Detection of low-conductivity objects using eddy current measurements with an optical magnetometer

Abstract

Detection and imaging of an electrically conductive object at a distance can be achieved by inducing eddy currents in it and measuring the associated magnetic field. We have detected lowconductivity objects with an optical magnetometer based on room-temperature cesium atomic vapor and a noise-canceling differential technique which increased the signal-to-noise ratio (SNR) by more than three orders of magnitude. We detected small containers with a few mL of salt-water with conductivity ranging from 4-24 S/m with a good SNR. This demonstrates that our optical magnetometer should be capable of detecting objects with conductivity < 1 S/m with a SNR > 1 and opens up new avenues for using optical magnetometers to image low-conductivity biological tissue including the human heart which would enable non-invasive diagnostics of heart diseases.

Sub-Sm–1 electromagnetic induction imaging with an unshielded atomic magnetometer

“…Based on Eq. (1), our record performance with the r el ¼ 0.91 Sm À1 sample of V ¼ 5:0 ml yields s ¼ 4:55 Â 10 À6 Sm 2 , an improvement of more than two orders of magnitude over our previous work 20 (s ¼ 3:13 Â10 À4 Sm 2 ) and of an order of magnitude with respect to recent detection reports 21 (s % 3:0 Â 10 À5 Sm 2 ). The r el ¼ 4.5 Sm À1 sample corresponds to s ¼ 2:25 Â 10 À5 Sm 2 , $26% better than previous works.…”

“…In a different context, current nitrogen-vacancy (NV) center magnetometers performing electromagnetic induction imaging 32 showed s % 1:8 Â 10 À1 Sm 2 . However, we note that assessing the smallest detectable object was not the scope of that work, 32 of previous ones, 16,20,21 or of this Letter.…”

“…31 The minimum imaged conductivity is improved by more than 50 times with respect to previous results 20 and by a factor 4 with respect to the recently reported detection of low-conductivity solutions in a shielded environment. 21 However, when considering the sensitivity of electromagnetic induction imaging instrumentation, the effective volume supporting eddy currents and thus contributing to the generation of the secondary field is also relevant. The larger the volume exhibiting a given conductivity, the bigger the signal produced.…”

“…[17][18][19] The higher sensitivity of the core sensor paved the path for applications with small volumes of low-conductivity materials. 20,21 Nevertheless, a direct demonstration of imaging of sub-Sm À1 specimens in unshielded environments, as required for applications in the biomedical field, 20 is still lacking.…”

“…The RF field is provided by a pair of 100 lH coils, of diameter 7.8 mm. They are placed at 647 mm with respect to the center of the vapor cell, in an arrangement similar to those recently used in other atomic magnetometer setups, 21,26 implementing the idea of Watson et al 27 The coils are antiparallel along the same axis. This creates two AC fields oscillating at the same frequency and with a stable p phase difference (anti-phase).…”