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.