Noise characteristics of high-T_c YBa₂Cu₃O_y SQUID gradiometers

We studied the field noise of high-T_c YBa₂Cu₃O_y SQUID gradiometers in moving environments. The superconducting quantum interference devices (SQUID) magnetometers were in planar and axial configurations. In shielded environment, the magnetometer shows a gradient noise of ∼0.17 pT/cmHz^1/2 at 1 Hz and ∼5.1 fT/cm Hz^1/2 at 1 kHz. In unshielded environments, a gradient noise of 6.9 pT/cmHz^1/2 was detected in our laboratory. The gradient noise at 1 Hz was increased to 0.38 nT/cmHz^1/2 when the gradiometer system is moving at a speed of 20 mm/s. A moving gradiometer nondestructive evaluation system was set up to detect deep flaws. It was found there is a linear dependence between the phase of the differential field and the depth of flaws. The results are consistent with the SQUID magnetometer NDE system in which the SQUID magnetometer system is fixed and the samples were scanned under the system.