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Magnetometer Utilizing Cooled Normal Pickup Coil and High-Tc Superconducting Quantum Interference Device Picovoltmeter for Nondestructive Evaluation

A magnetometer utilizing a cooled normal pickup coil and a high-Tc superconducting quantum interference device (SQUID) picovoltmeter was applied to nondestructive evaluation in an unshielded environment. The pickup coil of copper wire was cooled to T=77 K in order to decrease thermal noise from resistance. The magnetic field noise of the magnetometer was 130 pT/Hz1/2 and 10 pT/Hz1/2 at 100 Hz and 1 kHz, respectively, for the pickup coil with a 4.4 mm diameter. It was shown that the magnetometer could be moved in an unshielded environment without degradation of its performance. By moving the cooled pickup coil, we successfully detected a small crack on the backsurface of the Cu plate in an unshielded environment.