Abstract
The barrel part of the CMS electromagnetic calorimeter consists of about 75 000 Lead Tungstate (PbWO 4) crystals arranged in 36×4 modules which are assembled in two Regional Centres, in Rome and at CERN. Two automatic machines have been designed to check the crystal quality before assembly. The main crystal characteristics are compared to a set of specifications included in the contract with the crystal producers. The measurement stability and cross-calibration between the two machines is a fundamental issue, which has to be monitored throughout the construction phase. This paper describes comparisons between measurements made at the two regional centres to ensure a consistent and reliable crystal quality control.
Highlights
The CMS collaboration at the LHC has decided to construct a homogeneous electromagnetic calorimeter made of about 80000 lead tungstate (PWO) crystals
The light yield (LY) of the crystals must be higher than a given threshold, this to ensure a small stochastic term in the energy resolution
The transversal transmission (TTO) of the crystals should not vary as a function of the position, since it would be a proof for crystal inhomogeneities
Summary
The CMS collaboration at the LHC has decided to construct a homogeneous electromagnetic calorimeter made of about 80000 lead tungstate (PWO) crystals (see Ref. [1] and [2]). In the two Regional Centers for calorimeter assembly and test (one at CERN and one at ENEA near Rome), the staff thought about the design of a machine (called ACCOS machine) to rapidly measure the crystal properties.
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