Abstract

Large nuclear polarizations M, in the range of 80%, can now be achieved in gaseous ³He by optical pumping. Such gases have applications in many fields of physics, from quantum fluids to polarized targets for nuclear physics. The optical pumping makes use of the newly developed lamp pumped LNA lasers at λ=1.08µm which excites the 2³s₁ metastable atoms produced by an RF discharge in the helium gas.^[1] We report here on a new experiment which measures M with a high absolute accuracy. The method is based on the absorption by the metastable atoms of a weak probe beam propagating perpendicular to the direction of the magnetization (which in turn is parallel to an applied magnetic field${\overrightarrow{\text{B}}}_{\text{0}}$, see Fig. 1). The probe beam is generated by a single mode pumped LNA laser^[2] tuned to the C₉ helium transition (2³s₁, F=3/2 → 2³P₀). The frequency of the probe laser is modulated on and off resonance by a thin etalon mounted on a galvoplate driven at 15Hz. The axis of the linear polarizer P in Fig. 1 is set at 45° from the ${\overrightarrow{\text{B}}}_{\text{0}}$ direction; consequently the probe beam polarization is a combination of π and σ with respect to ${\overrightarrow{\text{B}}}_{\text{0}}$.

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