Longitudinal Spin Relaxation of 129 Xe

The principle mechanisms and features of longitudinal relaxation (T₁) of ¹²⁹Xe will be reviewed and discussed, particularly in light of practical aspects for production and application of hyperpolarized ¹²⁹Xe generated by spin-exchange optical pumping [1]. The spin-rotation interaction which couples the ¹²⁹Xe nuclear spin to the orbital angular momentum of a pair of interacting Xenon atoms, is responsible for almost all intrinsic T₁ relaxation for all xenon phases, all temperatures, and in all applied magnetic fields. The main extrinsic mechanism is collisions with the container walls (wall relaxation). In the gas phase, relaxation due to formation and break-up of Xe2 dimers (van der Waals molecules) and wall relaxation are the most relevant mechanisms. The limiting intrinsic T₁ for pure xenon at room temperature and fields below a few Tesla is ≈ 5 h. The persistent-dimer mechanism can be suppressed by diluting the xenon with a second gas and/or by going a hundred degrees or more above room temperature. Wall relaxation can be controlled to some degree through the use of polymer coatings (typically silane- or siloxane based); wall-collision-limited T₁ values typically range from 20 min to a few hours. In the solid phase, which is an important part of compact accumulation and storage of hyperpolarized ¹²⁹Xe in continuous-flow systems, relaxation comes from modulation of the spin-rotation interaction by lattice vibrations (phonons). In this talk, more attention will be paid to gas-phase mechanisms that have relevance to current applications of hyperpolarized ¹²⁹Xe in magnetometry and precision measurements.
[1] B. Saam, “T₁ Relaxation of ¹²⁹Xe and How to Keep it Long,” in Hyperpolarized Xenon-129 Magnetic Resonance: Concepts, Production, Techniques and Applications, T. Meersman and E. Brunner, eds., 2015, pp. 122-141.