A 37 channel DC SQUID magnetometer system

A 37-channel DC SQUID magnetometer system has been built for biomagnetic studies. The SQUID loop of each magnetometer serves as the active sensing element, thereby eliminating the need for flux coupling circuits. The magnetometers are located approximately=3 cm above the outer dewar bottom. The SQUIDs are directly coupled to a highly simplified readout electronics using only five wires per channel; no helium temperature impedance matching circuits are required. Each channel can be independently inserted into or removed from the dewar. Using a novel compensation technique, the system white and 1-Hz flux density noise values are typically 5 fT/ square root Hz and 10 fT/ square root Hz, respectively, including the noise contribution of the in-house fabricated dewar (about 2 fT/ square root Hz at 100 Hz) and the magnetically shielded room (about 1 fT/ square root Hz at 100 Hz). In addition, it is shown that due to the large dynamic range and high slew rate of the sensors it is possible to electronically form a gradiometric configuration that can be exploited in order to improve the signal-to-noise ratio.