High Tc SQUID System for Detection of Small Metallic Contaminant in Industrial Products

A three channel High-Tc SQUID system for detection of magnetic contaminants in industrial products was developed. Finding ultra-small metallic contaminants is a big issue for manufacturers producing commercial products such as lithium ion batteries. When the contamination does occur, the manufacturer of the product suffers a great loss to recall the tainted products. The outer dimension of metallic particles less than 100 micron can not be detected by X-ray imaging, which is commonly used as the inspection method. Therefore, a highly sensitive detection system for small contaminants is required. We developed a detection system based on a three channel high-Tc SQUID microscope with a high performance magnetic shield. Three SQUIDs were installed in one microscope-type cryostat with a 3" sapphire vacuum window which separates the SQUID and atmosphere. This design enables the SQUID to approach an object to be measured as close as 1 mm. The minimal detectable size of the particle is also highly dependent on the magnetic field noise of the SQUID. Therefore, we employed double transformer coupling, which lowers the noise. One transformer was placed at 77 K and the other was at room temperature (RT). As a result, the magnetic field noise of the SQUID was reduced by 36%. Finally we could successfully measure small iron particles as small as 30 micron. This detection level was difficult to achieve when using a conventional X-ray detection method.