TRANSACTIONS zyxwvutsrqp ON MAGNETICS, VOL. zyxwvutsrqpo 27, NO. 2, zyxwvutsrq MARCH 1991 A 37 CHANNEL dc SQUID MAGNETOMETER SYSTEM H. Koch, R. Cantor, D. Drung, S.N. Erne, K.P. Matthies, M. Peters, T. Ryhhen*, H.J. Scheer, and H.D. Hahlbohm Physikalisch-Technische Bundesanstalt, Institut Berlin, Abbestr. 10-12, D-1000 Berlin 10, FRG *on leave from: Helsinki University of Technology, Otakaari SA, 02150 Espoo, Finland Abstract A 37 channel dc SQUID magnetometer system has been built for biomagnetic studies. The SQUID loop of each magne- tometer serves as the active sensing element, thereby elimina- ting the need for flux coupling Circuits. The ma etometers are located = 3 zyxwvutsrqp cm above the outer dewar bottom. Re SQUIDs are directly coupled to a highly simplified read-out electronics using only 5 wires per channel; no helium temperature impedance matching circuits are required. Each channel can be indepen- dently 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 .ff/JHz and 10 ff/JHz, respectively, including the noise contnbubon of the in-house fabricated dewar (about 2 zyxwvutsrq ff/& at 100 Hz) and the magnetically shiel- ded room (about 1 ff/fiz 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 confi- guration that can be advantageously exploited in order to improve the signal-to-noiseratio. Introduction The standard approach in developing dc SQUID systems has been and still is to design the SQUID for the lowest achie- vable flux noise while maintaining optimal coupling to a flux transformer ~ircuit"~. In addition, significant effort has been directed towards the red tion of l/f-noise, either by improving the junc i n technology@ or using electronic noise reduction scheme@ and the elimination of resonances in the vicinity of the working point9"'. Such systems consist of relatively complex thin film circuits with integrated flux transformer coils, damping elements and the SQUID (often in gradiometric configuration for self shielding). The SQUIDs are usually coupled to the pre- amplifier using a cooled matching circuit and the linearized out- put signal is provided by a rather sophisticated flux locked loop (FLL) electronics (cf. Fig. la). For several applications, including biomagnetism, this line of development seems to be too ambitious, because in a system oriented approach one has to consider the additional constraint of minimizing the system costs and a realistic set of sensitivity specifications that are consistent with the signal strengths to be measured as compared to other sources of noise beside the intrinsic SQUID sensor noise. Additional interfering noise is contributed by the dewar wall material, the shielded room and/or the environment, the electronics and the patients body'=. It has to be stressed that for biomagnetic applications the effective magnetic field noise level is more relevant as a figure of merit than the commonly cited flux noise levels. Considering all noise sources, a s stem white noise level of < 5 ff/JHz and a field noise at 1 zyxwvutsrq dz of < 10 fT/JHz seem to be sufficient design goals for biomagnetic multichannel systems. As shown below this can be achieved with a radicallv sim- plified system (Fig. lb): The SQUID loop is utilized as the field ick-up device eli- minatine. the need for a flux transformer'! The SQTJID is directly coupled to a low noise reamplifier and does not require a cooled matching circuit'! The read-out electronics uses direct feedback and does not require a flu modulation scheme and a lock-in tech- nique13. Thus its size could be reduced to 25 cm2 still using conventional components. The SQUIDs are fabricated using the fast and simple four- level process developed in our laboratory''. Another unique feature of our multichannel system is the dewar which is described below in detail. The inset resembles the structure of a nuclear reactor core, allowing each SQUID sensor channel to be handled as an independent unit. The complete system was developed for use in our magne- tically shielded room. Thus magnetometers instead or gradio- meters were preferred. On the other hand, these magne- tometers may be com ined to electronically balanced gradio- problem oriented multisensor schemes. metric configurations' B leading to a high flexibility in choosing 8r z delay F zyx f Figure 1. SQUID-system schemes: (a) a typical state-of-the-art design; (b) simplified, yet high performance system introduced here. Manuscript received September 24, 1990 0018-9464/91/0300-2793$01.00 0 1991 IEEE