Improvement of Performance of SQUID Magnetometer System for Highly Sensitive Geomagnetic Field Measurements

We have been developing a low-temperature superconducting quantum interference device (SQUID) magnetometer system aiming at highly sensitive geomagnetic field measurements with an intended magnetic field noise floor of approximately 10 fT/root Hz in the frequency range below 100 Hz. However, our previous system using SQUID magnetometers with the noise of similar to 30 fT/root Hz had the system noise of similar to 200 fT/root Hz, and the measurable frequency bandwidth was limited up to similar to 30 Hz. This unsatisfactory performance was due to the thermal noise and cut-off frequency of an electromagnetic (EM) shield made of copper foil, which covered the SQUID magnetometers cylindrically, because its design was not optimum. To improve the instrument performance, we fabricated SQUID magnetometers having a noise floor of similar to 10 fT/root Hz. We also evaluated the thermal noise and cut-off frequency of pure copper foils with different values of thickness and purity. Adopting a 0.08-mm-thick copper foil as an EM shield for the new SQUID magnetometers, the system noise and frequency bandwidth were improved to similar to 25 fT/root Hz and similar to 1 kHz, respectively. Using this improved system, we measured geomagnetic fields and confirmed that the background signal in the Z-component was smaller compared with the measurement result obtained using the previous system.