Multi-energy X-ray Techniques for NDT: a New Challenge

Digital X-ray imaging is a well-known technique for NDT. Based on the energetic dependence of X-ray material absorption, multi-energy techniques can provide mat erial information. Dual-energy techniques have been developed for that purpose. Proved to be efficient for materials differentiation, limits are encounter ed for materials with close attenuation. Beam hardening ef fect and detector imperfections may induce artefact s unacceptable for accurate quantification. For a few years, new types of detectors based on se miconductor technology have emerged with energy discrimination abilities. They reduce acquisition n oise and provide multi-energy information. But impe rfections due to charge sharing and induction signal cannot b e neglected. Dedicated data processing methods are required. So then the question of improving dual-energy techn iques with such detectors arises. Which problems ca n be solved, and which new problems appear? In this paper we give some indications on potential benefits of spectral X-ray imaging for NDT. We present preliminary evaluations of performance and limitations. 1. Introduction: from standard X-ray imaging to multi-energy techniques Conventional X-ray imaging exploits the absorption of radiation through the examined object. This information may be sufficient, for instance in defect detection task considering an homogeneous mono-material sample, but is often inadequate when material characterization is required. More precisely, radiographic imaging p rovides a representation of the object in terms of line-integrals of the attenuation coeffici ent. Tomography provides, through a multi- view acquisition, a reconstructed volume in terms o f absorption values, and gets rid of items superimposition. But in both cases, a single exposu re using a standard detector which provides a single value per pixel can scarcely allo w material identification. In the usual inspection energy range, the attenuati on for X-ray radiation is a combination of two photon-matter interactions: the photoelectric e ffect and Compton scattering. These interactions and their relative contribution to the total attenuation are energy dependant. This dependence can potentially be used to characterize materials - as long as the employed acquisition system permits to obtain this informati on. It is not the case of a conventional X- ray detector, which is composed of a scintillator l ayer coupled to photodiodes array and supplies the deposited energy integrated over the f ull energy range of the generator spectrum. Dual-energy techniques have been developed for that purpose. In terms of system, energy discrimination can be done at source level or at de tector one. In the first case two acquisitions are performed successively, the energy switch being realized by voltage tuning associated with the use of filters. This method is efficient i n terms of energy separation, but assumes the object being static. The implementation using a 2D detector, either scintillator screen or flat panel, allows an acquisition rate at the detector s peed but induces influence of scatter, problematic for numerical accuracy. Dual-energy may also be implemented within a single exposure technique. Adapted detectors consist in tw o receptor layers of scintillator- photodiode type separated by an intermediate filter . The front (resp. back) layer absorbs the low (resp. high) energy photons. The two images are acquired simultaneously, allowing to