SQUID instruments and shielding for low-level magnetic measurements

Abstract

We have used a thick-walled aluminum enclosure to provide low-frequency shielding for biomagnetic studies with SQUID magnetic gradiometers. The shield provides an attenuation proportional to frequency above 1/3 Hz. A 24-hole fractional-turn SQUID gradiometer has been used in the enclosure to carry out studies of magnetoencephalography on humans. An ’’asymmetric’’ configuration for a second-derivative gradiometer has been developed which provides a significant increase in sensitivity and resolution over more conventional symmetric arrays.