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Anisotropic Spin-Exchange

Thad G Walker, Stephen J. Kadlecek, Ian A. Nelson

Published Nov 25, 2009• 0 citations
OpenAlex

Abstract

Using measured spin-transfer rates from alkali atoms to 3He, combined with spin-relaxation rates of the alkali atoms due to 3He and 4He, it should be possible to di?erentiate between isotropic and anisotropic spin-exchange. This would give a fundamental limit on the 3 He polarization attainable in spin-exchange optical pumping. For K-He, we ?nd the limit to be 0.90?0.11.

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Method for deducing anisotropic spin-exchange rates

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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.

•Jan 1, 2005•21 citations
Using measured spin-transfer rates from alkali atoms to 3He, combined with spin-relaxation rates of the alkali atoms due to 3He and 4He, it should be possible to di?erentiate between isotropic and anisotropic spin-exchange. This would give a fundamental limit on the 3 He polarization attainable in spin-exchange optical pumping. For K-He, we ?nd the limit to be 0.90?0.11.
Using measured spin-transfer rates from alkali-metal atoms to 3He^{3}\mathrm{He}3He, combined with spin-relaxation rates of the alkali-metal atoms due to 3He^{3}\mathrm{He}3He and 4He^{4}\mathrm{He}4He, it should be possible to differentiate between isotropic and anisotropic spin exchange. This would give a fundamental limit on the 3He^{3}\mathrm{He}3He polarization attainable in spin-exchange optical pumping. For K-He, we find the limit to be $0.90\ifmmode\pm\else\textpm\fi{}0.11$.
We use accurate ab initio and quantum scattering calculations to demonstrate that the maximum ³He spin polarization that can be achieved in spin-exchange collisions with potassium (³⁹K) and silver (¹⁰⁷Ag) atoms is limited by the anisotropic hyperfine interaction. We find that spin exchange in Ag-He collisions occurs much faster than in K-He collisions over a wide range of temperatures (10-600 K). Our analysis indicates that measurements of trap loss rates of ²S atoms in the presence of cold ³He gas may be used to probe anisotropic spin-dependent interactions in atom-He collisions.
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.
We present experimental studies on the spin-exchange optical pumping of 3He using alkali metal potassium vapor. High 3He polarizations are achieved using lower laser power than traditional rubidium-3He pumping studies. In addition, spin-exchange rate coefficient measurements for potassium-3He, rubidium-3He and cesium-3He pairs are reported. By spin-exchange optical pumping with potassium vapor, a high 3He polarization has been achieved in a mid-sized double-chamber glass target cell using only 5 watts of Ti:sapphire laser power. The spin-exchange rate coefficients of potassium-3He, cesium-3He and rubidium-3He pairs are measured and compared. The results are kSE = (4.0+/-0.3)E-20 cm3/sec for the potassium-3He pair, kSE = (6.5+/-0.4)E-20 cm3/sec for the rubidium-3He pair and kSE = (13.6 +/-1.3)E-20 cm3/sec for the cesium-3He pair. The results are consistent with theoretical predictions and confirm that the efficiency for spin-exchange polarization of 3He with potassium is significantly higher than with rubidium or cesium. The results motivate the development of high power diode lasers tuned to the appropriate potassium wavelengths. Applied to the production of polarized 3He as an atomic beam source for colliders, these developments could have significant impact on future studies of neutron spin structure. These studies may also have impact on magnetic resonance imaging using hyperpolarized 3He.
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.