Effets de rayonnement sur les detecteurs au silicium a pixels du detecteur ATLAS

Two detection systems are using pixel silicon detectors in the ATLAS detector: the Pixel, which is the subdetector closest to the interaction point, and the MPX network. The activation of the materials present in the Pixel produced by radiation has been measured in two experiments which we performed at CERF (CERN) and NPI-ASCR (Czech Republic). These experimental studies of activation are com- pared with GEANT4 simulations. The results of these comparisons show that the simulation can predict the activities with a precision of an order of magnitude. They also show that GEANT4 fails to produce certain radioisotopes seen in the experimental activation studies. The contribution to background and the resid- ual doses due to the desintegration of the radioisotopes produced by fast neutrons (category in which falls the expected average neutron energy of 1 MeV in ATLAS) are extrapolated to ATLAS conditions. It is found that this background in the AT- LAS Pixel subdetector will be negligible and that the doses are well below safety concerns for detector manipulation during maintenance and repair periods. The radiation field also inflicts damages to the silicon detectors thus reducing their detection efficiency. A modified Hecht model is presented using an electric field description which includes the double junction effect and a small exponential component in areas usually considered without electric field. This model allows the description of the detection efficiency as a function of applied bias voltage and irradiation fluence for several types of silicon detectors irradiated by particles of different types and energies. On top of validating the Hecht model proposed in this thesis, the studies of the radiation damage on silicon detectors has allowed to con- clude that the NIEL hypothesis has to be revised (study with different energies). Using the variation with irradiation fluence of the effective doping concentration and of the leakage current, it is shown that silicon detectors with a higher con- centration of oxygen have a radiation hardness slightly improved for low-energy protons compared to standard silicon detectors. It is also demonstrated that the radiation hardness of the Mesa detectors is similar to standard detectors for proton and neutron irradiations. The MPX network includes fourteen pixel detectors located in the ATLAS detector and its cavern. Their goal is to perform, within the ATLAS detector and cavern, real-time measurements of the particle flux (type and energy), the doses and the background signal due to neutrons and photons. It will be shown that these detectors can indeed adequately detect several types of particles produced in ATLAS: photons, neutrons and charged particles. Using low-energy photons, the energy calibration of the comparator threshold is done. A phenomenological model is elaborated describing the charge sharing among pixels which appears when a heavy charged particle stops in the detector.