Monte Carlo Simulations in NDT

Abstract

X-ray techniques are commonly used in the fields of non-destructive testing (NDT) of industrial parts, material characterization, security and examination of various other specimens. The most used techniques for obtaining images are radioscopy for 2D and computed tomography (CT) for 3D imaging. Apart from these two imaging techniques, where X-ray radiation penetratesmatter, other methods like refraction or fluorescence analysis can also be used to obtain information about objects and materials. The vast diversity of possible specimen and examination tasks makes the development of universal X-ray devices impossible. It rather is necessary to develop and optimize X-ray machines for a specific task or at least for a limited range of tasks. The most important parameters that can be derived from object geometry and material composition are the X-ray energy or spectrum, the dimensions, the examination geometries and the size of the detector. The task itself demands a certain image quality which depends also on the X-ray spectrum, the examination geometry and furthermore on the size of the X-ray source’s focal spot and the resolution of the detector. Monte-Carlo (MC) simulations are a powerful tool to optimize an X-ray machine and its key components. The most important components are the radiation source, e.g. an X-ray tube and the detector. MC particle physics simulation codes like EGS (Nelson et al., 1985) or GEANT (Agostinelli et al., 2003) can describe all interactions of particles with matter in an X-ray environment verywell. Almost all effects can be derived from these particle physics processes. The MC codes are event based. Every single primary particle is generated and tracked along with all secondary particles until the energy of all particles drops below a certain threshold. The primaries are generated one after another, since no interactions between particles take place. When simulating X-ray sources, in most cases X-ray tubes, the primary particles are electrons. The electron beam is parameterized by the electrons’ kinetic energy and the intensity profile along the cross-section of the beam. When hitting the target, X-rays are generated by interaction of electrons with the medium. The relevant magnitudes for imaging are the X-ray energy spectrum and the effective optical focal spot size (Morneburg, 1995). The most used imaging systems in the field of NDT are flat panel detectors. There are two basic types of detectors: Direct converting semiconductor detectors and indirect converting scintillation detectors. The type of particle interactions in the respective sensor layer determines the detection efficiency and effective spatial resolution. Interaction of X-rays in direct converting detectors produces electron-hole-pairs in the semiconductor materials. The free charge carriers drift to electrodes,where the current can bemeasured.MC simulations can Monte Carlo Simulations in NDT