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 t hat is strong enough to cause sufficient dephasing of the signal of mi grating spins in the time of its application. The spin-relaxation mech anism limits the duration of the gradient field, and whenever particle migration is slow, the strength of the required inhomogeneous magneti c 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definition of the magnetic-field gradient fails and the usual formula for self-diffusion attenuation of spin echoes is no longer valid.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 pa per an expression for the self-diffusion attenuation of the spin echo is derived that is valid for a strong nonuniform magnetic field, and i t is shown that the nonuniform magnetic field must have the appropriat e spatial symmetry and that only isotropic self-diffusion can be measu red with the new method. NMR measurement in the earth's magnetic field of the self-diffusion constants in some liquids confirms these result s. (C) 1994 Academic Press, Inc.