Improved Techniques for Structural NDT Using SQUIDs

Although the SQUID has been identified as having possible applications in structural nondestructive testing (NDT), the 10 to 20 mm minimum lift-off (or separation between pick-up coils and the test subject) imposed by a typical conventional cryostat makes high resolution SQUID inspection impossible. Here, we report on the development of a 4.2K cryostat which allows the lift-off to be reduced to 2 mm. A complementary planar gradiometric pick-up coil assembly with coil diameters of 2 and 4 mm is also outlined. The increased inspection resolution should significantly widen the scope of laboratory assessment of structural NDE using conventional SQUIDs. Preliminary results are presented to support this conclusion, and their relevance to SQUIDs operating at LN2 temperatures is noted.