Detection of magnetic contaminations in industrial products using HTS SQUIDs

Many products in the pharmaceutical and food industry are packaged in metallized wrappings. With standard high-frequency search coil metal detectors, they can only be tested for metal contaminations before they are wrapped. However, a key requirement of industrial quality control is the inspection of the products at the end of the production line. We have developed an inspection system for detecting the magnetic remanence of the contaminants. The system utilizes two HTS rf SQUID magnetometers with step edge junctions immersed in liquid nitrogen. The SQUIDs are arranged such that they cover the product channel in a rotated planar electronic gradiometer configuration. In order to suppress the low-frequency magnetic disturbances typically found in industrial environment, the product channel and the SQUID system were mounted inside a coaxial three-layer Mumetal shield. In combination with the gradiometric suppression, homogeneous low-frequency disturbance fields were attenuated by a factor of 400,000. The sensitivity of the system for small magnetic particles was determined experimentally, using numerous steel balls and splinters. A stainless steel particle of 3 mu g, corresponding to a sphere diameter of 0.09 mm, was detected with and without aluminized wrapping.