Magnetic noise due to environmental vibration in magnetically-shielded room

MEG (magnetoencephalography) and the MCG (magnetocardiography) signals measured by using SQUID magnetometers are several orders of magnitude weaker than the environmental magnetic noises generated by electric trains, motorcars and electrical devices. In addition to these environmental magnetic noises measured at low frequencies, magnetic sensitivity is limited by microtremor of magnetically-shielded room (MSR) used for the measurements. This paper describes magnetic noises in the MSR due to vibration of SQUID magnetic sensors and a method for reducing those noises. The largest and active isolation system (ACMI) was developed to reduce the magnetic noises due to microtremor. ACMI actively can control the microtremor of an isolation table by six air-operated actuators which immediately apply forces to the table against the microtremor. Results show that without the ACMI, a broad peak at a frequency of 16 Hz was observed. When the ACMI was operated, the observed peaks were clearly reduced in the magnetic signals. Reduction of magnetic noise by controlling vibrations with ACMI demonstrated that almost all peaks in magnetic signal were due to vibration of MSR. This study shows that ACMI is highly effective against microtremors at low frequencies of less than a few Hz which cannot be reduced by means of the passive air cushion supporting apparatus.