Simulation design of online Beam Monitoring for the MULTISCAN 3D Laser-Plasma X-Ray Source

¹ CEA, DES, IRESNE, DTN, SMTA, Nuclear Measurement Laboratory, France
² CEA, DES, IRESNE, DER, SPESI, Dosimetry, Sensors and Instrumentation Laboratory, France

^* eric.garrido2@cea.fr

Published online: 21 November 2023

Abstract

Many private and public actors in the security field use imaging systems with embedded X-ray accelerators for rail, seaports and border controls. These accelerators produce X-rays by the Bremsstrahlung effect, leading the output beam to have a very wide energy spectrum which precise form is difficult to know, especially the end-point energy. Mastering this energy distribution is nevertheless essential for certain applications like material discrimination. In addition, fluctuations over time or even from one pulse to another can disturb the analysis of the images and it is expected that the new X-ray sources based on laser-plasma accelerators, which are developed in the frame of MULTISCAN 3D H2020 project, will also increase this beam instability. In this context, we study a direct beam measurement at the exit of the target with a minimum footprint and a fast response, so that the results can be used in real time. A 50 mm × 50 mm × 2 mm scintillator is irradiated on the side by the X-ray beam (i.e. on a 50 mm × 2 mm face) and filmed from the top (i.e. on a 50 mm × 50 mm face). The light intensity of the image obtained by the camera is directly proportional to the dose deposition in this scintillator, and the depth profile is related to the X-ray energy spectrum. A study on different materials (PVT, CsI, BGO) is presented in order to determine the optimal scintillator. Several unfolding algorithms are examined to reconstruct the X-ray spectrum from monochromatic response functions of these detectors, which are calculated using MCNP code for a range of photon energies covering the expected X-ray spectrum. This study based on simulation results shows the feasibility of this technique, it appears that the Sequential Least SQuares Programming (SLSQP) minimization algorithm gives the most promising results and that the CsI scintillator provides better spectral information. It is probably due to the intermediate dose profile shape between PVT (flat falling tail) and BGO (steep rise). The binning of the depth profile (typically 100 µm or less) is also an important parameter to describe the initial steep march, and thus to keep the spectral information therein.