An ultra low noise DC SQUID system for biomagnetic research

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Abstract
A ultra low noise DC SQUID magnetometer designed and constructed for high resolution magnetocardiographic measurements is presented. Details of the design considerations and the construction of the single-channel system are given, focusing in particular on the matching of the DC SQUID with the gradiometer and the room temperature electronics. Using a first-order gradiometer as antenna for biomagnetic fields, a white noise level of 2 fT Hz-1/2 has been attained with a 1/f corner frequency at 3 Hz. The intrinsic magnetic flux noise of the SQUID system of 2*10-6 Phi 0 Hz-1/2 corresponds to a noise limited sensitivity of 700 aT Hz-1/2.