Large-area low-noise seven-channel dc SQUID magnetometer for brain research

Abstract

The design, construction, and performance of a new high-sensitivity dc SQUID magnetometer, covering a circular area of 93-mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first-order gradiometers, located on a spherical surface of 125-mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/√Hz, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject's head is described. Finally, some examples of measurements carried out with the new device are given.