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2.1 Introduction Computed tomography scanning with ionizing radiation provides three dimensional information about the internal inhomogeneous structure of the specimen under test in a nondestructive, noninvasive and rapid manner. This technique is a "cornerstone of materials characterization" (Kinney et al. 1988). The aim of computed tomography is to create a precise map of the internal inhomogeneities of the sample. This map is obtained through the determination of the attenuation coefficient of an ionizing radiation (X-ray, y-ray) in a single thin layer of the sample. The tomogram shows this layer in a perpendicular position to the main axis of the object, while conventional X-ray technique, called Rontgen densitometry, produces an image parallel to the object under inspection. A conventional radiographic image is produced by sample translation and offers a rapid scan of the specimen under test, and, at the same time, locates the defect zones in one plane. Tomographic images produced with ionizing radiation (X-rays and y-rays) are called tomograms of slices or CT images and are obtained from the translation and rotation of the source and detectors around the specimen. The slices produce spatial information (in 3D) able to discern zones of low attenuation contrast. First-generation tomograms have been obtained with one source and detector acting in parallel, by translation. The sample was rotated by a 1°-step angle and the whole image was obtained for 1800 of collected data. The secondgeneration tomograms were also obtained by translation, using an array of detectors that made simultaneous measurements through different angles during a single traverse inspection. The sample was rotated by the array beam angle. The third-generation tomograms are produced by a fanning movement, with a scanner provided with many detectors located on an arc focused at the X-ray source. The fourth generation of scanners also has a fan system of detection, and the detection array is located on a circle that surrounds the source and the sample. In inhomogeneous media like wood, the attenuation coefficient depends on both the quantum energy of the ionizing radiation and the chemical composition of the sample. The electronic signals corresponding to images taken on wood specimens with X-rays in the Compton energy range enable the precise recording of mass density variations, clearly distinguished from the effects of high atomic number constituents. A characteristic signature V. Bucur, Nondestructive Characterization and Imaging of Wood © Springer-Verlag Berlin Heidelberg 2003

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Ionizing Radiation Computed Tomography

of the specimen inspected with X-rays can be obtained (Polge 1966; Mothe et al. 1998). Because the high quantum energy of radiation and the low atomic numbers of chemical constituents of wood, the attenuation phenomenon is caused mainly by the Compton effect and consequently is proportional to the mass density of the wood. It is generally accepted that the density of the cell wall in oven dry wood is constant (I500kg/m3).