A COMPACT PLANAR GRADIOMETER SYSTEM FOR MEASURING TANGENTIAL COMPONENTS OF BIOMAGNETIC FIELDS

We present a compact and reliable planar gradiometer for a multichanne l system measuring tangential components of biomagnetic fields. The re adout electronics was simplified by using a double relaxation oscillat ion SQUID (DROS) which provides very large flux-to-voltage transfer of typically 3 mV/Phi(0), enabling direct readout by a dc preamplifier w ith modest voltage noise. The DROS wars made from hysteretic Nb/AlOx/N b junctions, To realize a compact and reliable gradiometer, the planar pickup coil is integrated on the same wafer with the SQUID. The plana r pickup coil consists of two 10 mm x 10 mm coils connected in series to form a first-order gradiometer and has a baseline of 30 nam. The fi eld gradient noise is 9 fT/cm root Hz at 1 Hz inside a magnetically sh ielded room. The gradiometers were arranged to measure simultaneously x and y components of biomagnetic fields.