Comparison of the Performance of Metal Contaminant Detection Using an Optically Pumped Magnetometer With an HTS SQUID

We have constructed a system to detect small metallic particles by using high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) gradiometers and have implemented it in a factory. Recently, there has been a remarkable improvement in the sensitivity of optically pumped magnetometers (OPMs) that operate at room temperature without cryogen, despite the limitations of the ambient magnetic field. Therefore, in this study, we tested the capability of the OPM by installing it in the inspection system. Since the OPM cannot operate unless the magnetic flux density in all directions (X, Y, and Z) is below 50 nT, a multi-layered magnetic shield with a higher shielding factor is required to keep the magnetic field sufficiently low. However, the inspection system has a strong permanent magnet to magnetize the metallic particles, making it even more difficult to achieve. To overcome this, two magnetic shields were placed in series, which separated the OPM from the permanent magnet. We prepared a small metallic sample for the test and compared its signal-to-noise ratio (SNR) with that of an HTS SQUID gradiometer. The distance from the sensor sensitivity position to the test object, i.e., the stand-off distance, was set to 2 mm for the SQUID gradiometer but necessarily 7.2 mm for the OPM because the center of sensitivity is 6.2 mm away from the sensor surface. Samples of SUS304 steel wire pieces were prepared. The samples were magnetized horizontally with a permanent magnet before the measurement. As a result, clear signals with SNR > 3 were obtained for the φ30 × L30 µm and φ20 × L61 µm samples with both sensors. With the SQUID, a clear peak signal was obtained in the smaller sample (φ20 × L38 µm), but with the OPM, the peak was somehow visible but buried in noise. Although OPMs do not require cooling facilities, they are still difficult to apply to a high-end metal contaminant inspection system at present. If further efforts are made to shorten the stand-off distance, the application of OPMs will be further expanded.