Magnetic shield for wide-bandwidth magnetic measurements for nondestructive testing and biomagnetism

Abstract

A magnetically shielded chamber has been designed and constructed for nondestructive testing and biomagnetic measurements with a high spatial resolution SQUID magnetometer. The shield consists of two layers of 1.6-mm-thick, high permeability alloy and two layers of 1.3-cm-thick aluminum, and has interior dimensions of 0.6×1.2×1.7 m. Except at 60 Hz, the noise level above 10 Hz is 12 fT/√Hz. The major noise, due to the 60 Hz power line, has been reduced to 8–36 fT/√Hz. The thermal noise generated by the shield does not affect the measurement inside the shield. At the center of the shield, the shielding factor against the earth’s static magnetic field is 500, and the ac shielding factor at 60 Hz is larger than 3×105. The results have been discussed and compared with the calculations.