極低周波微小磁界振動に対する増分透磁率µ_Δを用いた磁気シールド材料評価

抄録

Magnetoencephalography (MEG) requires a magnetically shielded room (MSR) for protection against very slow and micro magnetic noise. An index called initial permeability, µ_i, is generally used to evaluate the performance of high-quality magnetic shielding materials. However, the measurement conditions, such as the magnetic strength and frequency of the µ_i, are not suitable for an MEG magnetic shield; further, the effect of background geomagnetism (DC magnetic field) is not considered. In this paper, we propose incremental permeability, µ_Δ, as an index to evaluate a magnetic shield for very low-frequency micro magnetic field fluctuations. In addition, the µ_Δ measurement system is described in detail. The measurement conditions of the magnetic strength and frequency were set at 1 to 0.001A/m and 3 Hz, respectively. We experimentally confirmed that geomagnetism does not have much influence on the measurement of µ_Δ. Lastly, the effectiveness of this evaluation method was demonstrated via results obtained for three different samples.