Reduction of magnetic noises in cryocooler operated multichannel SQUID system

We developed a low-vibrational-noise cooling configuration using continuous liquefaction of evaporated helium gas from a multi-channel SQUID gradiometer system for whole-head magnetoencephalography (MEG) measurements. The initial reduction of the vibration of the cryocooler was done by rigidly mounting the remote motor and cold head chamber on a concrete wall. This significantly reduces transmission of vibrations to the transfer tube, especially compared with mounting them on vertical aluminum frames. In addition, to reduce vibrations from a mechanical SQUID array insert, we completely eliminated it by mounting each SQUID gradiometer separately on the helmet surface within the vacuum space. Thus, vibrations from the boiling liquid helium and transmission from the transfer tube were both greatly reduced. Magnetic field noise of the 150-channel SQUID gradiometer system showed that the vibration-induced interference peaks are comparable in amplitude to those from a conventional MEG system without helium reliquefaction.