Beam Conditions Monitor – ATLAS safety & luminosity

by Dr Bostjan Macek (University of Hawaii at Manoa)

Tuesday Jan 22, 2013, 3:00 PM → 4:00 PM Pacific/Honolulu

417A (Watanabe hall)

Beam conditions and the potential detector damage resulting from their anomalies have pushed the LHC experiments to implement their own monitoring devices. In ATLAS the role of beam protection is handled by the Beam Conditions Monitor (BCM). It is aimed at resolving background from collision particles by sub-ns time-of-flight measurement. It uses 16 1×1 cm^2 0.5 mm thick polycrystalline chemical vapor deposition (pCVD) diamond sensors arranged in 8 positions at a radius r ≈ 55 mm, ∼ 1.9 m up- and down- stream the interaction point. Time measurements at 2.56 GHz sampling rate are performed to distinguish between collision and shower particles from beam incidents. A FPGA-based readout system performs real-time data analysis and interfaces the results to ATLAS and the LHC systems. The system was employed in various modalities from the first physics LHC run in November 2009, and is adapting its performance to balance between the need to protect the sensitive ATLAS Inner Detector, and yet allow efficient operation of the collider. Though initially designed as a safety system its properties make it well suited also for physics measurements. Its appropriate acceptance, timing performance and data-purity have been exploited further by extending the system into a luminosity monitoring device. Though secondary functionality, this has proved to provide even bigger contribution to the ATLAS experiment. In the presentation the diamond sensors, the detector modules and their readout system will be described. Results of performance with LHC beams will be given, showing timing separation of collisions from beam related background. On this basis the beam-abort system will be presented and appropriate algorithms discussed. In the second part the focus will turn to luminosity infrastructure, calibration and final measurement results.