Characterization of the Radiation Field in the ATLAS Experiment With Timepix Detectors

We present a study of the radiation field at various locations in the ATLAS experiment, using compact detector systems based on pixelated silicon sensors assembled with Timepix readout chips. The hodoscope design of the ATLAS-Timepix (TPX) detectors includes neutron converters in between two sensors, allowing the characterization of both neutron and charged particle fields on a track by track basis. Thermal and fast neutrons are discriminated by segmenting the sensor area with dedicated sensitive materials. Using specific pattern recognition algorithms, clusters from electrons and photons above ~10 keV, minimum ionizing particles (MIPs), and highly ionizing particles are classified. A coincidence method using the time-over-threshold mode of the chip is developed to extract stopping power and directional information of energetic charged particles. Thermal neutron fluences are obtained for each ATLAS-TPX unit, illustrating the effect of detector material and shielding in the experimental cavern. Reconstructed trajectories of energetic charged particles point out radiation coming from the interaction point and other hot spots.