A. Vedda

Radiation induced formation of color centers in PbWO4 single crystals

The changes in the absorption spectra induced by Co60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O−, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co60 irradiation induced absorption changes is discussed.

Antisite defect-free Lu3(GaxAl1−x)5O12:Pr scintillator

Pr-doped Lu3(GaxAl1−x)5O12:Pr, x=0–1, single crystals were grown by the micro-pulling-down method. We study luminescence and scintillation characteristics of the sample set focusing on their dependence on the gallium content. For x=0.4 we obtain the high figure-of-merit material with elevated density, high efficiency, and very fast scintillation response below 20ns without any slower components. Improvement of scintillation performance is explained as due to the absence of the antisite LuAl defects that was for the first time realized in such bulk garnet single crystals grown from the high temperature melt.

Efficient radioluminescence of the Ce3+-doped Na–Gd phosphate glasses

Photo- and radioluminescence, decay kinetics, and thermoluminescence are reported for a set of Ce3+-doped phosphate glasses. The presence of Gd3+ ions in the glass host matrix at concentrations above 20% enhances the energy transfer towards the Ce3+ emission centers, which results in a remarkable enhancement of the radioluminescence light output.

Decay kinetics and thermoluminescence of PbWO4 : La3+

Correlated measurements of emission spectra, photoluminescence and scintillation decays, thermoluminescence, and light yield were performed on a selected set of undoped and La-doped PbWO4 single crystals. The samples were grown from 5N purity raw powders and show the blue emission component only. Distinct influence of La doping was found in the decays, thermoluminescence and light yield characteristics. It is discussed in the light of the direct influence of La doping on suppressing the creation of point defect centers in the PbWO4 lattice, which are involved in the energy transfer and storage processes in this material.

Scintillation characteristics of Lu3Al5O12:Ce optical ceramics

Optical absorption, luminescence, and scintillation characteristics of Lu3Al5O12:Ce optical ceramics were measured and compared with an analogous high quality single crystal. Optical absorption of the former shows an additional light scattering loss due to nanovoids and/or refraction index inhomogeneities. Even though radioluminescence intensity of the optical ceramics prepared by a coprecipitation route exceeds that of single crystal, its scintillation light yield is lower due to the charge carrier retrapping in the process of energy transfer towards the Ce3+ emission centers. Retrapping processes are evidenced by thermoluminescence measurements below room temperature. Lu3Al5O12:Ce optical ceramics does not show the presence of the LuAl antisite defects which decreases the scintillation figure of merit in single crystals grown from the melt [M. Nikl, Phys. Status Solidi A 202, 201 (2005)]. Furthermore, using the ceramic technology, one can achieve both the homogeneous doping and higher Ce concentration. ...

Enhanced efficiency of PbWO4:Mo,Nb scintillator

The effect of Nb codoping on the optical properties of the PbWO4:Mo scintillator is investigated by radio- and thermoluminescence, scintillation decay, and light yield measurements. Steady-state radioluminescence efficiency of PbWO4:Mo,Nb with optimized doping concentrations (2750 and 350 molar ppm, respectively) becomes up to 20 times higher with respect to that of undoped PbWO4 and is comparable to that of Bi3Ge4O12. However, slow components down to several tens of microseconds appear in the time decay. Their existence may be related to the presence of traps monitored by thermoluminescence.

Scintillator Materials—Achievements, Opportunities, and Puzzles

Participation of shallow and deep traps in the processes of energy transfer and capture is studied by means of time- resolved emission spectroscopy and thermoluminescence in several groups of the Ce3+ and Pr3+-doped complex oxide single crystal scintillators. Tunnelling-driven recombination processes are distinguished in all the groups of examined materials: closely spaced electron and hole traps give rise to the t-1 phosphorescence decays at low temperatures in the Ce-doped aluminum garnets and perovskites, while thermally assisted tunneling process is proposed to explain temperature independent trap depth in glow curve peaks within 50-250 degC in Ce-doped lutetium orthosilicates.

High-efficiency SiO2:Ce3+ glass scintillators

We present the effect of a rapid thermal treatment (RTT) at high temperature (1800 °C) on the radio-luminescence properties of Ce-doped SiO2 glasses prepared by the sol–gel method and previously densified at 1050 °C. Cerium concentrations ranging from 0.05 up to 1 mol % were considered. We found that, for all concentrations, the RTT induces a strong increase of the Ce3+ radio-luminescence efficiency; the x-ray-induced luminescence intensity of the SiO2:0.1% Ce is about twice that of Bi3Ge4O12. The decay time of the scintillation response, evaluated as ≈50 ns, is not affected by RTT. Infrared absorption measurements indicate that the luminescence increase cannot be related to significant release of OH groups during RTT. The conversion of Ce4+ ions into Ce3+ ions can also be ruled out since an increase of about 20% of the intensity of the 4.8 eV optical absorption band related to Ce4+ was observed after RTT. The occurrence of dissolution of rare-earth aggregates is suggested.

Cerium doped heavy metal fluoride glasses, a possible alternative for electromagnetic calorimetry

The article is an overview of the research activity performed in the framework of the Crystal Clear Collaboration to produce scintillating glasses. The manufacturing of heavy metal fluoride glasses doped with Ce3+ is discussed. The luminescence and scintillation characteristics as well as the radiation hardness properties are extensively studied in the case of Ce doped fluorohafnate , found to be the most convenient glass scintillator for high energy physics applications.

Complete characterization of doubly doped PbWO4:Mo,Y scintillators

We present correlated measurements of radioluminescence, photoluminescence, and thermoluminescence, as well as the measurements of decay kinetics both under ultraviolet and 22Na(511 keV) photon excitations for Mo-doped and doubly (Mo,Y)-doped PbWO4 single crystals. A wide concentration range of Mo doping (200–50 000 ppm) is covered. Y codoping results in significant suppression of trapping states induced by the Mo doping. Consequently, an improvement of temporal characteristics, such as acceleration of the scintillation decay process, of doubly doped material with respect to Y-free crystals, is achieved. For the optimum Mo concentration (2750–10 000 ppm), the temporal characteristics are fully comparable to an undoped PbWO4, while the light yield is increased by a factor of 2.2.