Études de cristaux scintillants de tungstate de plomb et de prototypes de calorimètre électromagnétique pour le projet CMS au LHC

The CMS detector which will be installed on the LHC is equiped with an electromagnetic calorimeter made of about 110.000 lead tungstate crystals (PbWO4) read out by avalanche photodiodes. The choice of such photodetectors with gain is dictated by the low level of light yield from the scintillation and by the presence of a high magnetic field. This thesis reports on the work done on the crystal in order to improve its characteristics in view of the construction for a high performance calorimeter. In this way, a systematic study of the main quality parameters of the crystal (light yield, uniformity of the response, radiation hardness, optical transmission) has been led. In the same time, the analysis of the data taken in high energy beam tests for several prototype matrices has allowed to confirm the performance, expected mainly related to energy and position resolution. This performance is particularly decisive in the Higgs's boson search in the intermediate mass range using the detection of the 2 gammas mode.