The luminosity monitor of the ATLAS experiment

The LUCID (LUminosity Cherenkov Integrating Detector) apparatus is composed by two symmetric arms deployed at about 17 m from the ATLAS interaction point. The purpose of this detector, installed in summer 2008, is to monitor the luminosity delivered by the LHC machine to the ATLAS experiment, and to provide an interaction trigger when running at low luminosity. An absolute luminosity calibration is needed and it will be provided by a Roman Pot type detector with the two arms placed at about 240 m from the interaction point. Each arm of the LUCID detector is based on an aluminum vessel containing 20 Cherenkov tubes, 15 mm diameter and 1500 mm length, filled with C4F10 radiator gas at about 1.1 bar. The Cherenkov light generated by charged particles above the threshold is collected by photomultiplier tubes (PMT) directly placed at the tubes end. The intrinsically fast response of the detector and its readout electronics makes it ideal to follow the number of interactions per LHC bunch crossing therefore providing also an interaction trigger to the ATLAS experiment. The detector took already some data from the first beam interactions produced by LHC. In this talk we will report the relevant details of the detector design and its associated monitor and trigger electronics.