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 gradiometers arranged on a spherical surface, and thus can conform to a human head. The white noise level of the system is in a range between 12 and 18 fT/Hz/sup 1/2/. The noise from the cryocooler controlled to a level of 15 pT by several improvements to the cryostat (e.g., using low electrical conductivity material). Since the latter noise have a periodic characteristics, it can be reduced by using a "noise template" (i.e., a digital process using a computer). The system is described here, and its performance demonstrated by using it to measuring the faint biomagnetic fields from a human brain.