CONSTANT-RELAXATION METHODS FOR DIFFUSION MEASUREMENTS IN THE FRINGE-FIELD OF SUPERCONDUCTING MAGNETS

A number of new NMR methods have been introduced for measuring molecul ar self-diffusion coefficients in the fringe field of superconducting magnets, avoiding the determination of longitudinal and transverse mag netization relaxation rates. Constant-relaxation methods based on the stimulated-echo product signals allow the determination of both consta nt and time-dependent diffusion coefficients. A density-operator forma lism taking into account phenomenologically the effect of relaxation a nd diffusion encoding on the Liouville coordinates has been used to in troduce and characterize an inhomogeneous mixed echo. This echo corres ponds to a spatial and temporal superposition of a conventional spin e cho and a stimulated echo. The mixed echo is generated by the action o f a pulse sequence based on a stimulated-echo three-pulse sequence wit h an insertion of a Carr-Purcell-like pulse sequence in the last pulse interval. A constant relaxation method using a Carr-Purcell mixed-ech o pulse sequence has been developed. It has proved to be advantageous when T-2 is not very small compared to T-1. The performance and limita tions of these methods are discussed and demonstrated experimentally. (C) 1994 Academic Press, Inc.