Laser monitoring system for the CMS lead tungstate crystal calorimeter

We report on the multiple wavelength laser monitoring system designed for the CMS lead tungstate crystal calorimeter read-out with avalanche photodiodes (Barrel calorimeters) and vacuum phototriodes (End Cap calorimeters). Results are presented for the test beam performance of the system designed to achieve 0.5% relative inter-calibration of the optical transmittance for lead tungstate scintillation emission over nearly 80 000 channels. The system operates in continuous measurement cycles to follow each crystal?s evolution under irradiation and recovery periods foreseen during operation at the LHC

Energy Resolution Performance of the CMS Electromagnetic Calorimeter

The energy resolution performance of the CMS lead tungstate crystal electromagnetic calorimeter is presented. Measurements were made with an electron beam using a fully equipped supermodule of the calorimeter barrel. Results are given both for electrons incident on the centre of crystals and for electrons distributed uniformly over the calorimeter surface. The electron energy is reconstructed in matrices of 3 times 3 or 5 times 5 crystals centred on the crystal containing the maximum energy. Corrections for variations in the shower containment are applied in the case of uniform incidence. The resolution measured is consistent with the design goals

Scintillateurs et autres détecteurs optiques de particules

Parmi les techniques de détection de rayonnement, les détecteurs optiques occupent une place importante et apportent des réponses aux exigences des expériences de physique et d'astrophysique. Je traiterai plus particulièrement dans mon exposé des techniques de détection basées sur l'émission Čerenkov ou la scintillation dans les matériaux inorganiques transparents, en les illustrant par les recherches, développements et réalisations. Les aspects de science des matériaux, d'optique, en particulier les problèmes causés par les radiations, les techniques expérimentales utilisées, les développements réalisés en matière de simulation numérique et de modélisation, et bien entendu les expériences de physique en cours ou projetées seront abordés. Je terminerai par quelques perspectives d'utilisation des cristaux scintillants dans les calorimètres hadroniques du futur

Simulation of light collection in the CMS lead tungstate crystals with the program Litrani: coating and surface effects

The accuracy of the lead tungstate electromagnetic calorimeter of the CMS experiment under construction at CERN relies, among other things, on the correction of the calibration parameter from variations dues to crystal ageing. This ageing will be measured by a so-called monitoring system, but the relation between monitoring and calibration parameter variations is not so trivial, and depends much on the overall optical characteristics of crystal and photodetector. We present here simulations done with the program Litrani based on real ageing data for a realistic CMS crystal with a defined surface quality (optically polished, with or without one lateral face slightly depolished), covered by coatings of various characteristics, from totally absorbing to nominal aluminum or diffusing medium. The correlation coefficient between monitoring and scintillation signals depends greatly o n these characteristics, and varies between about 1.3 and more than 10 (the optimum being one). Submitted to or Presented at: Contact e-mail - [email protected]

Modelisation and simulation of the light collection in the CMS lead tungstate crystals

The ageing of scintillating crystals under radiation leads to a deterioration of their transparency, thus of their effective light yield and their calibration. The link between the transparency variation, as measured by a fibre optic monitoring system, and the calibration variation is not trivial. A model describing the light collection process is proposed. It helps to understand the parameters which govern this correlation, mainly absorption length and light back-reflection or diffusion characteristics. This model describes well the results of numerical qualitative simulations of light collection in lead tungstate crystals. It also explains the differences between calibration and monitoring variations observed in test beam

Optical anisotropy effects in lead tungstate crystals

The anisotropic crystal structure of lead tungstate leads to strong consequences in its optical properties and their characterization. Beyond the variation of surface reflections due to the birefringence of the material, we report the observation that part of the bulk light absorption is sensitive to the light polarization direction. This variable part varies with the quality of the crystal, and is clearly related to internal structural defects. Irradiation experiments with gamma rays or fast neutrons confirm the recent improvement of the short term resistance of this scintillator. Its long term behaviour remains identical, and acceptable for the use in high energy electromagnetic calorimetry. A polarization direction dependant induced absorption is observed, specially after high levels of radiation. Ray tracing programs in development for the simulation of the light yield in lead tungstate should take into account all these anisotropic effects