Performances of TOMIS, a transportable LINAC-based X-ray tomograph

Le résumé fourni par la source

Industrial X-ray imaging exams based on linear accelerators (LINAC), while being a powerful tool for the physical inspection of large and/or thick objects, are difficult to implement due to the important radiological constraints associated to the photon spectrum and the irradiation dose rate. Indeed, as most of the interrogating photons have an energy higher than 0.5 MeV, thick concrete shielding (>1 m) is usually required, and the investment cost of such bunkers can be prohibitive: an industrial 9 MV LINAC would typically need 4 m thick concrete in the beam axis and 1.5 m everywhere else. Moreover, in case of large objects, some of the pieces to be inspected cannot, or only with difficulty, be transported. This is particularly true in the nuclear waste management field [1]. To overcome these constraints, CEA led, since 2017, the development of a high-energy low dosimetry impact transportable tomograph: TOMIS. Developed as part of an investment supported by the French Government, TOMIS aims at providing a Non Destructive physical characterization of large-volume packages (diameter < 140 cm, height < 130 cm, mass < 5 t) with millimeter spatial resolution in less than an hour directly on their production/storage site. The innovation of TOMIS is to integrate in a standard truck container all the elements of the tomograph: - the X-ray source is an industrial LINAC (Varex M9) offering a dose rate close to 30 Gy/min at 1 meter from the target in the beam-axis; - the linear and collimated detector which ensures a high efficiency of detection over 150-cm width; - the lifting unit to handle large objects. The truck container also brings most of the mandatory shielding for photons (lead) and neutrons (polyethylene) in order to provide the user with a mobile and autonomous tomograph (see Figure 1). After 6 years of development, TOMIS was commissioned in December 2023 (see Figure 2). In this paper, we report on the radiography and tomography performances of TOMIS and compare them to those achieved in a fixed irradiation cell with several materials (stainless steel, concrete, etc) and objects of various size [2], [3]. Performances are detailed in terms of Modulation Transfer Function, contrast-over-noise ratio, limit of detection. TOMIS is scheduled to be used over the decades for industrial measurement campaigns on alpha bearing-nuclear waste packages.

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