Polarization Enhancement of $^{129}$Xe in TEMPO-Doped Xenon at 2.0K and 0.31 Tesla.

Enhancements of $^{129}$Xe NMR signals up to 300 times their equilibrium value at temperature 2.0K and magnetic field $\sim $ 0.3T, corresponding to a spin polarization $>$1{\%}, have been obtained in solid xenon with TEMPO$^{1}$ impurity concentrations near 10$^{18}$ /cm$^{3}$. This dynamic polarization results from interaction of the nuclear Zeeman spin reservoir with the electron spin-spin reservoir$^{2}$, while irradiating near the electron Zeeman resonance frequency ($\sim $8.7 GHz) with up to 500 mW of microwave power. The mixing procedures in which TEMPO is introduced into liquid Xenon prior to freezing, and the \textit{electron} spin resonance line shapes which correlate with dynamic polarization enhancements are described. At higher magnetic fields up to $\sim $5 T, and temperatures still within the 1 - 2K range, the results suggest a route for fast and copious production of hyperpolarized $^{129}$Xe, with its multitude of applications$^{3}$. 1. Pluckthun, M. et al, Nucl. Instrum. and Meth. in Phys. Res. A \underline {400}, 122 (1997). 2. Abragam, A. and M. Goldman, Rep. Prog. Phys. \underline {41}, 395 (1978). 3. Oros, A-M and N. J. Shah, Phys. Med. Biol. \underline {49}, R105 (2004).