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) YBa₂Cu₃O₇ on symmetric 24° and 30° SrTiO₃-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 √S_B (77 K, 10 Hz) of less than 50 fT/√Hz and low-noise magnetocardiograms are recorded. Due to the low noise of the dc-SQUID sensors also electrically stimulated magnetoencephalography measurements were possible