Evaluation of Radiation Field Properties with Pixel Semiconductor Detectors Operating in Particle Tracking Mode

The usage of radiation detectors covers a wide range of applications from basic research towards industry or medicine. Continuous technological progress in electronic fabrication processes with help of the sophisticated data analysis methods now allows faster extraction of more complex information about interacting particles. This thesis defines the methodology of evaluation of complex radiation field properties using Medipix2 semiconductor pixel detector which has been successfully used in the network of 16 detectors installed and operated in the ATLAS experiment at CERN in the frame of ATLAS-MPX project. The aim of the network is to perform real-time measurements of spectral characteristics and composition of the radiation environment inside the ATLAS detector. With the devices, in two different modes of operation, a large dynamic range of particle flux can be covered, of at least 9 orders of magnitude, which corresponds to the highest luminosity, while also measurements of the radiation background outside the collisions can be made. An important goal is the determination of the neutron component of the mixed radiation field. To identify different types of neutrons, the 300 μm thick silicon sensor area of each ATLAS-MPX device has been divided into several regions, covered by different neutron converter materials. A $^{6}$LiF layer is used for thermal neutron detection and a polyethylene foil for fast neutron detection. The calibration of the detection efficiency of all devices has been performed with various known neutron sources: $^{252}$Cf, $^{241}$AmBe, thermal neutrons and 14 MeV neutrons from Van de Graaff accelerator. ATLAS-MPX devices have been operated almost continuously starting from early 2008 (background and cosmic radiation measurements and the first LHC beam appearances in 2008 and 2009) through the stable LHC operation during the 2010 and 2011 run periods up to nowadays. This thesis presents the first evaluations of the luminosity dependence, activation of the environment and comparison of the different radiation groups with the Monte Carlo simulations. Based on the several years of experience with ATLAS-MPX network operation and data analysis several ideas for the future upgrade of the detector network are presented including new Timepix readout chip, multilayer detector concept and possibility of usage of the CdTe sensor material. The results presented in this thesis have proven to be an important feedback for ATLAS community and will possibly allow the calculation of the safety factors to be used in the future to determine the viability of the ATLAS electronics exposed to growing doses of neutrons and photons, in particular in view of planned LHC upgrade. There is a growing interest in these results also from the other large LHC experiments.