Abstract
This paper is concerned with a detailed investigation of the dynamic polarization of the protons in ·24O which occurs when one saturates the "forbidden" microwave transitions that simultaneously flip a proton spin and a electron spin. The rate equations for the electron and nuclear polarization are solved for (a) a simple ideal model, (b) a model for the case where the forbidden lines are not resolved, and (c) a model taking into account nuclear-spin temperature diffusion. An apparatus for simultaneous observation of proton magnetic resonance and paramagnetic resonance at liquid helium temperatures is described. The spin-lattice relaxation time is directly measured by a transient method, and it is found that for temperatures in the range . In the same crystals, the proton relaxation time is also measured by a transient method and found to be and dependent on the concentration of ions. The relative magnitudes of and are best explained by a model intermediate between (a) and (c). At K and a microwave frequency kMc/sec, the proton polarization is observed for a number of different concentrations of . The magnitude of the polarization, its dependence on magnetic field and microwave power, and the transient behavior are studied and qualitatively explained. In a crystal containing 1% Ce, the proton polarization is observed to become greater than the thermal equilibrium value by the factor 150, which is about one-quarter of the theoretical ideal. At higher microwave frequencies ( kMc/sec) it should be possible to obtain in this crystal sufficient proton polarization (∼25%) to be useful for dynamic nuclear cooling experiments and nuclear targets.
- Received 2 February 1961
DOI:https://doi.org/10.1103/PhysRev.122.1781
©1961 American Physical Society