J.A. Mares

Ce3' or Tb3'-doped phosphate and silicate scintillating glasses

Abstract Optical characterization of scintillating phosphate and silicates glasses is presented including absorption, emission spectra and decay kinetics. A new concept of energy transfer sensitization is developed, based on nearly resonant energy migration through a rare earth ion subsystem (Gd 3+ ions at sufficiently high concentration) in the glass matrix, followed by a single-step transfer towards the emission centers created by Ce 3+ or Tb 3+ doping. While Ce 3+ doping in phosphates results in emission spectra in the near UV-violet, the Tb 3+ doping in silicates gives a four-band spectrum with a dominating one at 545xa0nm. Intense UV irradiation produces a reduction of emission intensity in phosphate glasses.

Ce-doped YAG and LuAG Epitaxial Films for Scintillation Detectors

Epitaxial garnet films of undoped and Ce³⁺ -doped yttrium aluminum garnet (YAG) and lutetium aluminum garnet (LuAG) were grown by the isothermal liquid phase epitaxy from two different fluxes: we used a standard PbO-B₂O₃ flux and, as a novelty, BaO-B₂O₃-BaF₂ flux. It is shown that the films obtained from the lead-free BaO flux exhibited superior luminescent properties comparable with those of Czochralski grown single crystals. A detailed comparison of optical, luminescent, and kinetic properties of films grown from these two types of fluxes is made.

A role of Gd3+ in scintillating processes in Tb-doped Na–Gd phosphate glasses

Abstract Sensitization role of Gd 3+ ions in energy transfer processes in radio- and photo-luminescence is described for Tb 3+ -doped and Gd-enriched Na x Gd y phosphate glasses. Limited (Gd–Gd) n energy migration is obtained and the Gd 3+ →Tb 3+ transfer is explained as due to mainly the (super) exchange interaction. These results are discussed and compared with the situation observed in similar glasses or crystals where transfer processes are mainly due to multi-polar interaction (Gd 3+ →Ce 3+ or Ce 3+ →Gd 3+ ) and more efficient migration among Gd 3+ ions that is observed.

Scintillation Properties of

We summarize the latest R&D state-of-art of Ce³⁺ and Pr³⁺-doped mixed (Lu_xY_1-x)₃Al₅O₁₂ (LuYAG) garnet scintillator crystals and compare their properties with those of Lu₃Al₅O₁₂ (LuAG) and Y₃Al₅O₁₂ (YAG) single crystal analogues. Light yield, energy resolution and proportionality were studied under radioisotope excitation within 8-1300 keV energy range. The highest light yield of about 27000 ph/MeV is obtained in the newest prepared Ce³⁺ -doped LuAG crystals while up to 17000 ph/MeV was measured in the Pr³⁺-doped ones. On the contrary, the energy resolution and proportionality are noticeably better in the Pr³⁺ -doped crystals. Mixed Ce³⁺- or Pr³⁺-doped Lu_xY_1-xAG (x=0.9-1) crystals show comparable light yield and energy resolution compared to those doped pure LuAG, but the content of slow components in scintillation pulses lower.

{\rm Ce}^{3+}

Abstract Ce 3+ and Tb 3+ -doped Na(K)-Gd phosphate glasses were synthesized and their radio-, photo- and thermoluminescence properties were investigated. Increased radioluminescence efficiency with respect to Gd-free glass matrix has been achieved at Gd concentrations above 20xa0mol%. This is explained by efficient capture of thermalised electrons and holes in the Gd 3+ -sublattice resulting in the formation of Gd 3+ excited states followed by Gd 3+ -Gd 3+ energy transfer and single-step transfer to the Ce 3+ (Tb 3+ ) emission centres. In such a way non-radiative losses in the phosphate glass matrix are reduced. Energy migration in Gd-sublattice becomes frozen below 10–15xa0K due to local field fluctuations in the glass matrix. Thermoluminescence measurements above room temperature show the presence of two groups of traps with concentrations strongly dependent on the glass composition.

- and

Abstract Spectroscopic properties and energy transfer processes in Ce 3+ -doped RE 3+ AlO 3 orthoaluminates (for RE 3+ = Lu 3+ , Gd 3+ or Y 3+ ) or for the mixed Lu x Y 1− x and Lu x Gd 1− x are presented and summarised. Generally, spectroscopic properties of these crystals are very similar but in their Ce 3+ fluorescence and scintillation decay slow decay components appear. Temperature dependence of Ce 3+ fluorescence decays in Lu x Gd 1− x AlO 3 :Ce crystals show Ce 3+ → Gd 3+ nonradiative energy transfer followed by the opposite transfer Gd 3+ → Ce 3+ (including migration through Gd 3+ sublattice) which is responsible for slow Ce 3+ fluorescence decay component. Energy barrier between Ce 3+ and Gd 3+ interacting levels is ~ 70 meV.

{\rm Pr}^{3+}

This work reviews our R&D of new types of scintillating screens based on the single crystalline films (SCF) of Lu,Y orthosilicate, perovskite and garnet compounds grown by the liquid phase epitaxy method. The scintillation and luminescent properties of the following types of SCF scintillators are considered in this work: (i) Ce-doped SCF of Y,Lu-based orthosilicates and perovskites with the Ce3+ emission in the 325-600 nm and 360-370 nm ranges with a decay time of 20-33 ns and 16-17 ns, respectively; (ii) Ce,Tb doped SCF of Y,Lu-based orthosilicates and perovskites with the simultaneous Ce3+ and Tb3+ emission in the blue or UV and green spectral ranges, respectively; (iii) Sc3+ doped SCF of Y-Lu-Al-garnets, emitting in the 290-400 nm range due to formation of the ScY,Lu and ScAl centers with a decay time of the order of hundreds of nanoseconds.

-Doped LuAG, YAG and Mixed

Abstract A newly developed hybrid photo-multiplier (HPMT) was used to measure scintillation photoelectron yield N phels ( E ) of well-known YAP:Ce and YAG:Ce scintillating crystals. N phels ( E ) measured by the HPMT or evaluated LY( E ) were compared with results obtained by the classical PMT. Generally, both crystals are characterized by roughly the same photoelectron and light yields which can reach up to 6400 phels/MeV or 24xa0000 ph/MeV, respectively. These yields were measured with cylindrical plates of shape ∅ 10xa0×xa01 or 2 mm 3 under 662 keV γ-ray radiation. The energy resolutions of both crystals at 662 keV are good and the best values reach ≅4.6% or ≅5.1% for YAP:Ce or YAG:Ce, respectively.

{\rm Lu}_{\rm x}{\rm Y}_{1-{\rm x}}{\rm AG}

We investigate the luminescence and scintillation characteristics of highly efficient cerium-doped scintillators, lutetium-(yttrium) orthosilicate (Lu2(1-x)Y2xSiO5:Ce, x=0-1). The radioluminescence, photoluminescence excitation (PLE) and emission (PL) spectra of the Ce1, Ce2 centres as well as the decay curves and their temperature dependences are measured in the 80-500 K range. Light yield values are provided as well. The influence of yttrium concentration on the Ce1, Ce2 luminescence characteristics is demonstrated. With increasing yttrium content the onset of the Ce1 and Ce2 decay time decrease as well as the Ce1, Ce2 delayed recombination integrals increase shift to higher temperatures. The 5d1 thermally induced excited-state ionization of both Ce1 and Ce2 centres is confirmed and studied by purely optical methods.

Garnet Crystals

A numerous set of Pr-doped YAP single crystals was prepared using Czochralski and micro-pulling down methods. Crystals codoped by bi-, tri- and tetravalent ions were prepared by the latter method as well. The scintillation performance of all the prepared materials was evaluated using radioluminescence, photoelectron yield and scintillation decay measurements at room temperature. The influence of shallow traps was studied by means of thermoluminescence in the 10-350 K temperature range. Though the intrinsic scintillation efficiency of Pr-doped YAP is higher than that of YAP:Ce, enhanced delayed radiative recombination processes are found to be responsible for lower-than-expected photoelectron yield of the former system. The possibilities of further optimization of Pr-doped YAP scintillator are discussed.