Characterisation and optimisation of a radiation detection system for the new Site Gate Monitors at CERN

The goal of this thesis work is to describe the methods used and the tests performed in the framework of the renovation of the Site Gate Monitors (SGM) system at CERN, in particular concerning the radiation detection and signal processing. The SGM is a permanently-operating system for the detection of gamma emitters at CERN site exits, aiming to prevent their illicit removal. It monitors slowly-moving vehicles, targeting specific isotopes which are usually produced by CERN accelerators or used as standard laboratory sources. Their gamma radiation lies in an energy range between 50 keV and 3 MeV. The detectors should be sensitive to the energy range in which the sources of interest emit their gamma radiation. Individual isotope identification is not required. Short measuring time, high sensitivity and low false alarm rate are needed. Reliable discrimination of the targeted sources from naturally occuring radioactive materials (NORM) and medical isotopes, which should be recognized but not intercepted by the system, is required. Among the main constraints there are the variety of background conditions the detectors are exposed to due to natural and artificial radiation, the background suppression phenomenon and the presence of pulsed radiation due to CERN's accelerators complex operation. Energy windowing algorithms will be implemented to discriminate the targeted sources providing a rough energy information, to increase the sensitivity of the system in presence of background suppression and to reduce the one to background variations due to environmental influences. Since the radiation fields produced by the accelerators can be non uniform in time, pulsed radiation rejection is a requirement at CERN, which will be faced via custom-made filters, whose parameters have to be adjusted according to the specific detector location.