This paper discusses the relaxation of proton spins in organic solids due to their interaction with paramagnetic free radicals. It has been shown both theoretically and experimentally that after complete saturation the nuclear magnetization increases initially as a linear function of time. A transient region follows in which the relaxation function is proportional to t1/2. Finally there is the well known exponential region as a consequence of spin diffusion. This characteristic behaviour of “diffusion-limited” relaxation has been found in several organic systems between 77°K and 4,2°K. From the experimental observation of the relaxation function the constants C of direct relaxation, the coefficients D of nuclear spin diffusion and other parameters of relaxation and spin diffusion have been determined. The values of D are of the order of 10-14 to 10-16 cm2/s depending on the particular system and on the vitreous or polycrystalline state of the solid. The results are remarkable with regard to the high barrier radii and the signal losses. These can be explained by only a small averaging of the local fields produced by the electron spins at surrounding nuclei.
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