Lead Tungstate For High Energy Calorimetry
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ABSTRACT The very large volumes needed to build a crystal calorimeter for High Energy Physics experiments bring cost considerations at the front of the stage. A reasonable compromise between cost and performances must be reached. One possible solution is to look at very dense materials like PbWO4, which will reduce the detector volume, even if the relatively low light yield will impose some limitations to the energy resolution. A review of the different results obtained by the Crystal Clear collaboration will be given for this crystal, including spectroscopic data and radiation damage measurements.
INTRODUCTION Although the scintillation properties of lead tungstate, PbWO4 (PWO), are known since the forties [1] and have been since that time intensely studied [2, 3], the material was not considered interesting for applications due to the strong thermal quenching, resulting in a low light yield at room temperature of typically 140photons/MeV for small samples and about 50photons/MeV for 20cm long crystals. This opinion has changed in the last few years with the growing interest for scintillators in High Energy Physics experiments. This study is part of the research programme undertaken by the Crystal Clear collaboration and deals only with those PWO properties which are directly connected with its possible use as scintillator material in the future electromagnetic calorimeter of one of the experiments (Compact Muon Solenoid) to be installed at the future Large Hadron Collider at CERN.
EXPERIMENTAL TECHNIQUES Systematic measurements of optical transmission, photo and radio luminescence, light yield and scintillation decay time were performed on more than 40 PWO crystals of different dimensions produced in Bielorussia. Details on experimental setups are given in reference [4]. For the temperature dependence of luminescence decay characteristics, a commercially available TRG/TBT cryostat free of oil vapour, with a BT200/TBT temperature controller were included in the decay time measuring chain. Data obtained were also used to extract the temperature dependence of the light yield, to which the integral signal is proportional in this kind of measurement. In order to study the homogeneity of the scintillation properties along the crystal growth direction, samples PW0519 to PW0530 were cut 99 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
perpendicularly to this direction from a 22 cm long crystal randomly chosen from a batch of ten crystals.
SCINTILLATION PROPERTIES 1. Non uniformity effects related to crystal composition It is well known that variations of stoechiometry are often present in binary oxide crystals grown by Czochralski technique. This is due to the different evaporation rates of the two components but also to the specific characteristics of their phase diagram. In order to avoid this effect, all PWO crystals studied in this work were grown at a temperature slightly inferior to the melting point of the stoechiometric PbWO 4 compound. For the same reason the raw material was rich in
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