Large lead tungstate crystals for calorimetry

PbWO//4 crystals have been selected for the CMS Experiment at LHC, rather than the higher light yield but bulkier CeF//3, because space is at a premium in the 4T supraconductive magnet. Early PbWO//4 samples exhibited severe low dose radiation damage, slow components or long term afterglow. Thus, major R&D efforts have been undertaken by the Russian and Chinese producers. Besides improving the control of the raw materials, the stoichiometry and the growth parameters, the R&D efforts explored two avenues: doping of the crystals and oxygen annealing. Both groups have produced full size crystals which satisfy the requirements of CMS, including the low production cost which is an overwhelming consideration and a severe constraint in the optimization of the production process. This paper concentrates on the development of Chinese crystals; it attempts to guide the reader through the complex development process by giving a very brief description of the main steps and issues addressed in the past five years, as an introduction to a few recent results and to a status report on PbWO//4 production in China. 16 Refs.