Magnetic field-to-voltage coefficient evaluation of axial SQUID gradiometer in unshielded environment

Due to the existence of earth magnetic field, it is difficult to detect weak magnetic field from some parts of the body like heart, muscle, nerve and so on. In addition to the requirement for high-sensitivity magnetic field sensors, it is also essential to take certain measures to suppress noise. The axial gradiometer based on low-Tc Superconducting QUantum Interference Device (SQUID) can achieve a noise suppression ratio of 30 dB through the spatial gradient difference of the magnetic field, but the calibration of the magnetic field-to-voltage coefficient (B-V coefficient) generally requires a magnetically shielded room (MSR), which is complex and costly. With finite element simulation and practical experiments, a calibration method for B-V coefficient of the axial hardware gradiometer without shielding was presented in this paper. That is, a larger calibration field was generated for coefficient evaluation in an unshielded environment. At the same time, when the test signal was far away, the spatial gradient difference was used to improve the authenticity of the evaluation results. Through simulation and experimental verification, for the same performance and configuration of the gradiometer, the calibration result was 1.6 nT/V with 0.1 nT/V difference from that in a shielding room. The results showed that the unshielded evaluation method proposed in this paper was practical and could be used to evaluate the B-V coefficient of gradiometer.