Electromagnetic induction imaging through metallic shields

Electromagnetic induction imaging has wide potential application in the disciplines of medicine, security, industry, geophysics and scientific research in general. The present study focuses on the applications in the security industry and in particular on providing a new tool for cargo screening in the context of the detection of illicit trafficking of special nuclear materials. The thesis reports a proof-of-concept study of electromagnetic imaging of metallic objects concealed inside electromagnetic enclosures. The sample object is imaged via phase variation measurements between the driver and sensor coils due to inductive coupling between the coils and the object, these images being proportional conductivity maps. For effective imaging through conductive barriers, subtraction of images at different frequencies was carried out in order to isolate the contribution of the concealed object. The present study validates electromagnetic induction imaging for nuclear security applications. The resolution of the system was determined using an edge detection algorithm applied to the images and found to be ~30 mm. The instrumentation employs Helmholtz coils for the driving field and an array of 20 × 20 sensor coils mounted on a wooden apparatus, with fixtures being non-metallic to magnetically isolate the experiment. Further studies were made to determine the compatibility of the modality to image in 3D by imaging Copper and Aluminium disks raised above the sensor array. The experiment gave a positive result being able to detect up to 80 mm depth (lift-off height) for 150 mm diameter disks and up to 40 mm depth for the 20 mm diameter disks. A study was performed to determine the penetrating power of the system by imaging through Aluminium enclosures of varying thickness. It was found that a Copper disk of 40 mm diameter by 2 mm thickness could be imaged through an Aluminium box even when the wall thickness was 20 mm, at 10 to 200 Hz driving frequency

Magnetic imaging: a new tool for UK national nuclear security.

Combating illicit trafficking of Special Nuclear Material may require the ability to image through electromagnetic shields. This is the case when the trafficking involves cargo containers. Thus, suitable detection techniques are required to penetrate a ferromagnetic enclosure. The present study considers techniques that employ an electromagnetic based principle of detection. It is generally assumed that a ferromagnetic metallic enclosure will effectively act as a Faraday cage to electromagnetic radiation and therefore screen any form of interrogating electromagnetic radiation from penetrating, thus denying the detection of any eventual hidden material. In contrast, we demonstrate that it is actually possible to capture magnetic images of a conductive object through a set of metallic ferromagnetic enclosures. This validates electromagnetic interrogation techniques as a potential detection tool for National Nuclear Security applications

Magnetic Imaging through Metallic Enclosures

Security applications may require the ability to image through electromagnetic shields. This is for example the case when trafficking of illicit material involves cargo containers. Thus, suitable detection techniques are required to penetrate a ferromagnetic enclosure. We report on the demonstration of the ability of a system based on electromagnetic interrogation techniques to create magnetic images of metallic objects concealed within metallic enclosures. The penetrating power through single and double ferromagnetic enclosures was investigated. The instrument employs a driver Helmholtz-coil assembly and an array of 20 × 20 sensor coils. The sample objects were imaged via phase variation measurements between the driver and sensor coils, due to inductive coupling between the coils and the sample object