Shallow traps in PbWO4 studied by wavelength-resolved thermally stimulated luminescence

The thermally stimulated luminescence ~TSL! properties of lead tungstate in the temperature range 10–250 K have been investigated by wavelength-resolved measurements performed on undoped and lanthanum doped crystals after x irradiation at 10 K. Glow peaks at around 10 and 20 K have been observed, followed by a strong and composite structure in the 40–70 K region. At higher temperatures TSL emission in the 85–100 K range has also been detected, whose intensity is higher in the lanthanum doped crystal with respect to the undoped one. Additionally, the undoped crystal displays a composite glow peak in the 180–200 K region. The emission spectrum of the TSL varies both as a function of temperature and doping: a band centered at 2.85 eV is found in the undoped sample below 70 K; a similar emission is found in the case of the lanthanum doped crystal, with the exception of a limited temperature interval around 60 K where another band at lower energy (E52.63 eV) is found. In the 200 K region, the emission is centered at 2.5 eV. These results are discussed in comparison with literature photoluminescence and scintillation data. A detailed investigation of the composite TSL structure in the 40–70 K range has been performed through several partial heating treatments: three distinct TSL peaks have been found at 50, 55, and 67 K, characterized by trap depths of 48, 65, and 104 meV, respectively. The comparison with recent literature electron paramagnetic resonance studies allows us to propose that the TSL peak at 50 K is related to the thermal disintegration of (WO4)32 electron centers followed by radiative recombination. Spatial correlation between traps and luminescent centers is suggested, leading to localized recombination not involving the conduction band