Scanning DC-SQUID system with high spatial resolution for NDE

We have developed a scanning DC-SQUID system with a high spatial resolution for NDE. The SQUID system is composed of a concentric multiloop DC-SQUID gradiometer using Nb/AlOx/Nb Josephson junction, a non-magnetic scanning stage, a cryostat and a computer for gathering data and controlling the system. We have developed the concentric multi-loop gradiometer (d/sup 2/Bz/dr/sup 2/, r/sup 2/=x/sup 2/+y/sup 2/) to have a higher spatial resolution and reduce background noise. The gradiometer was integrated on a single 3/spl times/3 mm/sup 2/ Si chip. To reduce the standoff distance between the specimen and the pick-up coil, a cryostat with a thin bottom space was developed. A scanning stage was made of nonmagnetic materials and was driven by air pressure using a rod-less cylinder to avoid making magnetic noise.