Hybrid technique for reduction of environmental magnetic field noise

1 Introduction There have been extensive studies on high-T c SQUIDs that are fabricated from high temperature superconductor YBa 2 Cu 3 O 7-δ (YBCO) films and utilize grain boundary junctions formed on a bicrystal-boundary or a step-edge of substrate. Single-chip magnetometers of high-T c SQUID coupled with thin-film pickup loop(s) have been developed to have low field noises below 100 fT/Hz 1/2 [1,2]. With such sensitivities, it is capable to measure MCG signals for diagnosis of cardiac diseases. In biomagnetic recordings, low-frequency noise (typically at 1-100 Hz) of the measurement system is crucial, including the system and environmental magnetic field noises. Magnetically shielded rooms (MSR) are commonly used in clinical examination of MEG, where residual environmental noise below 100 fT/Hz 1/2 is readily obtained with multi µ-metal layer shielding. However, such heavy-shielded room is not suited for MCG measurement system from the requirements of low cost, lightweight, and small space. We have been studying the noise reduction scheme that utilizes passive soft shielding and active compensation of environmental field noises. We refer here to soft shielding as the shielding of order of 20 dB that can be attained with single µ-metal layer surrounding, i.e., simplest MSR. In this paper we describe different methods of field detection and active compensation. A novel scheme is proposed using a normal conducting detection coil and a compensation coil, both of which are magnetically coupled to the soft-shielded MSR. MCG signals were successfully recorded within the soft-shielded MSR when this active compensation was applied. Figure 1 shows an example of environmental field noise in our laboratory measured with a flux gate magnetometer, where the field spectrum of MCG signal, which had been measured within a high performance MSR for MEG use, is also shown for comparison. If we assume 100 fT/Hz 1/2 intrinsic field noise of high-T c SQUIDs for measurement of MCG signals, we need more than 40 dB attenuation of continuous background noise between 1 and 100 Hz. Further, we need attenuation of 80-100 dB for line-spectrum noises, especially 50 Hz noises, which arise from electric lines and devices near the MSR. As the scheme to reduce such field noises for MCG measurement, we consider here active compensation combined with a MSR of soft shielding. For this purpose, a prototype MSR was fabricated and used in this study. It utilized single µ-metal layer (1 mm thickness), which was expected to have attenuation …