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

Abstract

Metastability exchange optical pumping (MEOP) is experimentally investigated in ³He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 2³S-2³P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of ³He 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 2³S 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 2³S state is observed to systematically lead to slower build-up of ³He 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 ³He 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.