High Dynamic Polarization of Protons

This paper is a detailed study of the dynamic polarization of the protons in the waters of hydration in the crystal ${\mathrm{}(\mathrm{N}\mathrm{d},\mathrm{L}\mathrm{a})\mathrm{}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O, which occurs when one saturates a microwave "forbidden" transition that simultaneously flips a proton and a ${\mathrm{Nd}}^{3+}$ ion. The proton relaxation, as well as the dynamics of the polarization process, is discussed in terms of a phenomenological shell-of-influence model. A polarization apparatus is described for use at frequencies up to 75 kMc/sec and temperatures $1.3<T<\mathrm{}4.2\mathrm{}$°K. Proton relaxation rates ${T_{1p}}^{-1\mathrm{}}$ are measured over a wide range of fields $H$ and temperatures, and are found to be in agreement with the predicted rate ${T_{1p}}^{-1\mathrm{}}\approx {T_{1e}}^{-1\mathrm{}}{\left(\frac{g_{\mathrm{Nd}}\beta }{H}\right)}^2{〈r^{-6\mathrm{}}〉}_{\mathrm{av}}$ where ${T_{1e}}^{-1\mathrm{}}$ is the relaxation rate for the ${\mathrm{Nd}}^{3+}$ ion, $r$ is the separation between proton and ion, and the average is taken over the shell of influence. At the Nd concentration studied (∼1%), the local fields apparently prevent the free diffusion of spin temperature. Dynamic proton polarization measurements are made over a wide range of frequencies and temperatures: at 75 kMc/sec and 1.5°K a polarization of at least 70% is observed. At these high polarizations, the proton magnetic-resonance line shape changes and a structure appears, due to the freezing-in of the local dipolar fields. At still higher polarizations a local-field computation predicts a series of sharp proton lines, suggesting the possibility of high-resolution nuclear magnetic resonance in solids. The crystal studied is a favorable one for construction of polarized proton targets.