A 61-CHANNEL SQUID SYSTEM FOR MEG MEASUREMENT COOLED BY A GM JT CRYOCOOLER/

A SQUID-based MEG measurement system cooled by a GM/JT cryocooler have been developed. The system consists of 61 channels of axial gradiomet ers arranged on a spherical surface, and thus can conform to a human h ead. The white noise level of the system is in a range between 12 and 18 fT/Hz(1/2). The noise from the cryocooler is controlled to a level of 15 pT by several improvements to the cryostat (e.g., using nonmagne tic, low electrical conductivity material). Since the latter noise hav e a periodic characteristics, it can be reduced by using a ''noise tem plate'' (i.e., a digital process using a computer). The system is desc ribed here, and its performance demonstrated by using it to measuring the faint biomagnetic fields from a human brain.