Flux/voltage calibration of axial SQUID gradiometers using an optimization procedure

A straightforward experimental procedure for calibration of axial SQUID gradiometers has been developed, based on numerical optimization techniques. A dc current carrying wire of finite length, whose magnetic field spatial distribution is well known, was scanned by a SQUID system at several lift-off distances. Initially, theoretical magnetic field parameters such as lift-off and scanning tilt angles were numerically optimized in order to match the normalized shapes of experimental and theoretical signals. After that, the calibration factor can be easily found as the ratio between the two non-normalized signals. Once the calibration factor was obtained, an experimental validation was made by using a current-carrying copper sheet and by comparing the calibrated experimental signal with a model prediction, leading to good results. The overall procedure is easily implemented and can be modified to account for different SQUID systems and gradiometer geometry.