A nondestructive evaluation (NDE) surface-flaw detection technique involving decoration of the samples with a ferromagnetic or paramagnetic tracer and subsequent scanning with a high-resolution magnetometer or susceptometer is considered. The authors present the results of a preliminary study demonstrating that this technique is sensitive enough to detect and image defects at least as small as 254 mu m*51 mu m*76 mu m deep in a nickel-alloy test block. Applied field strength and superconducting quantum interference device (SQUID) noise limit sensitivity, and sensing coil characteristics limit spatial resolution. >
Typical SQUID-based magnetometers are currently constructed with 15 to 25 mm diameter coils that are spaced back from the room temperature measurement surface 12 to 20 mm. Both of these dimensions limit the spatial resolution of these systems. To understand the fundamental determinants of magnetic fields produced by bioelectric activity, better spatial resolution is needed (Wikswo, 1988). To address this need we have developed a close-spaced, four-channel magnetometer having 3mm diameter first-order assymetric gradiometer pick-up coils spaced on a 4.4mm grid and residing < 1.5 mm from the measurement surface which is at room temperature: μSQUID™.
By using a superluminescent diode as the light source and a depolariser inside the fibre coil, a constant scale factor is achieved without using polarisation control elements. For long-term behaviour an RMS-bias drift of 10 degrees/h is obtained.