Xenon DNP in Inhomogeneous Solid-State Mixtures: Elucidation of the Spin-Diffusion Bottleneck

Mehrdad Pourfathi¹, Nicholas N. Kuzma¹, Rajat Kumar Ghosh¹, Stephen J. Kadlececk¹, Rahim Rizi¹

Dynamic Nuclear Polarization (DNP) of ¹²⁹Xe lags far behind that of [1-¹³C]pyruvate under similar experimental conditions. Understanding the interplay between DNP, spin diffusion and nuclear relaxation at the microscopic scale requires theoretical modeling as well as experimental probes of the local DNP parameters. We use the recently reported measurements in micro-clustered ¹²⁹Xe/1-propanol/trityl solid mixtures to validate our first-principles theory of the observed spin-diffusion bottleneck at the cluster boundaries. In our self-contained model, the factor of ~ 30 apparent drop in the local ¹²⁹Xe polarization at the cluster boundary is obtained analytically across a thin insulating layer of the matrix immediately surrounding the cluster, in good agreement with the measurements. We estimate cluster size, intrinsic temperature, and T₁ relaxation rates by fitting the model to the data. These findings have enabled us to increase the achievable ¹²⁹Xe polarization from 5% to 21%.