Development of Beam Conditions Monitor for the ATLAS experiment

If there is a failure in an element of the accelerator the resulting beam losses could cause damage to the inner tracking devices of the experiments. This thesis presents the work performed during the development phase of a protection system for the ATLAS experiment at the LHC. The Beam Conditions Monitor (BCM) system is a stand-alone system designed to detect early signs of beam instabilities and trigger a beam abort in case of beam failures. It consists of two detector stations positioned at z=±1.84m from the interaction point. Each station comprises four BCM detector modules installed symmetrically around the beam pipe with sensors located at r=55 mm. This structure will allow distinguishing between anomalous events (beam gas and beam halo interactions, beam instabilities) and normal events due to proton-proton interaction by measuring the time-of-flight as well as the signal pulse amplitude from detector modules on the timescale of nanoseconds. Additionally, the BCM system aims to provide a coarse instantaneous luminosity measurement complementary to the main ATLAS luminosity monitor. Radiation hard sensor devices capable of producing fast signals with 1MIP sensitivity were required for the BCM system. Two different sensor materials were considered: Polycrystalline Chemical Vapour (pCVD) diamond and n-type thin epitaxial (epi) silicon. The annealing study of irradiated epi samples at an annealing temperature of 20oC proved the epi material to be radiation hard in terms of effective dopant concentration. The results were found to agree with the results obtained by a different group at elevated annealing temperatures. However, the performance of epi devices was measured to be inferior to pCVD diamond sensors in terms of BCM detector module signal to noise ratio. Thus, the pCVD diamond was chosen for the BCM sensor material in the end. Numerous measurements have been performed during the development in order to optimise the BCM system performance. These incorporate characterisation of the diamond sensors, tests of the detector modules with test beam pions and 90Sr electrons as well as tests with the digitisation electronics included. The most relevant results are discussed here. An estimate of the expected BCM system performance at the end of readout chain in ATLAS is also given.