Unshielded use of thin-film Nb dc superconducting quantum interference devices and integrated asymmetric gradiometers for nondestructive evaluation

Novel nondestructive evaluation measurements made using niobium de superconducting quantum interference devices with integrated asymmetric first-order gradiometers are described. Comparative theoretical and experimental studies of their spatial response have been described, and it is shown that the gradiometric response makes operation possible in an unshielded and electromagnetically noisy environment. As a demonstration of their capabilities, subsurface defects in a multilayer aluminum structure have been located and mapped using induced eddy currents at 70 Hz, with no magnetic shielding around the specimen or cryostat. (C) 1997 American Institute of Physics.