Cross-calibration of two automatic quality control systems for the CMS ECAL crystals

The barrel part of the CMS electromagnetic calorimeter consists of about 75 000 Lead Tungstate (PbWO4) crystals arranged in 36x4 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