SCANNING DC-SQUID SYSTEM WITH HIGH-SPATIAL-RESOLUTION FOR NDE

We have developed a scanning DC-SQUID system with a high spatial resol ution for NDE. The SQUID system is composed of a concentric multi-loop DC-SQUID gradiometer using Nb/AlOx/Nb Josephson junction, anon-magnet ic scanning stage, a cryostat and a computer for gathering data and co ntrolling the system. We have developed the concentric multi-loop grad iometer (d(2)Bz/dr(2), r(2)=x(2)+y(2)) to have a higher spatial resolu tion and reduce background noise. The gradiometer was integrated on a single 3 x 3 mm(2) Si chip. To reduce the standoff distance between th e specimen and the pick-up coil, a cryostat with a thin bottom space w as developed. A scanning stage was made of nonmagnetic materials and w as driven by air pressure using a rod-less cylinder to avoid making ma gnetic noise.