The preservation of human achievements and progress is undoubtedly a pillar of our society. This includes detecting threats in time to prevent disasters that may occur from forces of nature, defective technology, human failure or malicious intent. In this respect, there is arguably no difference between safety and security, though different institutions are involved in the regulation and control of them. However, the unifying rationale ought to result in the building of bridges between them. This is the Working Group's mission.
An algorithm has been developed to remove reliably dotted interferences impairing the perceptibility of objects within a radiographic image. This particularly is a major challenge encountered with neutron radiographs collected at the NECTAR facility, Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II): the resulting images are dominated by features resembling a snow flurry. These artefacts are caused by scattered neutrons, gamma radiation, cosmic radiation, etc. all hitting the detector CCD directly in spite of a sophisticated shielding. This makes such images rather useless for further direct evaluations.
One approach to resolve this problem of these random effects would be to collect a vast number of single images, to combine them appropriately and to process them with common image filtering procedures. However, it has been shown that, e.g. median filtering, depending on the kernel size in the plane and/or the number of single shots to be combined, is either insufficient or tends to blur sharp lined structures. This inevitably makes a visually controlled processing image by image unavoidable. Particularly in tomographic studies, it would be by far too tedious to treat each single projection by this way. Alternatively, it would be not only more comfortable but also in many cases the only reasonable approach to filter a stack of images in a batch procedure to get rid of the disturbing interferences.
The algorithm presented here meets all these requirements. It reliably frees the images from the snowy pattern described above without the loss of fine structures and without a general blurring of the image. It consists of an iterative, within a batch procedure parameter free filtering algorithm aiming to eliminate the often complex interfering artefacts while leaving the original information untouched as far as possible.
The effective high neutron scattering absorption coefficient of hydrogen (48.5 cm²/g) due to the scattering allows neutrons to reveal hydrocarbon structures with more contrast than X-rays, but at the same time limits the sample size and thickness that can be investigated. Many planar shaped objects, particularly wood samples, are sufficiently thin to allow thermal neutrons to transmit through the sample in a direction perpendicular to the planar face but not in a parallel direction, due to increased thickness. Often, this is an obstacle that prevents some tomographic reconstruction algorithms from obtaining desired results because of inadequate information or presence of distracting artifacts due to missing projections. This can be true for samples such as the distribution of glue in glulam (boards of wooden layers glued together), or the course of partially visible annual rings in trees where the features of interest are parallel to the planar surface of the sample. However, it should be possible to study these features by rotating the specimen within a limited angular range. In principle, this approach has been shown previously in a study with fast neutrons [2]. A study of this kind was performed at the Antares facility of FRM II in Garching with a 2.6×107/cm² s thermal neutron beam. The limit of penetration was determined for a wooden step wedge carved from a 2 cm×4 cm block of wood in comparison to other materials such as heavy metals and Lucite as specimens rich in hydrogen. The depth of the steps was 1 cm, the height 0.5 cm. The annual ring structures were clearly detectable up to 2 cm thickness. Wooden specimens, i.e. shivers, from a sunken old ship have been subjected to tomography. Not visible from the outside, clear radial structures have been found that are typical for certain kinds of wood. This insight was impaired in a case where the specimen had been soaked with ethylene glycol. In another large sample study, a planar board made of glulam has been studied to show the glued layers. This study shows not only the limits of penetration in wood but also demonstrates access to structures perpendicular to the surface in larger planar objects by tomography with fast neutrons, even with incomplete sets of projection data that covers an angular range of only 90° or even 60°.
Reducing risks and confining them to an acceptable level can be regarded as an essential commitment of technical applications for safety purposes and the public security measures.
Rating the efficiency of such measures requires an assessment of the risk that is supposed to be reduced. However, approaching this subject means to face a plethora of existing literature and an ongoing discussion if and how this can be achieved. On one hand, it remains a rather uncertain estimate particularly if it comes to rare interrupting events. On the other hand, making a decision responsibly requires a certain rational base. Several approaches exist for assessing risks, from verbal to quantitative, depending on the individual case. The frequently quoted Delphi technique represents the verbal one, others such as the Bayesian statistics a quantitative one or the consequence/probability matrix in-between.
A problem remains with understanding probability. Even if an event is highly unlikely it may happen right now.
Stochastic artefacts are frequently encountered in digital radiography and tomography with neutrons. Most obviously, they are caused by ubiquitous scattered radiation hitting the CCD-sensor. They appear as scattered dots and, at higher frequency of occurrence, they may obscure the image. Some of these dotted interferences vary with time, however, a large portion of them remains persistent so the problem cannot be resolved by collecting stacks of images and to merge them to a median image. The situation becomes even worse in computed tomography (CT) where each artefact causes a circular pattern in the reconstructed plane. Therefore, these stochastic artefacts have to be removed completely and automatically while leaving the original image content untouched. A simplified image acquisition and artefact removal tool was developed at BAM and is available to interested users. Furthermore, an algorithm complying with all the requirements mentioned above was developed that reliably removes artefacts that could even exceed the size of a single pixel without affecting other parts of the image. It consists of an iterative two-step algorithm adjusting pixel values within a 3 × 3 matrix inside of a 5 × 5 kernel and the centre pixel only within a 3 × 3 kernel, resp. It has been applied to thousands of images obtained from the NECTAR facility at the FRM II in Garching, Germany, without any need of a visual control. In essence, the procedure consists of identifying and tackling asymmetric intensity distributions locally with recording each treatment of a pixel. Searching for the local asymmetry with subsequent correction rather than replacing individually identified pixels constitutes the basic idea of the algorithm. The efficiency of the proposed algorithm is demonstrated with a severely spoiled example of neutron radiography and tomography as compared with median filtering, the most convenient alternative approach by visual check, histogram and power spectra analysis.
Wood is the material that has accompanied the whole development of mankind in various applications, for manufacturing tools and weapons, for buildings and constructions and also as fuel. It has various appearances and is subjected to decomposing changes, so there are sufficient arguments for non-destructive testing of wooden objects in the same way as is common practice with other technologically used materials. However, even today wood is rarely tested. Moreover, artefacts of cultural heritage containing wood are rare and delicate, so dismantling these for studying purposes is undesirable. Radiological inspection technologies are the methods of choice to provide a non-destructive insight. Interrogations of a specimen with various kinds of radiation provide different answers about their internal structures. High-quality detailed images are achievable with X-rays or gamma radiation in cases where heavy metals or other dense materials are not involved. Neutrons behave complementarily; they are avidly absorbed by light elements such as hydrogen on the one hand and yet are capable of easily penetrating heavy metals on the other. This provides an alternative for X-ray radiography and tomography when material characteristics are of primary interest rather than structural details, or when shielding with plates or sleeves of heavy metal severely impedes inspections with X-ray or gamma radiation technologies. However, due to the moderating effect of wooden samples it is essential to use fast neutrons for radiography and tomography of voluminous objects. Some typical examples described here will show the difference between neutron and X-ray photon-based radiographic technologies.
Die Arbeitsgruppe 5 der Europäischen Föderation für Zerstörungsfreie Prüfung (EFNDT Working Group 5 'Public Security and Safety', EFNDT WG 5) verfolgt weiterhin das Ziel, organisatorisch weit auseinanderliegende Zuständigkeiten und Arbeitsgebiete zusammenzuführen, die weitgehend mit gleichen, zumindest ähnlichen Prinzipien und Methoden arbeiten. Auf der Suche nach neuen Wegen stoßen beide Seiten auf Gemeinsamkeiten. Dies gilt auch für neue Ansätze und Bewertungsmethoden von Techniken, mit denen Risiken vermindert werden sollen. Hierzu hat in Berlin ein Workshop im Oktober 2012 stattgefunden, auf dem parallele Projekte zur Container- und Frachtsicherheit zusammengeführt werden konnten. Herausgestellt wurde die Komplementarität der laufenden Projekte, gemeinsame Aspekte sollen künftig auch zusammen bearbeitet werden. Darüber hinaus wurden neue Methoden und Vorgehensweisen vorgestellt, aber auch die Sichtweise der Europäischen Kommission mit ihren Vorstellungen zu einem europäischen Markt für Sicherheitstechnik dargestellt. Angesprochen wurden bestehende und vorgeschlagene europaweite Regulierungen und das europäische Referenznetz zum Schutz kritischer Infrastrukturen (ERNCIP), welches ein Netzwerk von Laboratorien für den Test von Sicherheitstechnik zum Ziel hat. Auf technischem Sektor standen Methoden im Vordergrund, mit denen die Gesamtmenge eines Gefahrstoffes wie z. B. Sprengstoff erkannt werden kann, und nicht nur Spuren als indirekter Hinweis. Die Arbeitsgruppe konnte somit erstmalig ein neutrales Forum für parallel laufende Projekte zur öffentlichen Sicherheit zur Verfügung stellen. Die auf den Workshops 2010 bis 2012 in Berlin gehaltenen Vorträge sind auf der Webseite des EFNDT WG 5 im geschützten Bereich zu finden. Nicht alles davon ist geeignet, es der allgemeinen Öffentlichkeit zugänglich zu machen. Deshalb sind alle Vorträge aus den Workshops in einem passwortgeschützten Verzeichnis abgelegt. Die Erteilung des Passwortes erfolgt über den Convenor der Arbeitsgruppe durch die EFNDT. Eine Erweiterung dieser Struktur zu einem Austauschforum ist vorgeschlagen. Zur Teilnahme daran sind nicht nur Mitglieder in den Gesellschaften für zerstörungsfreie Prüfung eingeladen, sondern auch diejenigen, die in den Anwendungsgebieten zur öffentlichen Sicherheit tätig sind.
Radiological inspections, in general, are the nondestructive testing (NDT) methods to detect the bulk of explosives in large objects. In contrast to personal luggage, cargo or building components constitute a complexity that may significantly hinder the detection of a threat by conventional X-ray transmission radiography. In this article, a novel X-ray backscatter technique is presented for detecting suspicious objects in a densely packed large object with only a single sided access. It consists of an X-ray backscatter camera with a special twisted slit collimator for imaging backscattering objects. The new X-ray backscatter camera is not only imaging the objects based on their densities but also by including the influences of surrounding objects. This unique feature of the X-ray backscatter camera provides new insights in identifying the internal features of the inspected object. Experimental mock-ups were designed imitating containers with threats among a complex packing as they may be encountered in reality. We investigated the dependence of the quality of the X-ray backscatter image on (a) the exposure time, (b) multiple exposures, (c) the distance between object and slit camera, and (d) the width of the slit. At the end, the significant advantages of the presented X-ray backscatter camera in the context of aviation and port security are discussed.