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

Abstract

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 , and well below 1 fT for the large ones. Using a software gradiometer, we achieved a minimum noise level of 0.54 fT . 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.

Ultra-sensitive SQUID Systems for Pulsed Fields—Degaussing Superconducting Pick-Up Coils

“…A threshold behavior is observed in Nb1 as reported before [7]. Briefly, once the critical field µ 0 H c1 of 50 mT is exceeded, a significantly larger low frequency noise is observed.…”

“…This leads to field distortions and consequently to detrimental line broadening [5]. In addition, their rearrangement after quick removal of the pulsed field leads to a random telegraph signal which manifests itself as excess low frequency noise which limits the SNR [6], [7].…”

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

“…This has been conducted using either in situ electromagnets or ex situ permanent magnets. Strong pre-polarisation fields produced by electromagnets entail inducing eddy currents in the shielding walls and magnetise the superconducting pick-up coils, which may interfere with the detection of weak NMR signals [15,16,17,18]. Ex situ pre-polarisation requires the pneumatic shuttling of a sample from a remote location to the detection space.…”

An Optically Pumped Magnetometer Working in the Light-Shift Dispersed Mz Mode

“…However, for all measurement cases, where highest possible magnetic field resolution is needed, for a long time Superconducting Quantum Interference Devices (SQUIDs) were the only choice. SQUIDs can reach noise-limited magnetic field resolutions of B n < 1 fT/√Hz [11,12], but need cryogenic cooling for operation Therefore, also optically pumped magnetometers (OPMs) [13] have been used in magnetometry since several decades [14]. OPMs are based on the interaction of atomic spins with an external magnetic field B 0 to be measured [15,16].…”