Xenon-131 Surface Sensitive Imaging of Aerogels in Liquid Xenon near the Critical Point

In recent years, optically pumped xenon-129 has received a great deal of attention as a contrast agent in gas-phase imaging. This report is about the other NMR active xenon isotope (i.e., xenon-131, S = 3/2) which exhibits distinctive features for imaging applications in material sciences that are not obtainable from xenon-129 (S = 1/2). The spin dynamics of xenon-131 in gas and liquid phases is largely determined by quadrupolar interactions which depend strongly on the surface of the surrounding materi* als. This leads to a surface dependent dispersion of relaxation rates, which can be substantial for this isotope. The dephasing of the coherence due to quadrupolar interactions may be used to yield surface specific contrast for imaging. Although optical pumping is not practical for this isotope because of its fast qua* drupolar relaxation, a high spin density of liquid xenon close to the critical point (289 K) overcomes the sensitivity problems of xenon-131. We report the first xenon-131 magnetic resonance images and have tested this technique on various meso-porous aerogels as host structures. Aerogels of different densities and changing levels of hydration can clearly be distinguished from the images obtained.