Design of a decision unit for calibration of a particle detector using a laser system

The ATLAS detector conceived for particle physics studies will begin its operation on the Large Hadronic Collider (LHC) by 2007. It will be dedicated to search of Higgs boson and studies of the Standard Model and new physics beyond it. The scintillation tile calorimeter is a sub-assembly of the ATLAS detector. The 10,000 readout paths of the calorimeter are constituted of photomultipliers transforming light in electric signals. The reconstruction of the particle tracks and energy disposed into the detector must be accurately determined. Hence every sub-assembly is provided with several calibration systems. The calibration system of the scintillator tile calorimeter makes use of a laser which has its own calibration units providing accurate knowledge of the calorimeter light pulses feeding the photomultipliers. It consists of: - four photodiodes receiving a fraction of the laser beam; - a monochromatic alpha radioactive source passing in front of photodiodes; - a charge injector to test the photodiode preamplifier linearity. To control the laser system an electronic card was designed and built. This is inserted into a VME frame composed of three parts: - the first one is an interface with the soft via the VME bus; - the second one is a pulse shaper for the analog signals coming from photomultipliers and photodiodes; - while, the third one records the laser response time. The electronic signals issued from the analogue inputs are shaped by constant fraction discriminators. Particularly, they have the specific feature of giving response times almost independent of the input signal height. The laser response time is dependent on the height of light pulses. The SLAMA card embodies a function dedicated to this operation with the purpose of optimizing the calibrations in the absence of physical events. Other various functions are also integrated by the card by means of FPGA programmable digital components the following specific circuits: - an interface with the VME bus; - a time digital converter, TDC, ASIC, developed at CERN; - a unit for laser triggering delay control; - a software for autonomous operation. The preliminary tests before complete integration within the system were carried out in the laboratory. These allow confirming the feasibility of laser command and control. They showed also the high card's flexibility due to its possibility of in-situ re-configuration