A modular, extendible and field-tolerant multichannel vector magnetometer based on current sensor SQUIDs

• Published in: Superconductor science & technology Vol. 29; no. 9; pp. 94001 - 94009
• Main Authors: Storm, J-H, Drung, D, Burghoff, M, Körber, R
• Format: Journal Article
• Language: English
• Published: IOP Publishing 01.09.2016
• Subjects: Coils; MEG; Multichannel; multichannel SQUID system; Niobium; Nuclear magnetic resonance; Prototypes; Sensors; SQUID; SQUIDs; Superconducting quantum interference devices; ultra-low-field NMR
• ISSN: 0953-2048; 1361-6668
• DOI: 10.1088/0953-2048/29/9/094001

We present the prototype module of our extendible and robust multichannel SQUID magnetometer system. A large multi-module arrangement can be implemented by using up to 7 modules. The system is intended for high-precision measurements of biomagnetism and spin precession. Further demanding applications are magnetorelaxometry and ultra-low-field nuclear magnetic resonance (ULF NMR), where pulsed magnetic fields of up to 100 mT are typically applied. The system is operated inside the Berlin magnetically shielded room (BMSR-2) and equipped with 18 magnetometers consisting of niobium (Nb) wire-wound pick-up coils. A total of 16 small pick-up coils with 17.1 mm diameter form a regular grid with individual channels arranged to ensure system sensitivity covers all three orthogonal spatial directions. Two large hexagonal pick-up coils with an equivalent diameter of 74.5 mm sensitive in z-direction surround the grid at two different heights and are suitable for the detection of deep sources. Each pick-up coil is connected to the input of a thin-film Nb SQUID current sensor via a detachable superconducting contact. The SQUIDs are equipped with integrated input current limiters. Feedback into the pick-up coils is employed to minimise crosstalk between channels. The current sensor chip package includes a superconducting shield of Nb. The field distortion of the prototype and a multi-module arrangement was analysed by numerical simulation. The measured noise of the small magnetometers was between 0.6 and 1.5 fT Hz − 1 / 2 , and well below 1 fT Hz − 1 / 2 for the large ones. Using a software gradiometer, we achieved a minimum noise level of 0.54 fT Hz − 1 / 2 . We performed ULF NMR experiments, verifying the system's robustness against pulsed fields, and magnetoencephalographgy (MEG) on somatosensory evoked neuronal activity. The low noise performance of our 18-channel prototype enabled the detection of high-frequency components at around 1 kHz by MEG.