Design of a very efficient detector for High Energy Tomography

As part of its R&D programs on large objects characterization, the Nuclear Measurement Laboratory at the CEA-Cadarache center is commissioning a major upgrade of its High -Energy tomograph (MeV range). This setup would allow to handle very large objects, up to 160-cm diameter and 5-t mass. The mechanical bench, installed in 2017, exhibits very high performances in terms of spatial precision (40-μm precision is sustained, even with maximum load). The new X-ray source, a linear accelerator, is currently under validation tests. In the recent past, various solutions have been studied and tested for the detector: a linear array composed with CdWO4 scintillator needles and photodiodes as well as a CdTe multi-sensors. With these previous studies, an attenuation measurement capability corresponding to 10 to 15-cm lead was reached with a detector blur close to 1 mm. In order to improve these results, a new design has been studied. The new detector is based on a CsI:Tl plate, irradiated on the edge and imaged width low noise cameras. The designing process is based on the Quantum Accounting Diagram (QAD) analysis which allow to detail each detection step in order to ensure the best Detective Quantum Efficiency (DQE). Based on these simulation results, a demonstrator has been built and tested under irradiation. The obtained performances overcome the previous tests: attenuation capability reaches 20-cm lead with a detector blur close to 1 mm.The detector design principle, based on imaging CsI:Tl plates is first carefully described. For each detection step, the simulated or expected results are presented. Based on these simulations, the built demonstrator is then detailed as well as the obtained performance results with a 9-MeV X-ray linear accelerator. Finally, the final detector design and performances are discussed.