A PLANAR 2ND-ORDER DC-SQUID GRADIOMETER

In this work we describe a DC SQUID gradiometer, sensitive to the seco nd spatial derivative of the magnetic field. The sensitive area of the gradiometer is the inductive body of the DC SQUID itself. The isoflux line distribution generated by a dipolar source, obtained by performi ng magnetic measurements with an array of such detectors, is relativel y complicated, but its localisation capability is similar to that one usually achieves with axial detector arrays. Planar gradiometers also show a better resolution for near sources and a stronger rejection of far disturbances. The final device is expected to have an inductance o f a few hundreds of pH in order to obtain performances typical of a lo w noise DC SQUID. The pick-up coils will be the combination of four sq uare holes of 500-mu-m side with a 1.05 cm baseline. Due to the magnet ic field concentration (in the final device it can be a factor 10) the gradiometer will have a sensitivity of 10(-11) T m-2 Hz-1/2 and a fie ld sensitivity of about 2 fT Hz-1/2. Some preliminary results, obtaine d on detectors with an intermediate area between the prototype and fin al device, are reported here. The process used to fabricate this secon d-order gradiometer is based on Nb-NbOx-PbAuIn Josephson tunnel juncti ons. Some possible improvements will also be described.