Online Radiation Dose Measurement System for ATLAS experiment

Particle detectors and readout electronics in the high energy physics experiment ATLAS at the Large Hadron Collider at CERN operate in radiation field containing photons, charged particles and neutrons. The particles in the radiation field originate from proton-proton interactions as well as from interactions of these particles with material in the experimental apparatus. In the innermost parts of ATLAS detector components will be exposed to ionizing doses exceeding 100 kGy. Energetic hadrons will also cause displacement damage in silicon equivalent to fluences of several times 10e14 1 MeV-neutrons per cm2. Such radiation doses can have severe influence on the performance of detectors. It is therefore very important to continuously monitor the accumulated doses to understand the detector performance and to correctly predict the lifetime of radiation sensitive components. Measurements of doses are important also to verify the simulations and represent a crucial input into the models used for predicting future detector performance. The online radiation monitoring system in ATLAS experiment measures ionizing dose in SiO2, displacement damage in silicon and fluence of thermal neutrons at several locations in ATLAS detector. The on-line measurement of the Total Ionizing Dose (TID) is done with radiation sensitive MOS transistors (RADFETs). Displacement damage in silicon is monitored with diodes. Fluences of thermal neutrons are estimated from gain degradation in dedicated npn bipolar transistors. In this contribution the design of the online radiation monitoring system, results of measurements and comparison of measured integrated doses and fluences with predictions from FLUKA simulation will be shown.