Metastability exchange optical pumping of 3He gas up to hundreds of millibars at 4.7 Tesla

Abstract

Abstract. Metastability exchange optical pumping (MEOP) is experimentally investigated in3 He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 2 3 S-2 3 P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of 3 He atoms in the rf discharge plasma. The collisional broadening rate is measured (12.0 ± 0.4 MHz mbar −1 FHWM) and taken into account for accurate absorption-based measurements of both nuclear polarization in the ground state and atom number density in the metastable 2 3 S state. The results lay the ground for a comprehensive assessment of the efficiency of MEOP, by comparison with achievements at lower field (1 mT-2 T) over an extended range of operating conditions. Stronger hyperfine decoupling in the optically pumped 2 3 S state is observed to systematically lead to slower build-up of 3 He orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high pressure but in a less dramatic way than observed at 2 T in the same sealed glass cells. To date, thanks to the linear increase in gas density, they correspond to the highest nuclear magnetizations achieved by MEOP in pure 3 He gas. The improved efficiency puts less demanding requirements for compression stages in polarized gas production systems and makes high-field MEOP particularly attractive for magnetic resonance imaging of the lungs, for instance.

Optically polarizedHe3

“…The maximum rate is experienced by atoms in the velocity class v * that is resonant with the laser frequency, and is given by: $${\gamma }_{ij}(v^{\ast },\mathbf{r})=\frac{2\pi af}{m_{\mathrm{normale}}\omega {\gamma }^2}\frac{\gamma }{{\mathrm{normal\Gamma }}^{\prime }/2}{I}_{\text{las}}(\mathbf{r})T_{ij}\gamma ,$$ where I las ( r ) is the laser intensity, ω its angular frequency, α the fine structure constant, f the oscillator strength of the transition, and m e the electron mass; the numerical value of the first fraction is 0.149 m 2 /W. The total damping rate of the optical coherence of the transition, Γ′/2 = γ + πw , results from the combined effects of the radiative decay rate γ and of the pressure-dependent collisional broadening w , with a value for w / p of order 20 MHz/mbar (Vrinceanu et al , 2004) or 12 MHz/mbar (Nikiel-Osuchowska et al , 2013) that remains to be confirmed. In the low-pressure limit ( w ≪ γ ), γ ij = T ij γ for I las = 6.7 W/m 2 .…”

“…The photon efficiency of these high-field optical pumping schemes can be as high as in low field ( η ≈ 1 for ${\mathrm{normalf}}_2^{-}$ pumping at high pressure) but this remains to be systematically studied (Batz et al , 2011; Nikiel-Osuchowska et al , 2013). With modified He level structures and only weak hyperfine couplings remaining in the excited states, high-field MEOP has two distinct features: highly absorbing lines can be used, with absorption coefficients ≃ 10 times larger than for C 8 or C 9 pumping at low field, and the transfer of angular momentum to the ground state is orders of magnitudes slower than in low field at a given pressure.…”

“…Due to hyperfine decoupling, ME collisions imperfectly enforce the link between P He and the 2 3 S state populations, so that systematic measurement errors may appear even for moderate pump intensities. Fortunately, an adequate choice of optical transitions can be made to avoid such difficulties, by probing a pair of sublevels that are not addressed by the pump (Abboud et al , 2004; Nikiel-Osuchowska et al , 2013; Suchanek et al , 2007). This is achieved using the convenient line doublets highlighted in Fig.…”

“…Traditional low-field MEOP is performed in pure 3 He or 3 He - 4 He mixtures (Stoltz et al , 1996a) at pressures on the order of 1 mbar, whereas high-field MEOP has been performed at pressures up to a few hundred millibar (Nikiel-Osuchowska et al , 2013). An electrical discharge produced by external electrodes is employed to produce metastable densities on the order of 10 10 cm −3 .…”

“…More compact piston compression apparatus have been developed (Beecham et al , 2011; Kraft et al , 2014; Mrozik et al , 2011) and for MRI applications MEOP has been extended to optical pumping pressures up to hundreds of millibar at high magnetic fields (Nikiel-Osuchowska et al , 2013). The use of K-Rb mixtures has increased the polarizing rate for SEOP (Chen et al , 2007b; Singh et al , 2015) and large scale production of polarized 3 He via SEOP is under development (Hersman et al , 2013).…”