Limits of Low Magnetic Field Environments in Magnetic Shields

During the last decades, the development of increasingly sensitive magnetic sensors led to numerous applications of magnetic field measurements in neuro-science, medical diagnostics, brain-computer interfaces, metrology or for experiments testing the fundamental laws of nature. Especially the development of small, lightweight, rather mobile and easy to handle optically pumped magnetometers allows for more advanced and dynamic studies of, for e.g., the brain. The limitation for the quality of such magnetic signals are gradients and drifts in the measuring environment. This article combines the result of many years of work, resulting in calculations to quantitatively predict the areas with ultra-low magnetic fields inside passive magnetic shields, as well as the experimental confirmation. The calculation shows that a conventional three-layer chamber with an improved degaussing system will already provide environmental conditions never reached before. In the most recent implementation an unprecedented residual field of < 130 pT was repeatedly achieved over 0.5 x 0.5 x 0.5 m inside an easily accessible space, consistent with the numerical modeling.