Precise measurements of light yield and transmission of crystals at the INFN-ENEA regional centre

The barrel of the CMS electromagnetic calorimeter is currently under construction and will contain 61 200 PbWO 4 crystals. Half of them are being fully characterized for dimensions, optical properties and light yield in the INFN-ENEA Regional Centre near Rome. This paper describes the measurements and results from a sample of about 6000 crystals. Results are presented on long term stability and precision of light yield and transmission measurements. A strong correlation between crystal light yield and longitudinal transmission in the range 350–370 nm is observed. As it will not be possible to precalibrate with particles the whole calorimeter, the light yield and transmission measurements performed at the Regional Centre will be crucial to provide an initial intercalibration for most crystals. Thanks to the observed correlation, transmission measurements can be combined with those of direct light yield to improve the crystal intercalibration precision. Expectations on the achievable precision are derived.