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-T-c 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/Hz(1/2) and 10 pT/Hz(1/2) 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.