H at ultra-low magnetic field*

In the past decade, various hyperpolarization techniques have been developed to address the sensitivity limitations of thermal nuclear spin polarization at room temperature and typical magnetic field strengths. These techniques mostly focused on direct electron-to-nucleus spin order transfer; here, we investigate direct polarization transfer between nuclei via the spin polarization induced nuclear Overhauser effect (SPINOE). Specifically, we explore polarization transfer between ¹²⁹Xe nuclei, which have been efficiently polarized by spin-exchange optical pumping, and thermally polarized ¹⁹F and ¹H nuclei.

Both ¹⁹F and ¹H nuclear spins are difficult to polarize due to their short relaxation times, causing rapid return to thermal equilibrium. By bubbling hyperpolarized (HP) ¹²⁹Xe into solutions containing ¹⁹F atoms, we performed ¹⁹F NMR spectroscopy at ultra-low magnetic field (<50 mT). The SPINOE-enhanced ¹⁹F signal persisted nearly 200 times longer than its normal relaxation time, while polarization transfer to ¹H enabled simultaneous multinuclear imaging – otherwise infeasible at these field strengths.

Future work will focus on reducing image noise and investigating spin-polarization transfer between HP ¹²⁹Xe and other NMR-visible nuclei for ultra-low field applications.