Photon Flux Characterization of a New Electron LINAC in the CINPHONIE Irradiation Facility Through a Bayesian Framework

For several years, CEA has been developing high-energy imaging and photon interrogation techniques in the CINPHONIE irradiation bunker (CHICADE facility, CEA IRESNE, Cadarache, France). A new linear electron accelerator (K15 LINAC from Varex) has recently been commissioned and its irradiation field requires precise characterization. The manufacturer data for the mean electron energy is 15 MeV but we asked for a slightly higher dose rate, resulting in a higher end-energy that need to be characterized. In addition, the knowledge of the beam current is essential to ensure the reliability of both experimental results and numerical simulations. For that purpose, dose rate measurements in a water tank and delayed gamma-ray spectroscopy of thin metal foils activated in the X-ray field were carried out. A numerical model of the LINAC internals was built with MCNP6.3 to calculate dose rates in water and reaction rates in irradiation samples. A thorough characterization campaign was carried out to validate and calibrate this MCNP model against experiments. Finally, a Bayesian framework was applied to fine-tune the electron source energy distribution, estimate the average LINAC current and quantify the X-ray angular deviation. Finally, the best estimates are E = (17.8 ± 0.2) MeV and Ibeam = (107 ± 6) μA.