Gradiometer based on two single-hole SQUIDs

A gradiometer based on two ceramic superconducting quantum interference devices (SQUIDs) operating at liquid-nitrogen temperature is developed. The minimum gradient sensitivity is 3.1×10^-12 T/mHz^1/2 in a white-noise region. The minimum damping coefficient of the external disturbing factors is K=32. The gradiometer consists of two independent magnetometers separated from each other by 9 cm. It was tested by measuring the magnetocardiogram of a human heart. A typical magnetocardiogram was measured simultaneously with the magnetometer, with one lower channel of the gradiometer, and with the described gradiometer. The measurements were carried out with a bandwidth of 60 Hz, in real time, inside the ≃5-mm-wall aluminum barrel 50-m outside the nearest building. The comparison of these magnetocardiograms shows that the gradiometer considerably suppresses the low-frequency variations of the magnetic field of the Earth, the low-frequency industrial noise, and the influence of cryostat vibration