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 (Nd,0ex0exLa)₂{Mg}₃(NO₃)₁₂·{}24H₂O, which occurs when one saturates a microwave "forbidden" transition that simultaneously flips a proton and a Nd³⁺ 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 $1.3<T<4.2$^∘{}K. Proton relaxation rates T₁ₚ^-1 are measured over a wide range of fields H and temperatures, and are found to be in agreement with the predicted rate T₁ₚ^-1≈T₁ₑ^-1(gNdβH)²〈r^-6〉ₐᵥ where T₁ₑ^-1 is the relaxation rate for the Nd³⁺ 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.