High Dynamic Polarization of Protons

Abstract

This paper is a detailed study of the dynamic polarization of the protons in the waters of hydration in the crystal ${(\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{L}\mathrm{a})}_{2}$${\mathrm{Mg}}_{3}$${(\mathrm{N}{\mathrm{O}}_{3})}_{12}$\ifmmode\cdot\else\textperiodcentered\fi{}24${\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 $1.3<T<4.2$\ifmmode^\circ\else\textdegree\fi{}K. Proton relaxation rates ${{T}_{1p}}^{\ensuremath{-}1}$ are measured over a wide range of fields $H$ and temperatures, and are found to be in agreement with the predicted rate ${{T}_{1p}}^{\ensuremath{-}1}\ensuremath{\approx}{{T}_{1e}}^{\ensuremath{-}1}{(\frac{{g}_{\mathrm{Nd}}\ensuremath{\beta}}{H})}^{2}{〈{r}^{\ensuremath{-}6}〉}_{\mathrm{av}}$ where ${{T}_{1e}}^{\ensuremath{-}1}$ 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 (\ensuremath{\sim}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\ifmmode^\circ\else\textdegree\fi{}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.