Low-noise biomagnetic measurements with a multichannel dc-SQUID system at 77 K

Directly coupled dc-SQUID magnetometers made from the high-temperature superconductor (HTS) YBa/sub 2/Cu/sub 3/O/sub 7/ on symmetric 24/spl deg/ and 30/spl deg/ SrTiO/sub 3/-bicrystals have been optimized for low-noise operation in static magnetic fields. These magnetometers are encapsulated, for reliable operation in liquid nitrogen. In our small glass cryostat, allowing even mobile applications, up to 24 magnetometers can be mounted in three levels for gradiometer arrangements. With first-order gradiometers an external noise suppression of more than a factor of 100 is achieved. In a magnetically shielded chamber we find a typical noise /spl radic/S/sub B/ (77 K, 10 Hz) of less than 50 fT//spl radic/Hz and low-noise magnetocardiograms are recorded. Due to the low noise of the dc-SQUID sensors also electrically stimulated magnetoencephalography measurements were possible.