Effective combination of magnetic and conductive layers of magnetically shielded room [for biomagnetic measurements]

This paper describes the optimal design of magnetically shielded room used for measurements of extremely weak magnetic fields such as biomagnetic measurements, in which the higher shielding factor is required in the frequency range from 0.01 Hz to several ten Hz at a low cost. The 3D magnetic field analysis is carried out by using the finite element method taking account of eddy currents to estimate an effective combination of magnetic and conductive layers for the design of wall and door. It is confirmed that as regards the wall design, an Al layer itself can increase the shielding effect by a sandwiched PC-Al-PC (PC: permalloy, Al: aluminum) combination even at frequencies at which only Al layer has no shielding effect. When the wall is composed of four layers [PC(inner)-PC(middle)-Al-PC(outer)], the door composed by the Al and outer PC layers is supposed to be optimal from the standpoints of shielding effect and cost. Its shielding fatter is comparable to that of the door composed of Al, inner and middle PCs.