Ten years of lead tungstate development

Abstract

The CMS Collaboration at CERN has undertaken at the end of 1994, an ambitious R&D program on lead tungstate scintillating crystals for its electromagnetic calorimeter. All the parameters of this crystal have been extensively studied in order to optimize its performances in the context of the large hadron collider. Full-size crystals (23cm in length, up to 2.5×2.5cm2 in section) with the desired trapezoidal geometry can now be grown and mechanically processed with a yield in excess of 80%. A thorough investigation of the raw material preparation and of the growing conditions has led to a significant improvement in the optical transparency and in the light yield of the crystals. A detailed understanding of the light emission mechanism was established in 1995. A systematic analysis of the parameters influencing the structural quality of the crystals was at the origin of a considerable improvement of the radiation hardness of full-size crystals. All these progress has led several other experiments to select lead tungstate as the best scintillator for their detector. The conditions of these improvements will be discussed in the context of the mass production of nearly 80,000 crystals in a cost-effective way.