Development of a light-weight, high-performance magnetic shielding system for MEG measurements

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The main objectives of this three-year research are to apply a light-weight, high-performance magnetic shield of open-ended cylindrical structure (height=2.76 m, outer diameter=0.98 m and inner diameter=0.67 m), which was built by our own design using Co-based amorphous magnetic tapes and applying magnetic shaking technique, for the MEG measurements with first-order dc-SQUID gradiometers, and to study which parts of shielding system should be improved for the MEG measurement to be done. We have succeeded in measuring MEG alpha rhythm from a human brain of 23-year old male volunteer, however, we found magnetic noise inside the shield should be further improved for the measurement of evoked MEG. Other important results and future problems are summarized below :- Conductive cloth RF shield enclosing the magnetic shield was found to be successful to recover flux-voltage (φ-v) characteristics of dc-SQUID to the level of no RF interference.- Low frequency magnetic fluctuation noise is observed during magnetic shaking, which was found to be in strong relation to the Barkhousen magnetic noise. Metglas 2714A amorphous magnetic tape exhibits much smaller fluctuation noise than Metglas 2705M one.- Active compensation for openings of the cylindrical magnetic shell has been established for the helical shell and for the axial shell. 55 times improvement was obtained at 2 Hz of uniformly impressed magnetic field and 20 times at 10 Hz and 5 times at 50 Hz.- Stable fluxgate magnetometer was developed by incorporating bias reversal technique to the orthogonal structure.- A simple coil system for producing uniform magnetic field was proposed for active magnetic shields-Further magnetic noise cleaning should be needed, which can be realized by taking account of the second and the third-results in the design of the shield.