Photon flux characterization of a new electron LINAC in the CINPHONIE irradiation facility

Electron linear accelerators (LINACs) are versatile and powerful X-ray sources, that can be used in medical radiotherapy as well as in various industrial applications including non-destructive testing, imaging and security inspection. LINACs accelerate electrons by passing them through a series of oscillating electric fields within a vacuum tube. These high-energy electrons are then directed towards a metallic target, producing X-rays (bremsstrahlung radiation) when they decelerate upon impact. In the field of non-destructive radioactive waste characterization, high-energy photon imaging (radiography, tomography) is used on large cemented radiological waste containers, with a volume of the order of 1 m3, to check their integrity and assess their content [1][2]. However, for such packages, passive gamma-ray spectroscopy, passive neutron counting and even active neutron interrogation fail in measuring nuclear materials, like plutonium and uranium. Therefore, high-energy photon interrogation techniques is under study to detect and quantify nuclear materials through the detection of induced-photofission particles. For the past years, CEA has been developing high-energy imaging [3] and photon interrogation techniques in CINPHONIE irradiation bunker (CHICADE facility, CEA IRESNE, Cadarache, France). CINPHONIE was recently upgraded with a K15 Varex accelerator that can reach a maximum dose rate of 130 Gy/min at 1 m from the X-ray target [4]. For advanced techniques (high-energy photon and photoneutron activations, photofission, bi-energy imaging), it is paramount to simulate precisely the irradiation field. For that purpose, a numerical model of the LINAC internals was built (with MCNP 6.3). It aims at simulating photon and neutron fields in view to calculate dose rates and reaction rates in irradiation samples, waste packages, but also in the whole casemate. A thorough characterization campaign was carried out to validate and calibrate this MCNP model against various experiments, including dose rate measurements in a water tank and delayed gamma-ray spectroscopy of thin metal foils activated in the X-ray field. These experimental results were used to fine-tune the electron source energy distribution as well as to estimate the average beam current. Its high-energy part is indeed particularly crucial for photofission and bi-energy studies.