A. Nyanin

Influence of variable tungsten valency on optical transmittance and radiation hardness of lead tungstate (PWO) scintillation crystals

Abstract A new approach to interpret the radiation hardness of PbWO4 (PWO) scintillators is developed by revealing importance of the inclusions of tungsten oxides WO3−x with variable valency. It is demonstrated that the influence of the ionizing radiation on PWO is, in many aspects, similar to the effect of the high-temperature annealing in oxygenless ambient. In both cases, a valency change of the tungsten oxides is initiated and results in induced absorption and, consequently, in crystal coloration. In the PWO crystals doped with L2O3 (L=Y,La,Gd), the radiation hardness and the optical properties are mainly affected by inclusions of W1−yLyO3−x (0

Improving the timing resolution of an electromagnetic calorimeter based on lead tungstate crystals

Results of the beam tests of the prototype photon spectrometer PHOS for the ALICE experiment at the Large Hadron Collider (CERN) are presented. The spectrometer is based on detector elements composed of lead tungstate (PbWO4) crystals with dimensions of 22 × 22 × 180 mm and Hamamatsu S8664-55 (S8148) avalanche photodiodes. The beam tests have been performed on the secondary T10 beamline of the PS proton synchrotron. The main emphasis has been placed on the possibility of improving the PHOS timing resolution. Introduction of an additional timing channel with a silicon photomultiplier (SiPM) used as a photodetector is shown to improve the timing resolution for 1-GeV deposited energy from current value σt = 3 to 0.3 ns. Silicon photomultipliers of the Hamamatsu MPPC S10362-33 family with an active area of 3 × 3 mm2 are used in these measurements. Using fast photomultiplier tubes with an 8-mm-diameter photocathode, the timing resolution attainable in electromagnetic shower development in a lead tungstate crystal has been measured for a large-area photodetector. The timing resolution for a deposited energy of 1 GeV is 150 ps. The effect of the detector channel temperature on the timing resolution is investigated. Cooling the crystal results in an increase both in the scintillation intensity and in the decay time of the scintillator and fails to substantially improve the timing resolution.

Development of the technology for large-scale production of lead tungstate single crystals for physical experiments

The scientific principles underlying the technology for growing crystals of lead tungstate PbWO4 (PWO) with controlled properties are discussed. The developed technology is implemented at the North Crystals Company in mass production of PWO crystals with 100% reproducibility of operating parameters. This made it possible to solve the problem associated with the preparation of PWO crystals for the ALICE experiment at CERN and to produce the pilot batch of large-sized PWO crystals (diameter, 46 mm; length, 250 mm) with required characteristics for the CMS experiment at CERN. More than 10000 PWO scintillators of high optical quality for the ALICE experiment were fabricated over a short time according to the proposed technique.