INTEGRATED LTS GRADIOMETER SQUID SYSTEMS FOR UNSHIELDED MEASUREMENTS IN A DISTURBED ENVIRONMENT

Various types of thin-film do gradiometer SQUIDs with integrated picku p loops were investigated. The SQUIDs were prepared on a 5 mm x 10 mm chip in the well developed Nb-NbOx-Pb/Au/In technology, It was shown t hat the SQUIDs work in an unshielded environment. Our best SQUIDs have a field gradient sensitivity better then 30 fT cm(-1) Hz(-1/2). All g radiometers have some response to a homogeneous field, i.e., a parasit ic area. The effective pickup area, the field gradient sensitivity, an d the components of the parasitic area for the field perpendicular and parallel to the gradiometer plane were measured for all types of grad iometer. The calculations of flux coupling efficiency are compared wit h experimental results. The origins of parasitic area are discussed. A further reduction of external disturbances was achieved by using an e lectronic second-order gradiometer. Examples of magnetocardiographic m easurement in a physical laboratory without any shielding with first a nd second-order gradiometers are shown.