Contribution of the MONTE CARLO modelling for the minimum detectable activity of spectrometer gamma of large volume

The CEA DAM has a number of high-capacity (2 – 20 litres) sodium iodide gamma detectors used in the Intervention and global safety programs. The NaI crystals are positioned in packaging adapted to their use: transport suitcases, measuring container… The knowledge of the energy response and the minimum detectable activities of these systems makes it possible to ensure the adequacy of their employment to the needs. NaI Crystals of 2 to 20 litres were modelled using the Monte Carlo Transport Code of Radiation material interaction MCNP6 in different containers for gamma sources available at CEA DAM. The comparison of the results of the models to the experiments allowed to refine and validate the parameters of modeling (geometry, materials, physics…) and thus to have a "model" representative of the system. The purpose of the presentation is to show the interest of the simulation in understanding the response of the sensors used to the CEA DAM and in estimating the minimum detectable activities. One of the systems presented is the HELINUC Airborne gamma detection System. The container (see Figure 1) is loaded with 4*4-l NaI crystals. Two Ge detectors installed on either side of the container can also be deployed.