Large lead tungstate crystals for calorimetry

PbWO₄ crystals have been selected for the CMS Experiment at the Large Hadron Collider (LHC), rather than the higher light yield but bulkier CeF₃, because space is at a premium in the 4-T supraconductive magnet. Early PbWO₄ 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 O 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 PbWO₄ production in China