J. Stepisnik et al., STRONG NONUNIFORM MAGNETIC-FIELD FOR SELF-DIFFUSION MEASUREMENT BY NMR IN THE EARTHS MAGNETIC-FIELD, Journal of magnetic resonance. Series A, 107(2), 1994, pp. 167-172
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for self-diffusion attenuation of spin echoes is no longer valid.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.