Development of a Prefabricated-Magnetically-Shielded Room of Hexagonal Shape for Biomagnetic Measurement

This paper describes the design and development of a hexagonal prefabricated magnetically shielded room (PMSR) for highly sensitive measurements of neuromagnetic fields such as magnetoencephalography (MEG), using SQUIDs with first-order and/or second-order gradiometers. This PMSR consists of prefabricated plywood panels 1.2 m wide × 2.4 m high × 24 mm thick, and covers a floor area of about 9 m². These panels are sandwiched between two layers of high-m metal sheets 1 mm thick. The shielding factor at the center of the PMSR against external magnetic fields is 20 to 40 dB at frequencies from dc to 120 Hz. Magnetic noise inside the PMSR measured by a second-order gradiometer was less than several hundred fT at frequencies above 0.5 Hz, except for 50 Hz. For this measurement, the second-order gradiometer was able to eliminate the common-mode magnetic noise near 20 Hz produced by vibrations in the PMSR and observed on a magnetometer. These results are discussed, making comparisons with calculations.