Electron traps in the scintillator material PbWO4 and their correlation to thermally stimulated luminescence

Various efforts to improve radiation hardness and scintillation properties of lead tungstate with respect to its application in high energy calorimeters by doping with suitable ions like Nb, La, Gd or Lu have been checked for their success mainly by studying optical absorption and thermally stimulated luminescence (TSL) after irradiation at room temperature, in a few cases at liquid nitrogen temperature. Very shallow traps, which contribute to fast scintillation processes, are not stabilized under such conditions. Extending the temperature range for irradiation as well as for experimental studies down to liquid helium temperature, we found new TSL glow peaks and established a correlation to electrons captured by anionic WO4 complexes. Their electron spin resonance (ESR) data together with other experimental results like temperature dependence of photoconductivity give distinct hints with respect to the energy level scheme of PbWO4.