Precision determination of alkali-metal number density via Faraday Rotation*

Hyperpolarized ¹²⁹Xe produced by spin-exchange optical pumping (SEOP) has several important applications, including magnetic resonance imaging and precision magnetometry. Characterizing the spin-exchange process requires accurate determination of the alkali-metal number density in the glass vapor cells typically employed. Faraday rotation near the D₁ or D2 resonance has been shown to be an accurate method [1,2] and can yield results that differ significantly from the published vapor-density curves often relied upon in SEOP measurements.  SEOP of ¹²⁹Xe presents additional challenges (compared to ³He), since alkali-metal densities are much lower at typical operating temperatures, meaning lower signal-to-noise ratio and more interference from rotations generated by other materials (e.g., the glass of the vapor cell and the oven windows).  We present a scheme for accurate measurement of alkali-metal vapor density in the temperature range 80-140 °C (corresponding to Rb-Xe SEOP) by measuring the rotation per unit applied magnetic field as a function of wavelength detuning. The density is deduced from a parabolic fit with no need for precise knowledge of the Verdet constant of the glass. The ≈40 G magnetic field range was added to an existing Helmholtz-coil field through an additional pair of smaller concentric Helmholtz coils, such that the stable field normally used for SEOP is undisturbed. [1] B. Chann, et al., Phys. Rev. A 66, 032703 (2002). [2] E. Vliegen, et al., Nucl. Instrum. Methods Phys. Res., Sect. A 460, 444–450 (2001).