Electronic Readout of the Atlas Liquid Argon Calorimeter: Calibration and Performance

The Liquid Argon (LAr) calorimeter is a key detector component in the ATLAS experiment at the Large Hadron Collider (LHC) at CERN. The LHC is a proton-proton collider with a center-of-mass energy of 14 TeV. The machine has been operated at energies of 900 GeV and 2.36 TeV in 2009 and is expected to reach the energy of 7 TeV in 2010. The LAr calorimeter is designed to provide precision measurements of electrons, photons, jets and missing transverse energy. It consists of a set of sampling calorimeters with liquid argon as active medium kept into three separate cryostats. The LAr calorimeters are read out via a system of custom electronics. The electronic readout of the ATLAS LAr calorimeters is divided into a Front End (FE) system of boards mounted in custom crates directly on the cryostat feedthroughs, and a Back End (BE) system of VME-based boards located in an off-detector underground counting room where there is no radiation. The FE system includes Front End boards (FEBs), which perform the readout and digitization of the calorimeter signals, calibration (CALIB) boards which inject precision calibration signals, trigger boards which produce analog sums for the first level of the ATLAS trigger system, and control boards which receive and distribute the 40 MHz LHC clock as well as other configuration and control signals. The BE electronics are made up primarily of Read Out Driver (ROD) boards which receive the digitized signal s. The RODs perform digital filtering, formatting, and monitoring of the calorimeter signals before transmitting the processed data to the ATLAS data acquisition system (DAQ). In the front end system, the signals, coming from about 200,000 calorimeter cells, are first subject to several stages of analog processing. Preamplifier hybrids amplify the raw signals, which are then split and further amplified by shaper chips to produce three overlapping linear gain scales, with gain ratios of 10. The shaped signals are then sampled at the LHC bunch crossing frequency of 40 MHz by switched capacitor array (SCA) analog pipeline chips. For events accepted by the L1 trigger, typically five samples per channel are readout from the SCA using the optimal gain scale, and digitized using a 12-bit Analog-to-Digital Converter. The digitized data are formatted, multiplexed, serialized, and then transmitted optically out of the detector to the Readout Driver (ROD) in the counting room via a single 1.6 Gbps optical output link per front end board. The RODs synchronize the output of the front end boards with the L1 trigger and compute physical quantities such as energy, time phase and quality of the signal. A large energy dynamic range of the readout cells (from 50 MeV up to 3 TeV) and a good energy resolution are some of the main challenges of the Liquid Argon readout electronics. To compute the electronic gain of each individual channel a calibra t ion board has been designed. The board, hosted in the same crate as the front end boards, delivers a signal whose shape is close to the calorimeter ionization signal. This is achieved by applying an exponential voltage pulse across an injection resistor (of 0.1% accuracy) located in the cold, directly on the calorimeter electrodes. The LAr calorimeter has been installed in the ATLAS cavern and filled with liquid argon since 2006. Since then the detector has collected a large amount of data from random triggers, calibration, cosmic muons, LHC beam splash events and collisions (2009). We present here the full system performance of the LAr electronics. The measured performance of the precision readout of the individual calorimeter channels will be presented. After a brief overview of the LAr readout electronics, and a description of the methods used to reconstruct the calorimeter pulses, performance results are then presented, in term of pedestal, gain stability, noise, coherent noise, energy reconstruction (including linearity and resolution) and timing performance.