VIOLATION OF THE GRADIENT APPROXIMATION IN NMR SELF-DIFFUSION MEASUREMENTS

The gradient approximation of magnetic field fails at NMR self-diffusion measurement with a very strong nonuniform magnetic field. The standard formula for self-diffusion attenuation of the spin-echoes has to be replaced by a new one where the role of the gadient of one field component ist taken over by the gradient of the magnitude of the total magnetic field. It leads to a distribution of the spin-echo attenuation in specimens, and to a nonexponential relation between the spin-echo signal and parameters of the applied nonuniform magnetic field. The anisotropy of particle migrations furthermore enlarges the nonuniformity of the attenuation distribution.