Certifying procedures for lead tungstate crystal parameters during mass production for the CMS ECAL

Certifying procedures and fully automated equipment for testing of PbWO₄ (PWO) scintillators have been developed. The parameters to be verified are the optical transmission spectra in the longitudinal and transversal directions; the light yield and its non-uniformity along the crystal; the scintillation kinetics; the radiation hardness and the dimensions. Both the precision of measurements and the output rate meet the stringent requirements of the mass production stage of PWO scintillating elements for the CMS electromagnetic calorimeter at CERN (full characterization of several tens of crystals per day for five years). This is achieved by a) the implementation of a `start-stop´ technique with high count rate capability for scintillation decay measurement; b) the development of special compact fast scanning spectrophotometers; c) the application of a multi-axis movement system for crystal and spectrometers; d) the use of a standard programmable 3D machine for precise dimension measurement. Specific methods and an experimental set-up have also been developed for checking of radiation hardness on a sampling basis