Highly sensitive SQUID system

At PTB, SQUID (Superconducting QUantum Interference Device) magnetometers have long been used to determine extremely small changes in magnetic flux. The previously used multichannel SQUID systems were mainly applied in medical metrology to detect magnetic heart signals (magnetocardiography) and to measure and localize ion currents in the human brain (magnetoencephalography). For new applications in ultralowfield magnetic resonance or for magnetic relaxometry, samples must be pre-polarized with a magnetic field in the mT range shortly before the actual measurement is performed. This required the development of new SQUID magnetometers that are robust against the fields of the prepolarization phase (which are higher by a factor of 10¹²) without losing sensitivity during the measuring phase.

To achieve these requirements, a SQUID current sensor, which had already been developed at PTB, was combined with a separate field coil. The highly sensitive SQUIDs are located within a superconducting shielding to protect them from the high fields occurring during the pre-polarization phase. One sensor module consists of an array of 18 of those SQUID magnetometers. Each module is equipped with sensors in two measurement planes with individual field coils that are sensitive to all three spatial directions, and with coils of two different diameters. The larger coil improves the detection of sources that are located further away. The individual field coils are arranged in such a way that several sensor modules can be combined with each other.

It has been demonstrated that the new system is robust against pre-polarization fields into the mT range. The new SQUID measuring systems with the 17 mm and 75 mm field coils attain a minimum white noise of the magnetic flux density of 540 aT/√Hz and 61 aT/√Hz, respectively. The sensitivity of the sensors compared to the previous system has thus been more than doubled, and the application possibilities have considerably increased.