Magnetic susceptibility imaging for nondestructive evaluation (using SQUID magnetometer)

High-resolution superconducting magnetometers such as MicroSQUID (superconducting quantum interference device) have been shown to be effective for nondestructive evaluation. MicroSQUID can also be used with a room-temperature magnet to image the magnetic susceptibility of materials. A diamagnetic or paramagnetic sample is scanned in the applied field, and the local perturbations are measured. For thin samples, such as plates, sheets, or thin sections of rock, the data are deconvolved to generate two-dimensional susceptibility images. Three-dimensional structures can be imaged with magnetic susceptibility tomography: deconvolution of a large data set obtained by applying the field and scanning in multiple orientations. Extremely small surface defects on nonmagnetic or weakly magnetic samples are imaged by decorating the sample with paramagnetic microspheres prior to scanning. Magnetic susceptibility imaging demonstrates the feasibility of SQUID nondestructive evaluation on materials that could previously be examined only with X-rays or ultrasound.<>