Characterisation of a Planar Second-Order Gradiometer in a Lightly Shielded Environment

Abstract

In recent years biomagnetic studies using SQUID systems have mainly used axial or planar first-order gradiometers which measure the B_z component of the magnetic field. However, these systems tend to rely on large, expensive mu-metal shielding plus electronic channel-differencing to achieve reasonable signal-to-noise ratios. This is often inconvenient and in some situations can make measurements impossible. We have therefore chosen to concentrate on devices capable of operation with minimal magnetic shielding. We have developed a planar secondorder gradiometer, integrated with a first-order gradiometric SQUID, for B_x/B_y measurements of magnetic field in lightly shielded environments [1]. They rely on a high degree of intrinsic geometric balance and no external balance mechanisms, making them appropriate for unshielded multi-channel applications. The devices are fabricated on twoinch Silicon wafers using our fully-established niobium fabrication process at the University of Strathclyde[2]. Precise photolithographic patterning of the pick-up coils permits a high degree of intrinsic balance. We discuss the characterisation of our gradiometer by analysing its spatial response to magnetic sources and its immunity to background interference. The spatial response to a magnetic source, on-axis, is characteristic of the order and balance of the gradiometer and therefore indicates how well the device performs. We have modelled the response to an offaxis source, to better understand the spatial response of a tangential gradiometer. The noise Performance of the device has been analysed and the sources of many discrete frequencies identified. This design of gradiometer has been successfully introduced into our biomagnetism programme at the Wellcome Biomagnetism Unit in the Southern General Hospital, Glasgow. Biomagnetic measurements using it are reported elsewhere[3,4].