Performance of the ATLAS beam diagnostic systems

Abstract

The beam diagnostic system of the ATLAS detector comprises two diamond sensor based devices. The innovative Beam Conditions Monitor (BCM) is aimed at resolving background from collision particles by sub-ns time-of-flight measurement. The Beam Loss Monitor (BLM) is a clone of the LHC machine BLM system, replacing ionization chambers with diamond sensors. BCM uses 16 1 × 1 cm2 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 downstream 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 beam permit system. The diamond sensors, the detector modules and their readout system are described. Results of performance with LHC beams of increasing energy and intensity including timing separation of collisions from beam related background are be presented, and beam abort algorithms discussed. BLM utilizes 12 pCVD diamond sensors close to the beam pipe at z ≈ ±3.5 m. The radiation induced currents in the sensors are read by the LHC machine developed readout, averaging the current over various time constants from 40 μs to 84 s. Exceeding a preset threshold for any of the readings drops the beam permit and aborts the LHC beam. Both systems provide post-mortem data dump of approximately 1000 LHC turns prior to the anomalous condition, allowing to diagnose its development and to refine the BCM algorithms and BLM thresholds. The systems were employed in various modalities from the first physics LHC run in November 2009, and are adapting their performance to balance between the need to protect the sensitive ATLAS Inner Detector, and yet allow efficient operation of the collider.