Abstract
To develop a small, light and highly reliable cooling system for Nb-based superconducting quantum interference device (SQUID) magnetometers, we measured the magnetic noise produced by a Gifford-McMahon (GM) cycle cryocooler containing the magnetic regenerative material erbium-nickel (Er 3Ni) and calculated the magnetic field around the regenerator by integral computation. Both results showed that GM cryocoolers containing magnetic regenerative material such as Er 3Ni were suitable for use in SQUID systems for biomagnetic measurement. A GM cryocooler with Er 3Ni was installed in a glass fibre reinforced plastic (GFRP) cryostat. Two SQUID magnetometers were mounted on this cryostat and the magnetic noise produced by the GM cryocooler was measured in a magnetically shielded room. The intensity of the magnetic noise was 40 pT p-p, which was about six times the value expected from the results measured under the earth's magnetic field. We therefore eliminated the magnetic noise by digital filtering, and magnetocardiograms and auditory evoked magnetoencephalograms were successfully measured.
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