Chapter 7: T₁ Relaxation of ¹²⁹Xe and How to Keep it Long

The principle mechanisms and features of longitudinal relaxation (T₁) of ¹²⁹Xe are reviewed and discussed, particularly in light of practical aspects for production and application of hyperpolarized ¹²⁹Xe generated by spin-exchange optical pumping. The spin–rotation interaction, which couples the ¹²⁹Xe nuclear spin to the orbital angular momentum of a pair of interacting Xe 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, both relaxation due to formation and break-up of Xe₂ dimers (van der Waals molecules) and wall relaxation are most relevant. 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). Reliable T₁ values of ≈ 2.4 h are obtained in the solid, so long as care is taken to maintain the entire polycrystalline sample at 77 K. Longer T₁ values are obtained in samples that pass through the liquid phase to form “ice”, as compared to those that condense directly from the gas phase as “snow.”