pumping.

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.

We present measurements of the optical absorption of K vapor at 795 nm due to the presence of high-pressure He gas. The results set a limit on the polarization attainable in hybrid spin-exchange optical pumping of

^{3}\mathrm{He}

.

We report on extensive experimental measurements of the key rates that determine the efficiency for polarizing the nuclei of

{}^{3}\mathrm{He}

by spin exchange with optically pumped Rb vapor. In agreement with recent theoretical predictions, we find a strong temperature dependence of the electron-spin loss rates due to

{}^{3}\mathrm{HeRb}

collisions. We also find that the maximum possible efficiency for spin-exchange polarization of

{}^{3}\mathrm{He}

by K is 10 times greater than for Rb.

We report investigations of spin exchange between

^{21}\mathrm{Ne}

nuclei and optically pumped Rb vapor. Polarization of 30% (

〈{K}_{z}〉=0.46

) of 2\ifmmode\times\else\texttimes\fi{}

{10}^{19}

atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ has been achieved with a vapor pressure of 3\ifmmode\times\else\texttimes\fi{}

{10}^{14}

atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ of Rb. This high density of alkali metal can be efficiently optically pumped only when light trapping is suppressed by inclusion of at least 100 Torr of

〈{K}_{z}〉=0.46

. In addition, we have made the first accurate measurement of the binary-collision spin-exchange rate: $〈{\ensuremath{\sigma}}_{\mathrm{ex}}v〉=(4.66\ifmmode\pm\else\textpm\fi{}0.28)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$

{\mathrm{cm}}^{3}

-${\mathrm{s}}^{\ensuremath{-}1}$. Extension of these techniques to

^{3}\mathrm{He}

has yielded 5% polarization of 2\ifmmode\times\else\texttimes\fi{}

{10}^{19}

atoms-${\mathrm{cm}}^{\ensuremath{-}3}$ in a preliminary experiment.

The standard model of Spin-Exchange Optical Pumping of 3 He has come under close scrutiny as researchers try to maximize the quantity and polarization of 3 He. This paper will review four important effects that have come to light: 1) pumping with frequency narrowed diode lasers significantly improves the polarization and production rate as compared to off-the-shelf broadband sources 2) Rb-K mixtures again improve the production rate due to reduced collisional loss in K-He collisions, 3) a unexplained 3 He wall relaxation mechanism that scales closely with alkali vapor pressure limits the attainable 3 He polarization to about 80%, 4) a small collisionally-induced breakdown in the selection rules for optical pumping explains why narrowband lasers are so advantageous, and may explain some observed limitations for Rb-K pumping as well.