X-ray backscattering: Variable irradiation geometry facilitates new insights

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1 18 th World Conference of Non Destructive Testing, April 2012, Durban, South Africa X-ray backscattering: Variable irradiation geometry facilitates new insights Norma WROBEL 1, Kurt OSTERLOH 1, Mirko JECHOW 1, Uwe EWERT 1 1 BAM Federal Institute for Materials Research and Testing; Berlin, Germany; Phone , Fax ; norma.wrobel@bam.de, kurt.osterloh@bam.de, mirko.jechow@bam.de, uwe.ewert@bam.de Abstract: Currently applied methods for X-ray backscattering radiography use a scanning pencil beam and a large-area and a highly sensitive detector. The image coordinates are determined by the beam-direction and the intensity by the measured dose of radiation recorded by the detector system. The advantage of the widely used process is the low radiation dosage hitting the inspected object. However, the disadvantage remains that some structures remain undiscovered and details or even dangerous objects might be masked by a surrounding backscattering environment. The new X-ray backscatter camera with a special twisted slit collimator offers an imaging method for resolving those problems. The new technique allows variable irradiation geometry in difference to existing solutions. The capability to visualize silhouettes of absorbing details in front of scattering bulk materials will be beneficial in the inspection of luggage on airports or for screening cargo containers, especially in case of inspecting with unilateral access. Key Words: X-ray backscattering, irradiation geometry, security applications 1. Introduction The X-ray backscattering radiography is based on the inelastically scattered X-ray photons, known as Compton Scattering. With this radiation effect objects which are transparent to X- rays emit scattered radiation so that they appear shining. This occurs preferentially in organic and low-numbered materials based on their ranking in the periodic table of the elements. All elements with high atomic numbers (e.g. heavy metals) mainly absorb X-ray photons so they emit scattered radiation to the outside with a much lesser intensity, if detectable at all. X-ray images are generated by beam attenuation with the source on one side of the object and the image detector on the other side. The situation changes principally if the object to be investigated only allows access from one side. Here, the alternative approach to use scattered radiation provides a solution. The underlying fundament of the recently introduced x-ray backscatter camera is based on the principle of a simple pinhole camera. Figure 1. Principles of generating backscatter images.

2 Figure 1 shows two different approaches to generate a backscatter image: on the left side with a directed pencil beam with a large area high sensitive detector and on the right the illumination of a large area with a high intensity source and a camera with an image detector. Commercially available systems for scanning large objects like containers, or trucks are generally using the first principle, i.e. the pencil beam technique [1]. Because of the fact that no comparable optical device exists which can be used for X-rays more than 100 kev, the new X-ray backscatter camera has been developed comprising a novel twisted slit collimator. 2. The technical approach 2.1 The novel twisted slit collimator The new X-ray backscatter camera is equipped with a unique twisted slit collimator. This offers an imaging technique in spite of the thickness of the diaphragm required for shielding [2]. Figure 1 demonstrates the principle of generating images with a pinhole camera. With this application it is perspicuous that the diaphragm in the pinhole camera has to be as thin as possible, or the hole functions merely as a collimator for a pencil beam. To avoid this consequence a previous technical approach consisted of a hole that has been widened to a cone with an aperture of 8 [3]. Other approaches such as the Soller-like aperture try to enable imaging with series of parallel holes similar to a thick-walled sieve but only able for passing parallel rays [4,5]. The construction of the new X-ray backscatter camera consists of a diaphragm with a virtual continuous series of holes in shifted direction which merges together forming a twisted slit. This approach results in a large angular aperture with a wall thickness adequate for shielding hard radiation like high energy X-rays or even gamma rays. Figure 2. The twisted slit collimator. Figure 2 shows the drawing of the twisted slit collimator. On the front side the slit is inclined into one direction and on the back side into the opposite one. So the linear passage through the slit is possible only through a hole shaped gap in any vertical direction. 2.2 Development of the camera In the last two years the design of the introduced camera has passed through an ongoing development. The above described principle of the twisted slit collimator has been realized at first as a solid block of 50 mm thickness incorporated in a lead brick which can be easily integrated into a variable lead box (Figure 3).

3 Figure 3. Pictures of the originally camera and an example of the experimental setup. For the experimental setup the lead brick with the collimator is integrated in the middle of the front side of a box built of lead bricks. To have a look inside the lid has been removed (Figure 3 lower right panel). To obtain an X-ray backscattering image this box is equipped with a phosphor imaging plate as a detector. The X-ray source for illumination here is placed on the same side of the investigated object with an angle of ca. 45 to the central viewing of the axis of the camera. The first realisation of the camera, built of several lead bricks, weighs about 300 kg and is rather impractical in use. Because of the complicated handling the progress results in an advanced development of a next generation of the new X-ray backscattering camera. Figure 4. Next generation of the X-ray backscattering camera with the twisted slit collimator. In the picture in the right the image plate is replaced by a digital matrix detector. The next generation of the X-ray backscattering camera (Figure 4) consists of a much smaller housing made of tungsten with a weight of ca. 30 kg ten times less than the previous model. Because of the weight reduction and the compact construction the handling of the camera becomes easier. In addition, there is another big advantage using the new camera. For imaging the usual imaging plate can be replaced by a digital matrix detector which reduces the exposure time from about 30 min down to less than a minute. Table 1. Reduction of the exposure time. Camera (Lead Box) Camera (Tungsten) Camera (Tungsten) Exposure Time min min 40 s!!! Imaging Plate Imaging Plate Digital Matrix Detector

4 With this considerable time reduction making an X-ray backscattering image by using this new construction of the camera in combination with a digital matrix detector, the technique of X-ray backscattering will be more attractive. 3. Functionality and Application 3.1 Using X-ray backscattering with this technique Compared to the common methods for X-ray backscattering using a scanning pencil beam the location of the scattering bulk material that might hide some absorbing details appears less limiting. With this configuration of the new camera the object that has to be investigated will be fully illuminated by the X-rays from directions independently from the viewing one. This gives also the possibility to mask certain areas especially the scattering ones lying in front of absorbing details by additionally used shielding. With this procedure those embedded parts are also made visible as silhouettes. Figure 5. Changing of irradiation geometry and viewing direction. The experimental setup in Figure 5 shows that absorbing details from the inside of objects, here an absorbing bolt with a nut in the centre of scattering water, was made visible in the dependence of the irradiation geometry, i.e. the angle between the incident radiation and the viewing axis of the camera. In the first experimental setup with a small angle only the scattering water has been detected and the bolt inside remained invisible. The camera and the beaming were oriented here nearly in the same direction. Changing the setup like in the second panel of Figure 5 with the beaming direction and the camera perpendicular to each

5 other the bolt and the nut inside the water appears weakly but detectable. After modification of the beaming direction by shielding the part of the beam towards the camera with a lead board (third picture) the bolt with the nut inside the water became clearly identifiable. As a consequence, absorbing details of the object which have to be investigated with the X-ray backscattering technique will appear differently depending on their surroundings and illumination. They may appear as silhouettes in an X-ray image clearly, faint or not at all. Figure 6. Dependency of the object surrounding. Top row: the object and its backscattering Image stand-alone; bottom row: the object and its image with a marble plate in the back. Figure 6 shows another example of the dependency of the object surrounding using X-ray backscattering. Investigating objects without any influence from adjacent item show identifiably the object (top panels) with their scattering outer shell only. In contrast, some internal features of absorbing details become apparent as silhouettes if the object of interest is located interest is located directly in front of a scattering bulk phase like a marble plate. Moreover, the absorbing parts of the objects cause a shadow within the scattering wall in the rear (to the left of the object itself). As a consequence, some specimens appear differently depending on the scattering properties of the surrounding. With this freedom in varying the irradiation geometry the new X-ray backscattering camera offers the possibility of variable illumination to unravel certain features of absorbing details not directly visible due to their densities when applying the backscatter principle. 3.2 Using this technique for security application Considering all the presented facts this new technique will be beneficial for security application like in the inspection of luggage on airport or for screening cargo containers, especially if there is an access only from one side for the inspection and suspicious details may be disguised by a scattering shell. Another consideration of investigating luggage on airports pertains to the transportation of liquids in the hand luggage. The described method appears useful also for this purpose. Liquids are particularly prone to backscattering X-rays as shown above by the beaker full of water. This makes them detectable even in an unsorted environment.

6 Figure 7. Comparison of liquids and solids in an X-ray backscattering image. On the lower left the liquids: left water (glass), right acetone(glass); in the lower middle; left: water(glass), right: CaSO 4 (plastic); in the lower right: left: CaSO 4 (plastic), right: Ca-lactate(plastic). Figure 7 shows X-ray backscattering images of small bottles of different material (glass and plastic) with different fillings. In the images liquids appear bright but it cannot be discriminated between the different types (in the left panel water and acetone). In the middle it is obvious that there is a liquid in the left bottle (here water) while the other one contained a solid (here crystalline CaSO 4 ). In the left panel both bottles, both are made of plastic, are filled with solids, on the left with crystalline CaSO4 and on the right with powdered Calactate. As a result, there is a difference between liquids and solids identifiable based on the comparison of the bright intensities from the scattering liquids on the one hand and the lightly one from solids on the other hand. The water scattering is more intensely and there is no clear difference between the solids, as a powder or a crystalline. Also it is distinguishable that plastic bottles are better visible than glass bottles. So with this method liquid and in hand luggage or inn cargo containers could be detectable with this X-ray backscattering technique shown above even in a rather crowded environment with solids. Principally it can be used to interrogate objects like suitcases, parcels or container that might be placed at a wall or in a corner with access only from one side. It is complementary to the conventional X-ray inspection because it is sensitive for low Z materials and liquids. 4. Conclusion and Outlook This backscattering technique with the novel twisted slit collimator enables generating backscatter images of small to medium sized objects with image detectors following basically the pinhole camera principle. Gradually there are consistent new capabilities for using that camera also for the mobile application. There is a continuous advancement of that method which bring new technical expertise and results. Because of the independency of the irradiation geometry and viewing direction this method facilitates new non predictable insights in investigating object under different kinds of aspects.

7 The latest technical developments attend to the refinement of the quality of the images. So the next steps are the application of new digital detectors to improve the image quality. Also is planned the construction of the next camera following that reported principle a camera with a multiple slit for X-ray backscattering. References [1] W.W. Sapp Jr., P. Rothschild, R. Schueller, A. Mishin: New, low dose 1 MeV cargo inspection system with backscatter imaging, Penetrating Radiation Systems and Applications II (ed. F.P. Doty et al.), Proceedings of SPIE Vol.4142 (2000), [2] K. Osterloh, U. Ewert, H.-J. Knischek, Blende für eine bildgebende Einrichtung, Patent DE , granted 21. August [3] Non-destructive testing Characteristics of focal spots in industrial X-ray systems for use in non-destructive testing Part 2: Pinhole camera radiographic method, EN : [4] J. Wong, P.A. Waide, J.W. Elmer, A.C. Thompson: Spatially resolved diffraction using a Soller Collimator-Imaging plate assembly, Nucl. Instr. & Metth. Phys. Res. A 446 (2000), [5] J.S. Iwanczyk, B.E. Patt: radiation Imaging Detector: US Patent No Granted June 30, 1998.