The upgrade of the laser calibration system for the ATLAS hadron calorimeter TileCal

The Tile Calorimeter (TileCal), the central section of the hadronic calorimeter of the ATLAS experiment, is a key detector component to detect hadrons, jets and taus and to measure the missing transverse energy. TileCal is built of steel and scintillating tiles coupled to optical fibers and read‐out by photomultipliers (PMT). The performance of TileCal relies on a continuous, high resolution calibration of the individual response of the 10,000 channels forming the detector. The calibration is based on a three level architecture: a charge injection system used to monitor the full electronics chain including front-end amplifiers, digitizers and event builder blocks for each individual channel; a distributed optical system using laser pulses to excite all PMTs; and a mobile Cesium radiative source which is driven through the detector cell floating inside a pipe system. This architecture allows for a cascade calibration of the electronics, of the PMT and electronics, and of full chain including the active detector, PMTs and electronics. Due to the complex procedures involving the Cesium source movement, the full calibration can be done only monthly during collision operations. On the contrary, the laser calibration can be done at any time both in inter‐bunch and intra‐bunch modes. The laser calibration system is therefore a key tool to monitor the PMT gain drift, one of the main source of systematics uncertainty in establishing the calorimeter energy scale. With the present laser system, the PMT gain was controlled at the percent level. The upgraded system aims at reaching sub‐percent level stability. The innovative solutions adopted for the up‐graded system as well as results from benchmark tests will be presented. All tests made before installation show an overall monitoring stability at a few per mille level and a uniformity in light distribution/equalization at the sub‐percent level.