A niobium planar SQUID gradiometer operating in an unshielded environment

We present an integrated thin-film gradiometric SQUID sensor in Nb-Al/Al2O3-Nb technology. The de-SQUID converter is designed as a parallel inductance double-washer SQUID. It is inductively coupled to a planar parallel inductance gradiometer antenna with 15 mm baseline by two series input coils with eight turns each. The field gradient sensitivity is 0.5 nT cm(-1) Phi(0)(-1). The intrinsic field gradient resolution down to 1 Hz is 5 fT cm(-1) Hz-(1/2), measured in a shielded environment. The sensor can be operated reliably without any magnetic shielding. Under these conditions the transfer function dV/d Phi is decreased by about a factor of three, resulting in a field gradient resolution of 15 fT cm(-1) Hz(-1) in the white noise region. Coherent noise sources produced by a power line and railway were suppressed by means of electronic subtraction using a reference channel and selective filtering. The resolution of the system is demonstrated by MCG measurements without any shielding.