Performance of 9 to 15 MV Computed Tomography of large objects in industrial and nuclear field

As part of its R&D programs on large objects characterization, the Nuclear Measurement Laboratory at the CEA-Cadarache center has equipped its high-energy tomograph with a new linear accelerator (linac): a VAREX K15 (9 to 15 MV range). This linac delivers a very high dose rate: up to 130 Gy/min at 1 m from the target. Combined with a mechanical bench and optimized detectors, this X-ray source allows handling very large objects for radiographies and tomographies, up to 1600 mm in diameter and 5 t in mass [1]. Compared with the kilovoltage range, MV energies offer two advantages: higher photon flux and deeper penetration capabilities (steel from 100 to 400 mm). The new X-ray source has been fully characterized in terms of dose rate, focal spot size and photon spectrum using water attenuation measurements. Depending on the geometry of the object to be scanned, two detectors can be used. The first is dedicated to larger objects and is a lens-based detector with different scintillator screens (Gadox or CsI, as described in [2]) specially designed for this configuration (with a screen size up to 800x600 mm2). The second, a commercial flat-panel with a small pixel pitch (0.1 mm) is used for the smallest but densest objects, where the spatial resolution is critical [3]. The performance of these detectors is characterized, compared and discussed. The tomograph set-up and its final performance at low (9 MV) and high (15 MV) energies are detailed in terms of MTF curves, and contrast-over-noise ratio obtained on specific mock-ups. Examples of tomography on real objects (industrial packages produced by metal additive manufacturing or radioactive waste drums) are also presented. Finally, the main drawbacks in this energy range are listed and detailed: 1) the scattering background caused by the Compton effect, 2) the thickness of the scintillator, which must be optimized according to the spatial resolution or expected efficiency and 3) the size of the X-ray source (limited to 1.5 mm). To overcome these limitations, various studies are currently underway, and the expected solutions are presented and discussed.