Effect of Niobium Cylinder on Output of the Three-Dimensional SQUID Gradiometer

Because of the large ambient noise in biomagnetic measurements, it is usually necessary to use a gradiometer to reduce the environmental noise [A. Hayashi, Y. Hirata, and S. Kuriki, “Coil cancellation scheme with an electronic gradiometer for the reduction of magnetic field noise,” J. Magn. Soc. Jpn., vol. 29, no. 4, pp. 477-481, 2005.]. In order to prevent noise coupling into the SQUID from the on-chip input coil, it is also necessary to cover the chip with a niobium cylinder [S. L. Zhang et al., “A novel superconducting quantum interference device for biomagnetic measurements,” Sci. Bull., vol. 58, no. 24, pp. 2917-2919, 2013.], [J. R. Claycomb and J. H. Miller, Jr., “Superconducting magnetic shields for SQUID applications,” Rev. Sci. Instrum., vol. 70, no. 12, pp. 4562-4568, 1999.]. This paper focuses on the possible impact of introducing a niobium cylinder on the output of a SQUID gradiometer and how to reduce this effect. In this paper, the effect of the niobium cylinder on the output is equivalent to imbalance in simulation and measurement. Changing the relative position of the niobium cylinder makes the equivalent imbalance of the SQUID gradiometer coil less than 1%, which is caused by manual winding.