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Charge trapping phenomena and recombination centers were studied in three Cs2HfCl6single crystals of slightly different stoichiometry grown by the vertical Bridgeman method. Electron paramagnetic resonance (EPR) spectra measured both before and after X-ray irradiation show creation of two distinct Vkcenters. One of them was Vk(a) already known from a recent work by “R. Král, V. Babin, E. Mihoková, M. Buryi, V. V. Laguta, K. Nitsch, and M. Nikl, Luminescence and Charge Trapping in Cs2HfCl6Single Crystals: Optical and Magnetic Resonance Spectroscopy Study, J. Phys. Chem. C 121,12375–12382 (2017)”. Its quantity was different in each of the samples studied; the smallest, however, was in the one with the best stoichiometry. The second Vkcenter presently observed has never been described before. Its existence at an almost undetectable level was observed only in two of the three crystals. Thermally stimulated luminescence (TSL) spectra measured in the three samples were evidently composed of at least 2–3 strongly overlapped components within the 10–500 K temperature range. This suggested the existence of several recombination centers activated by the depletion of specific charge carrier traps. The corresponding TSL glow curves composed of seven complex peaks demonstrated significant decrease of the peaks amplitude in the sample with the best stoichiometry. Along with decreased radioluminescence amplitude, the combined EPR and TSL study allowed us to assume the reduction of both the recombination and trap center concentration with the increased crystal quality.

Single crystals of the undoped Cs2HfCl6were grown using the vertical Bridgman method. Photoluminescence spectra and decays were measured in the extended temperature range of 8–500 K and approximated by a phenomenological model. Self-trapped exciton and defect-related emissions were clearly distinguished with the onset of thermal quenching above the room temperature. Electron paramagnetic resonance experiment revealed the formation of two configurations of Vkcenter (Cl2–molecular ion) at low temperatures. One of the Vkcenters is created by a hole self-trapping at two neighboring Cl ions. In the temperature range 70–80 K, this state is thermally disintegrated and reappears in another configuration formed most probably by an interstitial Cl ion (an analog of the H-type center in alkali halides).

Indirect detection is a versatile way to detect hard x-rays. It is based on an x-ray-to-light converter, optical coupling, and a visible light detector. The converter screen, known as a scintillator, is deployed in both imaging and point detection, using either signal integration or counting. Two applications are explored in this review—sample examination and x-ray beam diagnostics for synchrotron sources. A large variety of scintillators are available to fulfill the needs of synchrotron applications. High dynamic range, small pixel size, and radiation hardness are the major advantages of scintillators. This article provides a review of the technical and scientific aspects of scintillators used in synchrotron radiation (i.e., storage rings and x-ray free-electron lasers). The advantages and drawbacks of implementation of the most popular scintillators on synchrotron beamlines are described.

Abstract

A set of Eu-doped Y3Al5O12(YAG) phosphors in the powder form was successfully synthesized by multiple-step solid-state reaction under the reducing Ar:5%H2atmosphere. Their physical properties were investigated by means of X-ray diffraction, time-resolved luminescence spectroscopy, and electron paramagnetic resonance (EPR). Special attention was given to well-grounded confirmation of Eu2+occurrence in the YAG structure. X-ray diffraction confirmed the presence of a major YAG phase in all samples. The influence of Eu concentration, form of doping (EuS, EuF2, or Eu2O3) and number/form of annealing steps was studied. Corresponding characteristics were measured and evaluated in a broad temperature range (8–800 K). The correlated measurements showed that the single Eu2+ions stabilized in YAG in the yttrium position are responsible for the 440 nm emission. The corresponding lowest energy excitation band is situated at 300 nm, and the radiative lifetime of Eu2+emission is about 500 ns. A phenomenological model of excited state dynamics was used to explain the experimentally observed features. Possible practical exploitation of the Eu2+center in the YAG host is discussed.

Electron paramagnetic resonance (EPR) signals of Bi2+ions have been detected in the EPR spectrum of manganese-, bismuth-, and tin-doped PbWO4single-crystals irradiated by xenon and mercury lamps at 100 K. The parameters of the Zeeman, hyperfine, and superhyperfine interactions and the localization of Bi2+ions have been determined.

In manganese-doped PbWO4crystals, low-intensity signals of triclinic clusters Mn4+-VOand Fe3+-VPbhave been revealed in addition to signals of Mn2+tetragonal centers. The Mn4+-VOcluster is formed by a Mn4+ion in the W6+position, which is associated with a vacancy of the nearest neighbor O2-ion, and the Fe3+-VPbcluster consists of a Fe3+ion substituting for Pb2+with a local compensation of by a lead vacancy. It has been shown that, in PbWO4: Mn, there is also a small amount of Mn4+tetragonal centers located in the Pb2+position with a nonlocal compensation of an excess charge.

Two topics were focused. The first one was about the gamma-ray scintillator, Pr3+:Lu3Al5O12(LuAG). The second one was about neutron scintillator, Ce3+:6LiCaAlF6and Eu2+:6LiCaAlF6(6LiCAF). Those scintillators have been developed very recently for modern imaging applications in the medical and homeland security fields. In both cases, the rare earth ions are playing the crucial role as emission centers. Pr3+in LuAG provided fast 5d→4f transition providing noticeably shorter decay time than that of Ce3+. Among several candidate hosts, LuAG showed the best performance. Bulk crystal growth, basic scintillation properties, two-dimensional gamma-ray imaging and positron emission mammography (PEM) application were demonstrated. Due to the international situation, the homeland security was compromized by illicit traffic of explosives, drugs, nuclear materials, etc. and the ways to its improvement became an important R&D topic. For this purpose the Ce and Eu doped LiCAF appeared competitive candidates. Especially, when substitution of 3He neutron detectors was considered, the discrimination ability of gamma-ray from alpha-ray was important. Bulk crystal growth, basic scintillation properties and two-dimensional neutron imaging were demonstrated.

Single crystalline films SCFs of undoped and Ce3doped YAlO3YAP perovskite were crystallized by liquidphase epitaxy method onto YAP substrates for the first time from the novel leadfree BaOB2O3BaF2fluxes. The absorption, luminescence, and scintillation properties of these films are measured and compared with the films grown from the standard PbOB2O3flux. It appears that the films obtained from the BaObased flux exhibit better luminescent and scintillation properties with respect to films grown from the PbObased one.

Electron and hole trapping phenomena below room temperature were studied by simultaneous use of electron paramagnetic resonance (EPR) and thermostimulated luminescence (TSL) experiments in Ce-doped YAlO3, Y3Al5O12 and Lu3Al5O12 single crystals. Charge-carrier trapping associated with O– centers and antisite Y(Lu)Al defects in perovskite and garnet structures, respectively, was evidenced by EPR and associated with the dominant TSL peaks. Existence of F+ centers was proved by EPR in YAlO3. A Ce3+ ion at the Al site and spatially correlated CeLu and LuAl centers were evidenced in Lu3Al5O12 by the same method. Spatial correlation of the latter two centers is further supported by the associated tunneling-driven radiative recombination process evidenced by phosphorescence decay. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The radioluminescence (RL) properties of Ce-doped silica glasses prepared by sol-gel method were investigated as a function of the dopant amount in a wide range of concentrations (Ce:Si molar ratios from 1 × 10-5to 5 × 10-2). The effects of xerogel densification temperature and of a postdensification thermal treatment were also considered. In order to understand the microstructural features governing Ce3luminescence efficiency, optical absorption measurements in the ultraviolet, visible, and infrared regions and time-resolved photoluminescence experiments were performed. The complex dependence of RL intensity upon rare-earth (RE) concentration and thermal treatment was attributed to the role of OH vibrations as nonradiative recombination channels, as well as to the formation of RE aggregates in the silica matrix. Specifically, the segregation of CeO2nanoparticles, in which Ce4does not supply any radiative emission, has been revealed by infrared absorption measurements, in agreement with previous Raman, transmission electron microscopy, and X-ray diffraction data. Moreover, an optical absorption band, centered at 2.4 eV and whose intensity increases with the square of cerium concentration, was observed and tentatively assigned to an intervalence electron-transfer transition involving Ce3-Ce4dimers. Postdensification thermal treatments markedly reduce the intensity of this band and increase RL intensity. The relationship between the 2.4 eV band and the RL properties will be outlined. In particular, the increase in RL intensity will be discussed, mainly as a consequence of microscopic modifications leading to an improvement of the charge-transfer efficiency toward emitting centers.

Luminescence of the excitons and the role of anti-site defects, as well as the peculiarities of energy transfer to the Ce3+ activator ions were investigated in Lu3Al5O12:Ce compound. The measurements of time–resolved emission spectra and decay kinetics at 9 K and 300 K under the excitation by synchrotron radiation were used. The characteristics of single crystals and single crystalline films of Lu3Al5O12:Ce were compared and differences explained in the light of substantially different concentrations of the anti-site defects in the same material prepared by different technologies. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Shaped single crystals of Bi:Gd3Ga5O12 (Bi = 0.102, 0.126 and 0.141 mol.%) and Bi:Y3Ga5O12 (Bi = 0.041, 0.047 and 0.061 mol.%) were grown by the micro-pulling-down method. Measured optical absorption spectra show an absorption band round 290 nm ascribed to the lowest energy 6s2→6s6p transition of Bi3+. Difference in the position of the emission spectra of Bi-related bands in Gd3Ga5O12 (470 nm) and Y3Ga5O12 (314 nm) was explained by the lower local distortion of Bi polyhedron in Y3Ga5O12 host leading to smaller Stokes shift of only 0.36 eV. Unsuitability of Bi-doping in the Gd3Ga5O12 host to get energy transfer from Gd3+ towards Bi3+ centers was concluded, while Bi-doped Y3Ga5O12 could compete against Bi4Ge3O12 scintillator. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence and scintillation dynamics can be extensively influenced by the energy transfer processes, which is demonstrated at three selected scintillator materials: PbWO4, Ce-doped aluminium garnets and PrF3:Ce single crystals. Charge carrier retrapping processes can delay radiative recombination at the luminescence centers in an extended time scale, which makes a substantial part of generated scintillation light technically unexploitable. Correlated measurements based on time-resolved emission spectroscopy and thermoluminescence are of great help to evaluate these phenomena and to understand their microscopic origin. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Room temperature photoconductivity measurements were performed for PbWO4 crystals grown under different conditions. In the near-band-edge region three bands of photoionization absorption (310, 330 and 338 nm) were detected. The intensity of these bands depends on crystal perfection and quality. The value of a photocurrent under excitation within the 330 nm band depends on preceding irradiation of a PbWO4 crystal by a laser beam (λex = 337 nm). The nature of defect centers related to the photoionization absorption bands is discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence spectra and scintillation decays under steady-state and pulsed X-ray excitations, respectively, are presented for undoped and Ce-doped Lu3Al5O12 single crystals at room temperature. The energy transfer from the trapped exciton state towards the Ce³⁺ centers appears rather inefficient. A negative influence of the shallow trapping states is evidenced, which results in delayed radiative recombination at the Ce³⁺ centers and slows down the scintillation response of the material. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Absorption, emission and excitation spectra and luminescence decay kinetics of Na(K)GdCe, NaGdTb and NaGdMn phosphate glasses and their dependence on the temperature (in the 1.7–300 K range) and on the glass composition have been studied. The processes of energy migration through the Gd³⁺ ions and following energy transfer in Gd³⁺-impurity pairs have been compared for the three types of the glasses studied. It has been suggested that the Gd³⁺ → Tb³⁺ energy transfer occurs only in the closest pairs through a very short-range exchange interaction, while the Gd³⁺ → Ce³⁺ and Gd³⁺ → Mn²⁺ energy transfer is possible also in more separated pairs mainly due to the longer-range multipolar Gd³⁺-impurity interaction.

The creation of defects in CsI:Pb crystals by ultraviolet radiation at 4.2 K has been investigated by thermally stimulated and optically stimulated luminescence methods. The origin of the optically created defects has been established. The dependences of the defect creation efficiency on the irradiation energy and time, on the uniaxial stress, and on the concentration of the impurity (Pb²⁺ and Na⁺) ions have been found. The charge-transfer processes taking place under irradiation of CsI:Pb crystals in the Pb²⁺-related absorption bands and resulting in the appearance of the self-trapped and localized exciton luminescence and in defect creation have been explained.

Emission and excitation spectra, and the luminescence decay kinetics have been studied for PbCl2 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 PbCl2 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 Pb²⁺ centres in lead-doped ionic crystals. A conclusion has been drawn that the luminescence of lead halides cannot be connected with single Pb²⁺ ions. It is proposed that the radiative decay of the self-trapped excitons of the type of {(Pb2)³⁺ + hole} is responsible for the UV and the B emission of PbCl2 crystals, while the lowest-energy (BG) emission arises from the tunnelling recombinations between the (Pb2)³⁺-type self-trapped electrons and VK-type self-trapped holes.

AbstractThe creation of Pb2+-based ternary nanophases was evidenced in co-evaporated CsX-PbX2(X = Cl, Br, I) thin films using X-ray diffraction spectroscopy (XRD). The optical properties of CsPbX3and Cs4PbX6nanostructures were determined. CsPbX3(X = Cl,Br) are wide gap semiconductors with direct band transitions and a pronounced Wannier free exciton emission showing subnanosecond decay times and very small Stokes-shift. On the contrary, it is supposed that, in CsPbI3, the auto-localization of the exciton state results in a broad and much slower emission with large Stokes shift. Cs4PbX6is characterized by the auto-localized exciton emission in the near UV/violet spectral region and two component decay kinetics closely similar to that of Pb2+emission centers in KX:Pb2+(X = Cl, Br, I).

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 CsPbI3 and Cs4PbI6 single crystals as well as with the characteristics of CsPbI3 and Cs4PbI6-type aggregates in CsI–PbI2 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 CsPbI3-type aggregates dispersed in a CsI 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 Cs4PbI6-type aggregates. The mechanism of the creation of aggregates in a CsI:Pb crystal has been discussed.

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.

Absorption, emission and excitation spectra, polarization and decay kinetics of luminescence and their temperature dependences have been studied at 4.2–300 K for several CsBr:Pb samples with various lead concentrations. The origin and structure of the relaxed excited states responsible for the 3.33, 2.8 and 2.43 eV emission bands have been discussed. It has been concluded that the relaxed excited state model proposed for Tl+-doped caesium halides and CsCl:Pb is not valid in case of CsBr:Pb. To establish the origin of the relaxed excited states in a CsBr:Pb crystal, the luminescence characteristics of CsBr:Pb have been compared with those reported for Cs4PbBr6, PbBr2 and CsPbBr3 bulk crystals. It has been assumed that owing to a strong interaction between Pb2+ and Br— ions, the molecular-like states, which are similar to the excited levels of some stable lead-related compounds, are responsible for the luminescence of CsBr:Pb crystals.

The influence of Gd3+ doping on the optical properties of PbWO4 has been investigated by radio- and thermoluminescence, photoluminescence time decay, optical absorption and light yield measurements. The samples were grown by the Czochralski method and the concentration of Gd in the melt was in the range 0 to 135 mol. ppm. The results demonstrate that a dopant concentration of around 100 ppm in the crystal appears as the best one in order to optimise the scintillation performance of the material.

Lead tungstate PbWO4 single crystals were grown from sodium metasilicate gel and water solutions of lead acetate and sodium tungstate as the reagents. Crystals grown were several micrometer in size and by X-ray diffraction measurements it was determined that they are only of the scheelite structure. No evidence of lead tungstate low temperature modification – raspite – was found. The emission properties of crystals were measured over the temperature interval 80 to 300 K and it was found that they are similar to those in PbWO4 crystals grown by the Czochralski method.

The transient absorption and luminescence under pulsed electron beam excitation were studied for undoped and La3+doped PbWO4crystals. The absorption band at ∼1.0 eV is suggested to be due to self-trapped electrons and the absorption band at 3.5 eV may be due to self-trapped holes. The formation of luminescence centers via electron–hole recombination is affected by La3+in doped crystals. The large fraction of electrons and holes undergoes thermostimulated recombination within geminate pairs and the spatial separation of geminate pairs components is important in recombination process. The intrinsic (blue) luminescence arises in this recombination. The mechanism of La3+influence on recombination is discussed.

Luminescent aggregates of the type of Cs4PbBr6 dispersed in a CsBr crystal lattice have been observed for the first time in as-grown and in annealed (at 160 to 260 °C for 20 h) CsBr:Pb single crystals with a sufficiently large lead concentration (0.3 to 1.8 mol% in the melt). Absorption spectra as well as spectral, polarization and kinetic characteristics of their luminescence peaking near 3.3 eV and excited near 4.0 eV and 5.2 to 5.1 eV have been studied at 4.2 to 200 K. The characteristics of the Cs4PbBr6 aggregates have been compared with those of a Cs4PbBr6 single crystal and with the characteristics of Pb2+ centres in a quenched CsBr:Pb crystal. The origin and the structure of the corresponding relaxed excited states have been discussed.

The induced absorption spectra were measured for equally grown CaWO4 and CaWO4 : La single crystals after 60Co radioisotope irradiation by doses within 1 to 230 Gy. La doping increases considerably the radiation hardness of CaWO4 material similarly as was observed for isostructural PbWO4 single crystals recently. Transmission of as grown undoped CaWO4 shows a noticeable decrease below 450 nm, which gives the evidence of deep trapping states. These trapping centres are effectively diminished by La doping as well. Induced absorption bands are discussed in the light of known and hypothesized color centers in CaWO4 and PbWO4 single crystals, which were reported earlier in the literature.

Luminescence and scintillation characteristics of four selected material systems, namely CsI : Tl(Na), CeF3, PbWO4 and Ce-doped aluminium perovskites XAlO3 : Ce (X=Y, Lu, Y–Lu) are reviewed. The progress in their physical understanding and related optimisation of their characteristics and technology are demonstrated. The important role of various defect states in the scintillator performance of these materials is stressed, which has led to the need for a deeper study of the processes of energy transfer and storage to achieve their intrinsic limits and full exploitation in scintillation detectors.

Abstract: Silicon nanocrystals were prepared by Si+-ion implantation and subsequent annealing of SiO2 films thermally grown on a c-Si wafer. Different implantation energies (20–150 keV) and doses (7 × 1015 − 2 × 1017 cm−2) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of µs at room temperature. The changes of PL in the range 70–300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si+-implanted SiO2 films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials.