Studies of PbWO_4 crystals and construction and tests of a compact crystal calorimeter for ZEUS

The ZEUS experiment at the electron(positron)/proton collider HERA is planning to replace a small electromagnetic tungsten/scintillator sampling calorimeter by a lead tungstate (PbWO_4) crystal calorimeter. The ''beampipe calorimeter'' (BPC) directly adjacent to the beam intercepts electrons with small scattering angles. It measures the inelastic electron-proton scattering in the interesting kinematic region 0.045 #<=# Q"2 #<=# 0.80 GeV"2 and 3 x 10"-"7 #<=# x #<=# 10"-"3 where the transition from photoproduction to deep inelastic scattering occurs. An improvement of the current electromagnetic energy resolution from 17%/#sq root#(E/GeV) to 5%/#sq root#(E/GeV), which is expected for a PbWO_4 calorimeter, would result in a much higher precision of for example the proton structure function F_2 and would further extend the accessible kinematic region. The so-called crystal BPC has to meet stringent requirements: the size is restricted, the light yield must be very high, the scintillation light pulses have to be short (95% within 96 ns), and the crystals have to be sufficiently radiation stable. Bench stests have shown that 20 PbWO_4 crystals (La doped) with dimensions 200 x 23.8 x 23.8 mm"3 which were received in autumn 1997 from BTCP, Bogoroditsk, Russia, fulfil the demands. A full size calorimeter of 4 x 4 crystals has been built and tested in a CERN SPS beam with energies 3-100 GeV. The crystals are read out by small photomultipliers which can work in a magnetic field. A LED transmission monitoring system proved its functionality when two crystals were interchanged with irradiated crystals and the recovery was tracked. (orig.)SIGLEAvailable from TIB Hannover: RR 8919(2000-024) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman