A SQUID-based metal detector—comparison to coil and x-ray systems

Abstract

The presence of foreign metal bodies and fragments in foodstuff and pharmaceutical products is of major concern to producers. Further, hidden metal objects can pose threats to security. In particular, stainless steel is difficult to detect by conventional coil metal detectors due to its low conductivity. We have employed an HTS SQUID magnetometer for the detection of stainless steel particles which is based on the measurement of the remanent magnetization of the particle. Our aim was to determine the detection limits of HTS SQUID-based remote magnetometry, especially for food inspection purposes, and to make a comparison of this technique to commonly used eddy current coil and x-ray inspection systems. We show that the SQUID system's sensitivity to stainless steel fragments is significantly higher than that of coil systems if the samples are magnetized in a 100 mT magnetic field prior to detection. Further, it has a higher sensitivity than x-ray systems, depending on the density distribution of the product under inspection. A 0.6 mg piece of grade-316 stainless steel (a fragment of a hypodermic needle 0.5 mm long and 0.65 mm diameter) represents the detection limit of our system with a 150 × 150 mm 2 inspection orifice.

High-<i>T_c</i> Superconducting Electronic Devices Based on YBCO Step-Edge Grain Boundary Junctions

“…Fig. 11 Comparison of the measured detection limits of SQUID, coil, and X-ray system for SS316 needle pieces [34].…”

“…CSIRO developed a prototype metal detector using DC SQUIDs to detect stainless steel fragments for quality assurance in manufacturing [34]. Figure 11 shows a comparison of the detection limits between the SQUID, coil, and X-ray for stainless steel particles in different packages.…”

An overview of some of CSIRO’s “Underground Science” capabilities

“…Projects undertaken by the group include the use of SQUIDs for biomagnetometry (magnetic mapping of brain activity), detection of metal contaminants in food [2], SQUID sensors for Time Domain Electromagnetic geophysical prospecting [3], magnetic anomaly detection for Geophysical and Defence applications [4], and THz imaging using Josephson Junctionbased detectors [5] to name a few. An overview of these activities follows:…”

Metallic contaminant detection system using multi-channel high Tc SQUIDs

“…Therefore, a highly sensitive detection system for small contaminants is required. Some SQUID detection systems for detecting contaminants in food have been developed [1][2][3][4]. One of these systems is commonly available now [5].…”