We give an overview of our research programme on the use of atomic magnetometers to detect and image concealed conductive objects via electromagnetic induction. The extreme sensitivity of atomic magnetometers at low frequency, several orders of magnitude higher than a coil-based system of similar size, allows for their operation in such a frequency range, thus permitting deep penetration through different barriers. This overcomes the limitations usually associated with electromagnetic detection. Applications in security and surveillance are discussed

Renzoni, F

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UCL Discovery

PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieElectromagnetic imaging with atomicmagnetometers: applications insecurity and surveillanceFerruccio RenzoniFerruccio Renzoni, "Electromagnetic imaging with atomic magnetometers:applications in security and surveillance," Proc. SPIE 12274, EmergingImaging and Sensing Technologies for Security and Defence VII, 122740P (7December 2022); doi: 10.1117/12.2635309Event: SPIE Security + Defence, 2022, Berlin, GermanyDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023  Terms of Use: https://www.spiedigitallibrary.org/terms-of-useElectromagnetic imaging with atomic magnetometers:applications in security and surveillanceFerruccio RenzoniDepartment of Physics and Astronomy, University College London, Gower Street, LondonWC1E 6BT, UKABSTRACTWe give an overview of our research programme on the use of atomic magnetometers to detect and imageconcealed conductive objects via electromagnetic induction. The extreme sensitivity of atomic magnetometersat low frequency, several orders of magnitude higher than a coil-based system of similar size, allows for theiroperation in such a frequency range, thus permitting deep penetration through different barriers. This overcomesthe limitations usually associated with electromagnetic detection. Applications in security and surveillance arediscussed.Keywords: Atomic magnetometers, electromagnetic induction imaging1. INTRODUCTIONThis paper reviews our progress in electromagnetic induction imaging with atomic magnetometers to detect andimage concealed conductive objects, with direct application in security and surveillance. Atomic magnetometersare ideal sensors for electromagnetic induction detection as they retain extreme sensitivity at low frequency,thus overcoming the limitations of coil-based systems. Operation at low frequency is thus possible, and allowsfor deep penetration through different barriers. The combination of the penetration capabilities and the lack ofionising radiation makes electromagnetic induction imaging very attractive for large scale security screening andfor surveillance applications, despite the inherent low resolution of the technique.The paper is organised as follows. We first introduce the basic principles of electromagnetic induction imagingand of atomic magnetometers. We then describe proof-of-concept demonstrations of imaging through barriers ofdifferent nature with direct applications in security and surveillance. We finally discuss the current state of thetechnology, in particular describing our new compact setup for imaging with atomic magnetometers. Electromag-netic induction imaging was demonstrated with an unshielded, portable radio frequency atomic magnetometerscanning over the target object. This configuration satisfies standard requirements in typical applications insecurity screening and surveillance applications.2. BACKGROUNDThis section provides a background of the key concepts and technologies that underpin electromagnetic inductionimaging with atomic magnetometers. We first introduce the basic concepts of electromagnetic induction imaging(EMI) (or magnetic induction tomography (MIT)).1 Then, we introduce the principles of atomic magnetometers2(AMs) – focussing in particular on radio-frequency AMs which are widely used for EMI applications.Further author information: (Send correspondence to F.R.)F.R.: E-mail: f.renzoni@ucl.ac.uk, Telephone: +44 (0)207 679 7019Emerging Imaging and Sensing Technologies for Security and Defence VII, edited by Gerald S. Buller, Richard C. Hollins, Robert A. Lamb, Martin Laurenzis, Proc. of SPIE Vol. 12274, 122740P© 2022 SPIE · 0277-786X · doi: 10.1117/12.2635309Proc. of SPIE Vol. 12274  122740P-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023Terms of Use: https://www.spiedigitallibrary.org/terms-of-use2.1 Electromagnetic Induction ImagingEMI is a non-contact technique which allows one to probe a sample’s electromagnetic properties. It can providemaps of the electromagnetic properties of a target object, i.e. the conductivity σ, the relative permittivity ϵr,and the relative permeability µr.The key principles of electromagnetic induction imaging are shown in Fig. 1. An oscillating magnetic field –referred to as the primary field – is applied to induce eddy currents in the target object. Eddy currents generate aresponse magnetic field component - the secondary field. The sample’s electromagnetic properties determine theproperties of the secondary field. Therefore, its detection, via a magnetic field sensor, allows a measurement ofthem. Electromagnetic induction images of samples can be generated by taking spatially resolved measurementsof the secondary field.Figure 1. Principles processes in electromagnetic induction imaging: the application of an oscillating magnetic fieldinduces eddy currents in the target objects and leads to the generation of the secondary field which carries informationabout the sample.An interesting feature of electromagnetic induction imaging is the possibility of penetrating barriers, bylowering the frequency of the applied oscillating magnetic field. Indeed, a barrier with electromagnetic propertiesµr, ϵr and σr can be penetrated by the magnetic field by lowering its frequency ω so that the skin depthδ =√2σωµ√√1 +(ωϵσ)2+ωϵσ(1)becomes comparable or larger to the thickness of the barrier. This allows detection and imaging of target objectsconcealed behind the barrier, as it will be further discussed in the following.2.2 Atomic magnetometersAtomic magnetometers (AMs)2 constitute an ultra-sensitive approach to the detection of magnetic fields. Impor-tantly, AMs fundamental limit of sensitivity is not frequency-dependent, so AMs can retain extreme sensitivityat low frequency. The ability to tune AMs while retaining high sensitivity is the key to applications in securityand surveillance, as it allows for lowering the operational frequency so to achieve the desired penetration depth.Several different types of atomic magnetometers exist. All the results reviewed here3–6 were obtained usingradio-frequency atomic magnetometers (RF-AMs).7 They detect oscillating magnetic fields by bringing theminto resonance with Zeeman transitions in the ground state. The operating frequency can be tuned with thehelp of a bias magnetic field over a wide range, from near DC to the MHz band. The core of our RF-AMis a 25 mm cubic quartz cell cell containing rubidium vapour and 20 Torr of Nitrogen as a buffer gas. Thelaser fields arrengement follow the standard configuration for RF-AMs.7 A circularly polarised laser beam,propagating along the direction of the applied DC bias field, polarises the atomic vapour. The magnitude of thebias field determines the desired operational frequency of the atomic magnetometer. An oscillating magneticfield perpendicular to the bias field drives the atomic precession. This field, applied at the frequency specifiedProc. of SPIE Vol. 12274  122740P-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023Terms of Use: https://www.spiedigitallibrary.org/terms-of-useby the bias field so to be resonant with the ground state Zeeman transitions, also serves as the primary field forEMI. The atomic precession is measured by a weak linearly polarised probe beam, propagating perpendicularlyto the pump beam. The precession of the atomic spins is transferred to the probe beam as rotations in its planeof polarisation. Such rotation is monitored with the help of a balanced polarimeter and a lock-in amplifier.Figure 2. Configuration of the RF-AM and set-up for EMI measurements.The variation in phase and amplitude of the measured total magnetic field with respect to the primary fieldcarry information on the electromagnetic properties of the target object. Images are created by displacing thetarget object with respect to the sensor.3. IMAGING THROUGH BARRIERSThe ability to operate the AMs at low frequency while retaining high sensitivity constitutes an importantadvantage with respect to coil-based EMI systems. In fact, low-frequency operation leads to large penetrationdepths, thus enabling a wealth of applications in security and surveillance. In the following, proof-of-conceptdemonstrations in these two areas are reviewed.3.1 Detection and imaging through metallic shieldsImaging through metallic shields is a requirement in many security procedures, from cargo and vehicle screeningto parcel inspection. Through-barrier EMI for security applications was first demonstrated with a coil-basedsystem,8 with successive developments4 with RF-AMs showing the full potential of the technique to penetratethick metallic barriers. That work demonstrated imaging of a Cu disk of 30 mm diameter and 2 mm thicknessthrough a combination of a 2.5 mm steel shield and 2 mm Al shield, at an operating frequency of 330 Hz so toguarantee penetration through the barriers.It is interesting to compare EMI with the gold standard for imaging through barriers: X-ray imaging. WhileEMI is inherently a low-resolution technique, it has the significant advantage over X-ray imaging of not requiringionizing radiation. This is a crucial advantage for large scale security screening.3.2 Underwater tracking and localisationLow-frequency operation of RF-AM based EMI systems also allows for underwater detection applications. Activedetection, localization, and real-time tracking of submerged conductive, nonmagnetic targets was demonstratedusing an array of RF-AMs operating in EMI-modality5 .EMI-AM has some advantage for underwater detection with respect to existing techiniques. With respect tothe use of atomic magnetometers in passive mode, active detection does not rely on the spontaneous magnetisationof the target object, and thus makes de-gaussing techniques ineffective. With respect to acoustic waves basedtechniques, EMI does not suffer from reflections from the sea bed, and thus it is effective also in shallow waters,of interest for harbour security. Finally, EMI-AM is also interesting for safe navigation, as it has great potentialto effectively detect submerged metallic objects near a ship hull.Proc. of SPIE Vol. 12274  122740P-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023Terms of Use: https://www.spiedigitallibrary.org/terms-of-use4. STATE OF THE TECHNOLOGYAll the proof-of-concept demonstrations discussed so far relied on laboratory-based set-ups, with the atomicmagnetometer being fixed in the laboratory frame and images formed by displacing the object with respectto the RF-AM. Real-world applications correspond to different settings. In security, a compact sensor headis required to move over the target object of interest - a cargo container, a vehicle or a parcel for example -to detect and localise suspicious items. Surveillance applications require the installation of a set of sensors atwanted locations in space so to be able to localise and track the target object within the region of interest. Workby the University College London (UCL) team over the last few years towards the realisation of a suitable sensorhas culminated with the development of an unshielded portable RF-AM, shown in Fig. 3, which was shown tobe able to spatially scan over a fixed target object6 . This satisfies the requirements for real-world applications.Figure 3. Photo of the UCL compact RF-AM with the cover removed.A few features of our compact atomic magnetometer are essential for its operation while being displaced atdifferent locations. First, the sensor head includes all the required lasers sources. This avoids the use of opticalfibres which make motion problematic. Second, the sensor head includes a set of magnetic field coils for theactive compensation of stray magnetic fields. This allows for the compensation of the unwanted backgroundmagnetic field after any displacement of the sensor. Finally, a procedure was implemented to reduce the impactof any residual spatial variations in the magnetic environment experienced by the sensor head while scanningover the target object.5. CONCLUSIONSElectromagnetic induction imaging has tremendous potential for security and surveillance application. It allowspenetration through barriers, without the involvement of ionising radiation. However, its development has beenhindered by poor performances of coil sensors at low frequency, as required for through-shield imaging. The workby the UCL team reviewed here demonstrated that the use of RF-AMs overcomes the limitations of coil-basedEMI systems, thus realising the full potential at low frequency, in particular allowing imaging through metalshields. Progress in the development of the EMI-AM technique was accompanied by the design and realisationof a portable sensor, suitable for real-world applications.Future work will include the development of new RF field sources, which will allow improved localisationand steering of the oscillating magnetic field over the target object, with increased potential for resolution andtomographic capabilities. This will be implementated used multiple coils arrangements.9Proc. of SPIE Vol. 12274  122740P-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023Terms of Use: https://www.spiedigitallibrary.org/terms-of-useREFERENCES[1] Griffiths, H., “Magnetic induction tomography,” Measurement science and technology 12(8), 1126 (2001).[2] Budker, D. and Romalis, M., “Optical magnetometry,” Nature Physics 3(4), 227–234 (2007).[3] Deans, C., Marmugi, L., Hussain, S., and Renzoni, F., “Electromagnetic induction imaging with a radio-frequency atomic magnetometer,” Applied Physics Letters 108(10), 103503 (2016).[4] Deans, C., Marmugi, L., and Renzoni, F., “Through-barrier electromagnetic imaging with an atomic magne-tometer,” Optics Express 25(15), 17911–17917 (2017).[5] Deans, C., Marmugi, L., and Renzoni, F., “Active underwater detection with an array of atomic magnetome-ters,” Applied Optics 57(10), 2346–2351 (2018).[6] Deans, C., Cohen, Y., Yao, H., Maddox, B., Vigilante, A., and Renzoni, F., “Electromagnetic inductionimaging with a scanning radio frequency atomic magnetometer,” Applied Physics Letters 119(1), 014001(2021).[7] Savukov, I. M., Seltzer, S., Romalis, M., and Sauer, K., “Tunable atomic magnetometer for detection ofradio-frequency magnetic fields,” Physical Review Letters 95(6), 063004 (2005).[8] Darrer J., B., Watson, J. C., Bartlett, P., and Renzoni, F., “Magnetic imaging: a new tool for UK nationalnuclear security,” Scientific Reports 5, 7944 (2015).[9] Cubero, D., Marmugi, L., and Renzoni, F., “Exploring the limits of magnetic field focusing: Simple planargeometries,” Results in Physics 19, 103562 (2020).Proc. of SPIE Vol. 12274  122740P-5Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 15 Jan 2023Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Electromagnetic imaging with atomic magnetometers: applications in security and surveillance

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Electromagnetic imaging with atomic magnetometers: applications in security and surveillance

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