STRONG NONUNIFORM MAGNETIC-FIELD FOR SELF-DIFFUSION MEASUREMENT BY NMR IN THE EARTHS MAGNETIC-FIELD

Self-diffusion measurement by NMR requires a magnetic-field gradient that is strong enough to cause sufficient dephasing of the signal of migrating spins in the time of its application. The spin-relaxation mechanism limits the duration of the gradient field, and whenever particle migration is slow, the strength of the required inhomogeneous magnetic field may exceed that of the main magnetic field. In this case, the definition of the magnetic-field gradient fails and the usual formula for self-diffusion attenuation of spin echoes is no longer valid. This always happens with NMR in the earth's weak magnetic field. In the paper an expression for the self-diffusion attenuation of the spin echo is derived that is valid for a strong nonuniform magnetic field, and it is shown that the nonuniform magnetic field must have the appropriate spatial symmetry and that only isotropic self-diffusion can be measured with the new method. NMR measurement in the earth's magnetic field of the self-diffusion constants in some liquids confirms these results. (C) 1994 Academic Press, Inc.