Fast megavoltage X-rays radioscopy

As part of its R&D programs on severe accidents, CEA is commissioning a major upgrade of its radioscopic X-ray setup in the cell named KROTOS, at Cadarache. This radioscopic tool allows high-energy radiography acquisition on a large phosphor surface (30×40 cm²) with fine resolution (1920×2560 pixels), high frame-rate (200 fps) and during few minutes. Since these performances aren't available with any commercial detector, a prototype has been built with state-of-the-art components. The X-ray source is a 9 MeV LINAC which produces short pulses of Bremsstrahlung X-ray (15 Gy/min at 1 meter in the beam axis). The phosphor screen is made with GADOX deposition on a brass plate. In order to obtain the expected framerate, four low-noise scientific-CMOS cameras are running simultaneously. Each camera output (1920×2560 16 bits-pixels at 50 Hz) provides large data flow (3.7 Gbit/s per camera). This data flow is buffered in computer RAM, then saved on fast solid state drives. The setup can sustain image acquisition during 1.5 min, corresponding to 18k images and 1.3 Tbit. The last step is the four-camera-output merging, accounting for parallax correction and lens distortion. The multi-camera radioscopy principle and synchronization constraints are presented. Image merging methods are then discussed. Experimental performances on phantoms are presented and compared to simulation results.

I. Introduction

The characterization of water-corium interaction is an important issue in R&D programs on severe accidents. Among technics used to address this characterization, High-Energy Xray radioscopy allows real-time measurement of corium fragment mass & velocity as well as void fraction estimation [1].