Gas-phase spin relaxation of 129Xe

We have completed an extensive study of 129Xe longitudinal spin relaxation in the gas phase, involving both intrinsic and extrinsic mechanisms. The dominant intrinsic relaxation is mediated by the formation of persistent Xe2 van der Waals dimers. The dependence of this relaxation on applied magnetic field yields the relative contributions of the spin-rotation and chemical-shift-anisotropy interactions; the former dominates at magnetic fields below a few tesla. This relaxation also shows an inverse quadratic dependence on temperature T; the maximum low-field intrinsic relaxation for pure xenon at room temperature (measured here to be 4.6 h, in agreement with previous work) increases by ~60% for T=100 °C. The dominant extrinsic relaxation is mediated by collisions with the walls of the glass container. Wall relaxation was studied in silicone-coated alkali-metal-free cells, which showed long (many hours or more) and robust relaxation times, even at the low magnetic fields typical for spin-exchange optical pumping (~3 mT). The further suppression of wall relaxation for magnetic fields above a few tesla is consistent with the interaction of 129Xe with paramagnetic spins on or inside the surface coating. At 14.1 T and sufficiently low xenon density, we measured a relaxation time T1 =99 h, with an inferred wall-relaxation time of 174 h. A prototype large storage cell (12 cm diameter) was constructed to take advantage of the apparent increase in wall-relaxation time for cells with a smaller surfaceto- volume ratio. The measured relaxation time in this cell at 3 mT and 100 °C was 5.75 h. Such a cell (or one even larger) could be used to store many liters of hyperpolarized 129Xe produced by a flow-through polarizer and accumulator for up to three times longer than currently implemented schemes involving freezing xenon in liquid nitrogen.