The CMS Electromagnetic Calorimeter Readout

CMS is a general-purpose detector designed for use in the large hadron collider (LHC) at CERN. The electromagnetic calorimeter will play an important role in electron and photon energy measurements. The benchmark process to evaluate and optimize its performance is H −→ γγ in the intermediate mass range (90 < mH < 150 GeV). For this purpose, each element of the readout chain has to be optimized. This paper discusses the readout chain of the CMS electromagnetic calorimeter and preliminary results obtained from prototype components. Presented at International Conference on Advanced Technology and Particle Physics, (Villa Olmo) Como Italy, October 5-9, 1998 Submitted to Nuclear Physics BProceedings supplements a) on behalf of the CMS collaboration. 1 CMS Electromagnetic Calorimeter Overview The CMS electromagnetic calorimeter (ECAL) [1] will be situated between the central tracker and the hadronic calorimeter, within a 4T magnetic field. The ECAL will consist of a barrel detector and two endcaps, corresponding to a length of 21.6m and a diameter of 15m. In order to obtain the best possible energy resolution, lead tungstate (PbWO4) scintillating crystals have been chosen as the active medium. Some characteristics of this scintillator are given in table 1 in comparison with other materials such as NaI(Tl) and BGO (used for the electromagnetic calorimeter of the L3 experiment at LEP). Table 1: Comparison of scintillating materials NaI(Tl) BGO PbWO4 density (g/cm) 3.67 7.13 8.28 radiation length (cm) 2.59 1.12 0.89 interaction length (cm) 41.4 21.8 22.4 decay time (ns) 230 300 30 emission peak (nm) 410 480 450 relative light output 100 18 1.3 The advantages of PbWO4 scintillator include its density which permits a compact calorimeter (small Molière radius and radiation length) and its short light decay time. The electromagnetic calorimeter will be composed of upwards of 76000 23cm-long truncated pyramidal crystals (∼26 radiation lengths). Each crystal will be placed in a 5×2 (η × φ) rigid alveolar structure (in order to maintain a crystal-to-crystal distance of 500μm, made of glass fiber and covered with a special coating to diffuse the scintillation light. From the mechanical point of view, these alveolar structures will be assembled to form modules (of either 400 or 500 crystals) and supermodules (a group of 4 modules representing 85×20 crystals in η × φ). Each half-barrel constains 18 supermodules disposed in φ. Figure 1 gives a 3D-view of the CMS electromagnetic calorimeter. Figure 1: 3-D view of the electromagnetic calorimeter. Overall dimensions are 21.6m length and 15m diameter.