Nuclear Quadrupole Resonance as a Non-Destructive Testing Tool

Nuclear pure quadrupole resonance (NQR) is a resonance technique that provides distinctly different information from that provided by nuclear magnetic resonance (NMR). In NMR the splitting of the energy levels, and therefore the frequency observed, occurs because of the interaction of the nuclear magnetic moment with an external magnetic field. Information about the system under study comes from perturbations on this magnetic interaction. These perturbations lead to a broadening of the line, or to relaxation effects on the interchange of energy between the spins and the lattice, and among the spins. In NQR the primary interaction is between the electric quadrupole moment of a nucleus and the electric field gradient at that nucleus. The field gradient is provided by internal interactions in the sample itself, arising from the chemical bonds, rather than by an external field. Anything that changes the bonding environment, such as tensile stress, will cause shifts in the quadrupole resonance frequency. All nuclei with spin greater than 1/2 have a nuclear quadrupole moment, in addition to their magnetic moment. The nucleus used as an example in this paper is 75As, which has spin 3/2.