V. Babin

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.

Polarized luminescence of CsPbBr3 nanocrystals (quantum dots) in CsBr:Pb single crystal

Abstract Polarization of the free exciton luminescence of CsPbBr 3 -type semiconducting nanocrystals of perovskite structure dispersed in CsBr:Pb crystal has been observed for the first time. The polarization spectra, temperature and angular dependences of the degree of polarization as well as the dependence of polarization characteristics on size and shape of CsPbBr 3 nanocrystals have been studied. The conclusions have been made about the orientation of anisotropic CsPbBr 3 nanocrystals with respect to CsBr crystal axes, the origin of optical transitions responsible for the exciton emission as well as about the mechanism of the production of the nanocrystals in the process of thermally stimulated diffusion of Pb 2+ ions in CsBr:Pb crystal lattice.

Coexistence of the impurity and perturbed exciton levels in the relaxed excited state of CsCl:Pb crystal

Four bands, all attributable to the main lead centres, have been detected in the emission spectrum of CsCl:Pb crystals. Their spectral and kinetic characteristics have been studied from 1.8 K up to 300 K, and the parameters of the corresponding relaxed excited states (RES) are calculated. It has been shown that the new model proposed by us recently for the RES structure of the luminescence centre in CsI:Tl is valid for CsCl:Pb as well. Two ultraviolet emission bands (at 4.0-3.9 eV and 3.55 eV) excited mainly in the A absorption band of centres (4.65 eV) are ascribed to electronic transitions from Jahn-Teller minima of two types of the triplet RES of the ion. The higher-energy band is split into two components due to the presence of a cation vacancy near . Two visible bands (at 2.6 eV and 1.97 eV) are connected to two different off-centre configurations of excitons localized near the ion.

Time-resolved spectroscopy of exciton states in single crystals and single crystalline films of YAlO3 and YAlO3 : Ce

Luminescence characteristics of single crystals (SC) and single crystalline films (SCF) of YAlO3 and YAlO3 : Ce are studied at 4.2–300 K under photoexcitation in the 4–20 eV energy range. The origin and structure of the intrinsic and impurity defects responsible for various exciton-related emission and excitation bands are identified. The ≈5.6 eV emission of YAlO3 SCF is ascribed to the self-trapped excitons. In YAlO3 SC, the dominating 5.63 eV and 4.12 eV emissions are ascribed to the excitons localized at the isolated antisite defect and at the defect associated with the nearest-neighbouring oxygen vacancy, respectively. The thermally stimulated release of the electrons, trapped at these defects, takes place around 200 K and 280 K, respectively. The presence of -related defects and isolated oxygen vacancies (AlO5 units) in YAlO3 SC is confirmed by NMR measurements. The formation energies of various -related defects are calculated within the density functional theory. The influence of various intrinsic and impurity defects on the luminescence characteristics of Ce3+ centres is clarified.

Luminescence of CsPbCl3-like Quantum Dots in CsCl : Pb Crystals

Absorption and luminescence characteristics of CsPbCl3-like quantum dots in as-received as well as in quenched and subsequently annealed CsCl : Pb crystals have been studied with a spectral resolution of 0.2 nm at temperatures 12–300 K. A complicated structure of absorption, emission and excitation spectra has been found and its dependence on the annealing temperature and time duration is examined. The temperature dependences of the intensities, positions and halfwidths of the spectral bands have been measured. An anisotropic quantum dot growth has been proposed for the explanation of the structural character of the spectra and their temperature dependences.

Effect of the Pr3+ → Gd3+ energy transfer in multicomponent garnet single crystal scintillators

Luminescence processes in the undoped and Pr3+-doped (Gd,RE)3(Ga,Al)5O12, RE?=?Lu,Y, multicomponent garnets are studied by time-resolved photoluminescence spectroscopy. Energy transfer processes between Pr3+ and Gd3+ causing significant deterioration of the scintillation performance are considered in detail. As is shown in current work, an overlap of the 5d1?3H4 emission transition of Pr3+ and 8S?6Px absorption transition of Gd3+ results in unwanted depletion of Pr3+ 5d1 excited state and is further intensified by the concentration quenching in the Gd3+-sublattice. This process explains a drastic decrease of light yield in Pr3+-doped Gd3+-containing multicomponent garnets observed in a previous work.

On the Interpretation of Luminescence of Lead Halide Crystals

Emission and excitation spectra, and the luminescence decay kinetics have been studied for PbCl 2 crystals at the temperatures 0.45-100 K. It has been found that even at the lowest temperatures, where the non-radiative decay of exciton states in PbCl 2 does not take place, the decay times of both the ultraviolet (UV) and the blue (B) exciton emission are by a few orders of magnitude shorter than the decay times of the emission arising from the triplet relaxed excited state of Pb 2- centres in lead-doped ionic crystals. A conclusion has been drawn that the luminescence of lead halides cannot be connected with single Pb 2+ ions. It is proposed that the radiative decay of the self-trapped excitons of the type of [(Pb 2 ) 3+ + hole} is responsible for the UV and the B emission of PbCl 2 crystals, while the lowest-energy (BG) emission arises from the tunnelling recombinations between the (Pb 2 ) 3+ -type self-trapped electrons and V K -type self-trapped holes.

Luminescent CsPbI3 and Cs4PbI6 Aggregates in Annealed CsI:Pb Crystals

Luminescent aggregates of various types have been produced in quenched CsI:Pb single crystals by their annealing in vacuum at temperatures from 220 to 370°C for 18 to 137 h. It has been found that the relative concentrations of different aggregates depend on thermal treatment conditions. Their optical characteristics obtained at 4.2-300 K have been compared with the corresponding characteristics of CsPbI 3 and Cs 4 PbI 6 single crystals as well as with the characteristics of CsPbI 3 3 and Cs 4 PbI 6 -type aggregates in CsI-PbI 2 thin films. The absorption band located near 411 nm and the emission bands peaking at 465. 535 and 600 nm have all been ascribed to the CsPbI 3 -type aggregates dispersed in a Csl single crystal matrix. The emission band peaking at 435 nm and the corresponding excitation bands at 362 and 288 nm have been ascribed to the Cs 4 PbI 6 -type aggregates. The mechanism of the creation of aggregates in a CsI:Pb crystal has been discussed.

Lu3Al5O12-based materials for high 2D-resolution scintillation detectors

About 20 μm thick Ce-doped Lu 3 Al 5 O 12 thin films grown by Liquid Phase Epitaxy and thin plates of similar thickness prepared by mechanical cutting and polishing from Czochralski grown crystals are used in 2D-imaging experiment down to μm 2D-resolution. Their scintillation response is also measured under α-particle excitation and performance of film and bulk material is mutually compared. Furthermore, scintillation and thermoluminescence characteristics of UV emitting Sc-doped LuAG grown by Czochralski method are presented since this system is a candidate material for UV emission-based 2D sensors with improved diffraction limit with respect to the presently used Ce-doped aluminum garnets.

Luminescence and ESR Study of Irregular Ce

Luminescence of new irregular Ce³⁺-related centers: the unperturbed and perturbed Ce³⁺ ions located in Al³⁺ sites and the Ce³⁺ _Lu ions perturbed by Lu³⁺ _Al ions was found. The systematic study of these centers by means of the ESR and luminescence methods and their influence on scintillation characteristics was performed. The structure and symmetry of the irregular Ce³⁺ centers was established and their spectral characteristics were explained.