Calibration, testing, commissioning and first data of ALFA at LHC

The ALFA detector system (Absolute Luminosity For ATLAS) aims for measuring the absolute LHC luminosity with high precision by using pp-scattering under small angles. Scintillating fibers tracking detectors are positioned 240 m from LHC interaction point 1 inside Roman Pots at millimeter distance from the LHC beam axis. The detectors consist of layers of 64 scintillating square fibers of 500 µm size in a U-V configuration. The layers are staggered 10 times to improve the spatial resolution to about 30 micrometers. A total of 11680 scintillating fibers are read out on the 8 ALFA detectors through 184 Multi Anode Photomultiplier Tubes, MAPMTs, of type Hamamatsu R7600-00-M64, 64 channels. Each detector has dedicated scintillating trigger tiles read out by 8 mm Hamamatsu Photomultiplier Tubes (R7400P and R9880U-110) via clear fibers. The gain of the MAPMTs differs up to a factor 3 inside a MAPMT and an additional factor 2 between different MAPMTs. For compensation, ALFA has developed a front-end electronic based on the MAROC chip, which features independent amplifier gain settings for its 64 channels. Exposure to pulsed light from a LED allowed measuring the 1 photoelectron peak for each of the 11680 channels and the amplification factor could be determined to equalize the overall gain of all readout channels. Prior to installation, all ALFA detectors were individually tested with cosmic particles and the light yield of each fiber determined. The detectors were then installed in a hadron test beam at CERN, which allowed characterizing efficiency, spatial resolution, cross-talk. The latter was found to be higher than expected. A dedicated laboratory study was able to reproduce the fiber cross talk and trace it down to reflections on the Roman Pot windows. Before installation the Roman Pot windows was therefore blackened. During the LHC technical stop winter 2010-2011 the full ALFA detector system was installed at ATLAS/LHC. It is currently undergoing a commissioning program and we expect to report first results from measurements with LHC beams. We will also give an outlook on possible upgrades of the system, in particular the trigger scintillator readout.