Using a 77 K SQUID to measure magnetic fields for NDE

A bare HTS SQUID of commercial design was used in 77 K experiments concerning NDE. The SQUID was operated with flux-locked instrumentation to provide a noise floor of 80 pT//spl radic/Hz. The effective sensor area was measured to be approximately 70 /spl mu/m/sup 2/ equivalent to an ideal point detector for NDE. The SQUID was used unshielded in a normal laboratory environment in a special purpose LN/sub 2/ cryostat positioned above a motorized computer-controlled scanning system. We measured magnetic fields associated with current flowing in wires and compared them with calculations. We also detected a simulated flaw in an aluminum plate using an eddy current technique and made a preliminary depth assessment by frequency sweeping. Although developments in electronic gradiometers and gradiometric SQUID's should make the use of single bare magnetometer SQUID's unnecessary, we show that these already have sufficient sensitivity for NDE research, even without flux-focusing washers or pick-up coils.<
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