Multichannel magnetography in unshielded environments.

Any biomagnetic instrumentation requires a very sensitive sensor. As the strength of the magnetic field of interest ranges from about 10 fT to 50 pT, the only field sensor having the required sensitivity and small sampling volume is the superconducting quantum interference device (SQUID) e.g. thin-film DC SQUIDs. For transforming the signal from the antenna to the SQUID, a thin-film coupling coil is used. The SQUID itself is shielded and therefore insensitive to external noise. In a five-channel system second-order gradiometers are used in an unshielded environment. The measuring system enables us to balance each channel by means of lead plates without removing it from liquid helium. The lead plates can be handled by a revolver system from outside the dewar. By an iterative balancing procedure inside an artificial uniform field, imbalances less than 10(-4) could be achieved. These results are confirmed by mathematical calculation of mechanical balancing. Sensitivities down to 20 fT Hz-1/2 could be achieved during 'quiet' hours. Another method for the suppression of disturbances is electronic balancing. One of the most important problems in the multichannel system is the cross talk between the single channels. With respect to the geometry of our five-channel device the cross talk coefficient was calculated to be 3.9% and measured at 3.8%.