Deep nondestructive testing using a bulk high T/sub c/ RF-SQUID

We report the utilization of a high T/sub c/ symmetric two-hole RF-SQUID (superconducting quantum interference device) operating at liquid nitrogen temperature as a gradiometer in nondestructive testing (NDT). This technology offers a possibility of solving simultaneously four requirements: sensitivity, penetration depth, resolution, and portability of the device. We used the simple dipole model, and showed that the signal attenuation in the near zone agreed with our experimental results and is less than that of a plane wave. In our experiment we used a low frequency current of 30 Hz and demonstrated that the amplitude of the signal was reduced only by 20% after the insertion of 1" (2.54 cm) of aluminum alloy plates between the mock crack and the SQUID in agreement with the theoretical calculations.<>