VIOLATION OF THE GRADIENT APPROXIMATION IN NMR SELF-DIFFUSION MEASUREMENTS

The gradient approximation of magnetic field fails at NMR self-diffusi on measurement with a very strong nonuniform magnetic field. The stand ard formula for self-diffusion attenuation of the spin-echoes has to b e replaced by a new one where the role of the gadient of one field com ponent ist taken over by the gradient of the magnitude of the total ma gnetic field. It leads to a distribution of the spin-echo attenuation in specimens, and to a nonexponential relation between the spin-echo s ignal and parameters of the applied nonuniform magnetic field. The ani sotropy of particle migrations furthermore enlarges the nonuniformity of the attenuation distribution.