Flux/voltage calibration of axial SQUID gradiometers using an optimizationprocedure

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