PLANAR GRADIOMETERS WITH HIGH-T-C DC SQUIDS FOR NONDESTRUCTIVE TESTING

We investigated planar gradiometers on the basis of galvanically coupl ed high-T-c. DC SQUIDs for application in non-destructive testing (NDT ). The stability of the sensor of better than 1% permits NDT investiga tions in unshielded environments. The layout of the gradiometer sensor s and a corresponding determination of their effective areas and basel ines will be discussed as well as the properties of the Josephson junc tions in the DC SQUID based on step-edge and bicrystal grain boundarie s. We show measurements with this type of sensor applied in a testing system for NDT to determine spatial and field gradient resolution as w ell as the influence of sensor position and experimental environment s uch as the dewar material on the performance of the sensor and the who le testing system. As an example first investigations of the permanent magnetization of hardened valves will be presented where we look for a correlation between hardening and magnetic field distribution.