Integrated low-temperature superconductor SQUID gradiometers for nondestructive evaluation

We describe an eddy current nondestructive evaluation system using a low-temperature superconductor magnetic field sensor in an electromagnetically unshielded environment. The sensor comprises a niobium dc superconducting quantum interference device (SQUID) integrated with a first-order gradiometric pickup coil to reject spatially uniform interference fields but remain sensitive to flaw induced fields. We demonstrate its use in locating and mapping subsurface flaws in a multilayered aluminum structure using an eddy current excitation field at a frequency of 70 Hz, with no magnetic shielding around the specimen or cryostat.