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Plastic scintillators are gaining attention as alternatives to inorganic scintillator crystals owing to their low fabrication cost, customable shape/size, and substantially lighter weight that make them suitable for various radiation detection technologies. These include scintillation panels for national security and industrial monitoring, radiation screens for medical diagnostics, and calorimeters for high energy physics. Because of their low density, plastic scintillators are typically doped with high atomic number (Z) sensitizers that enhance the interaction probability with ionizing radiation and excite molecular emitters. Although effective, such a two-component design suffers from incomplete sensitization, intrinsically limited efficiency due to multiple radiative steps with non-unity quantum yield, detrimental phase segregation effects and the fragility of organic emitters to ionizing radiation. In this work, an alternative single-component scheme is proposed based on high-Z reabsorption-free CdZnS/ZnS semiconductor nanocrystals (NCs) doped with manganese embedded in a polyvinyltoluene (PVT) waveguide. Optical-grade nanocomposites free from optical reabsorption of the scintillation light and with performance comparable to commercial products are obtained through a post-synthesis resurfacing procedure that maximizes the compatibility between the NCs and PVT and preserves their optical properties upon curing.

Charge trapping phenomena and recombination centers were studied in three Cs2HfCl6 single 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 Vk centers. 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 Cs2HfCl6 Single 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 Vk center 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

Scintillators, materials that produce light pulses upon interaction with ionizing radiation, are widely employed in radiation detectors. In advanced medical-imaging technologies, fast scintillators enabling a time resolution of tens of picoseconds are required to achieve high-resolution imaging at the millimetre length scale. Here we demonstrate that composite materials based on fluorescent metal–organic framework (MOF) nanocrystals can work as fast scintillators. We present a prototype scintillator fabricated by embedding MOF nanocrystals in a polymer. The MOF comprises zirconium oxo-hydroxy clusters, high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from 9,10-diphenylanthracene ligand emitters. Their incorporation in the framework enables fast sensitization of the ligand fluorescence, thus avoiding issues typically arising from the intimate mixing of complementary elements. This proof-of-concept prototype device shows an ultrafast scintillation rise time of ~50 ps, thus supporting the development of new scintillators based on engineered fluorescent MOF nanocrystals. Composites of fluorescent metal–organic framework nanocrystals in a polymer are exploited to create fast scintillators with a rise time of about 50 ps.

Lead halide perovskites (LHP) are rapidly emerging as efficient, low-cost, solution-processable scintillators for radiation detection. Carrier trapping is arguably the most critical limitation to the scintillation performance. Nonetheless, no clear picture of the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process has been reported to date, as well as on the role of the material dimensionality. Here, this issue is addressed by performing, for the first time, a comprehensive study using radioluminescence and photoluminescence measurements side-by-side to thermally-stimulated luminescence (TSL) and afterglow experiments on CsPbBr3 with increasing dimensionality, namely nanocubes, nanowires, nanosheets, and bulk crystals. All systems are found to be affected by shallow defects resulting in delayed intragap emission following detrapping via a-thermal tunneling. TSL further reveals the existence of additional temperature-activated detrapping pathways from deeper trap states, whose effect grows with the material dimensionality, becoming the dominant process in bulk crystals. These results highlight that, compared to massive solids where the suppression of both deep and shallow defects is critical, low dimensional nanostructures are more promising active materials for LHP scintillators, provided that their integration in functional devices meets efficient surface engineering.

7Li enriched Cs2LiYCl6:Ce3+ (CLYC) is a promising inorganic scintillator for real-time γ-ray and fast neutron spectrometry. The neutron/γ-ray discrimination is usually accomplished exploiting the different quenching effects of high Linear Energy Transfer (LET) particles on different scintillation mechanisms, usually by means of the time analysis of the pulse shape. In principle, the emission wavelength information could be used to address the same task. However, a systematic study of the correlations between the CLYC decay time, its radio-luminescence spectrum and the LET of the impinging particle has not yet been performed. We therefore investigated the CLYC scintillation process under neutron and γ-ray irradiation, correlating the time response to the scintillation wavelength spectrum using a 1–inch right cylinder>99% 7Li enriched CLYC. We found that the relative intensity of the Core to Valence Luminescence (CVL) is almost constant with photons in the energy range 20–660 keV, i.e. 0.5–5 keV/μm LET, and is only partially quenched by neutrons. Instead, the direct electron-hole capture scintillation mechanism is completely cut under neutron irradiation. The luminescence in between the deep-Ultraviolet and the Near Ultraviolet region (250–350 nm) might be attributed to both the CVL and the host luminescence, also in thick highly Ce3+-doped crystals.

In this study, we propose an original approach for particle discrimination in a mixed radiation field using scintillators. The method relies on the possibility to identify the point of interaction in a phoswich detector from the emission spectra of the produced scintillation light, using optical filters as passive selectors. As a proof of concept, we developed a compact prototype thermal neutron active dosimeter for mixed neutron/gamma fields. The detector was characterized in terms of energy calibration, neutron efficiency and gamma rejection. The obtained results show that more than the 99% of counts due to gamma rays are rejected if the color discrimination is employed, thus demonstrating the feasibility of the proposed approach to be used in radiation detection.

Scintillators are materials able to efficiently convert the energy deposited by high energy photons or ionizing particles into a great number of photons in the visible or near ultraviolet range, that can be detected by conventional photodetectors. They are extensively used for various applications, like for example medical diagnostics, high energy physics experiments, security, or industrial controls. The investigations on scintillators started soon after the discovery of X-rays, and faced a notable improvement especially in the last decades with the advent of several novel materials [1]. Among inorganic materials, fluorides, chlorides, bromides, iodides, and oxides single crystals are being developed. In several cases rare earth ions (RE) are present, either as lattice constituents and, especially, as luminescent activators. RE-doped scintillating glasses and, more recently, optical ceramics are also investigated. Indeed, RE ions present electronic configurations that permit a rich variety of radiative transitions and make them useful for different optical applications, including scintillation. For the purpose of scintillation, the allowed 5d-4f radiative transition is particularly useful, due to its high quantum efficiency and fast decay in the 10-7-10-8 s time scale. For this reason, Ce3+ , Pr3+ or Eu2+, featuring 5d-4f transitions in the visible or near ultraviolet in most host materials, are frequently considered. Scintillation is a complex process involving i) the absorption of the primary radiation beam and the production of a great number of free carriers; ii) the diffusion of free carriers and their transport to the luminescent centers, like RE ions; and finally, iii) the radiative recombination of electron-hole pairs giving rise to photon emission (Fig. 1). Good ionizing radiation absorption, radiation resistance, efficient and fast response are crucial properties to be considered for the engineering of a novel scintillator material. This lecture will first describe the scintillation phenomenon and the parameters that characterize scintillator material performances; the role of RE ions is then introduced, providing several examples in which RE are present, like lutetium/yttrium silicate single crystals [2], silicate glasses [3], complex garnet optical ceramics [4]; the particular case of PbWO4, the material used in the Large Hadron Collider at CERN for the discovery of Higgs Boson and in which RE ions were used as optically inactive dopants, is also presented [5]. A look to future scintillator needs is finally presented, especially concerning positron emission tomography medical diagnostics apparatuses in which a time response faster than 1 ns will be required; possible solutions involving RE doped materials, or nanocomposites made by luminescent nanoparticles embedded in a polymeric material [6] are outlined. [1] C. Dujardin et al., “Needs, trends and advances in inorganic scintillators”, IEEE TNS 65, 1977 (2018). [2] S. Blahuta et al., “Evidences and consequences of Ce in LYSO:Ce,Ca and LYSO:Ce,Mg Single Crystals for Medical Imaging Applications”, IEEE TNS 60, 3134 (2013). [3] F. Cova et al., “Dual Cherenkov and scintillation response to high-energy electrons of rare-earth doped silica fibers” Phys. Rev. Appl., 11, 024036 (2019). [4] S. Liu et al., “Towards bright and fast Lu3Al5O12:Ce,Mg optical ceramics scintillators”, Adv. Opt. Mater 4, 731 (2016). [5] S. Baccaro et al., "Influence of La3+ - doping on radiation hardness characteristics of PbWO4", Phys. Stat. Sol. 160, R5 (1997). [6] M. Gandini et al., “ Efficient, Fast and Reabsorption-free Perovskite-based Sensitized Plastic Scintillators", Nature Nanotechnology 15, 462 (2020).

This review paper provides a description of direct recombination processes between carriers from traps to luminescent centers, involving the occurrence of a tunneling mechanism between their space-correlated localized electronic levels. The experimental evidences that allow to recognize tunneling recombination in phosphorescence time decay and thermoluminescence experiments are first outlined, and compared with those characteristic of recombination processes involving the transfer of carriers in the delocalized bands prior to their recombination. Simple explanations are also proposed for the phenomenological observations in both athermal and thermally assisted tunneling processes. The importance of such recombinations in three material classes – scintillators, persistent phosphors, and dosimeters, is then outlined and discussed in relation to the material requirements in their distinct application fields. For each application, numerous literature examples are reported. Finally, the paper is complemented by a brief illustration of literature investigations describing experimental evidences of direct trap-center recombination different from tunneling, mostly based on the inspection of emission spectra and thermoluminescence glow curve shapes.

A new inorganic scintillation material based on Ba-Gd silica glass doped with cerium (BGS) is fabricated and studied. With the highest light yield among heavy glasses at the level of 2500 ph/MeV and fast scintillation response, the new scintillator ensures a good coincidence time resolution of γ -quanta from a 22Na source and SiPM readout. In addition to good performance in γ -quanta detection, the material demonstrates capability for efficient detection of low-energetic​ neutrons. The scintillator is produced by exploiting the standard industrial glass technology, which allows for an unlimited scaling up the conversion of raw material into a high-quality scintillator at a high rate. The glass can be casted in application-specific molds, so minimizing the material losses. The presented glass scintillator has potential for further improvement of its light output and scintillation response time.

Six Mg2+ co-doped Lu3Al5O12:Ce scintillating garnet films prepared by liquid phase epitaxy, with Mg concentration from 0 to 3000 ppm, were irradiated by X-rays and underwent a series of thermoluminescence measurements from 10 to 720 K. The thermoluminescence signal of epitaxial films originated purely from electron recombination at Ce ions. It was found that the area of all the peaks present in thermoluminescence glow curves decreased rapidly with the increase of Mg concentration; for 3000 ppm, the decrease was by 4–5 orders of magnitude. Moreover, the bright burn effect was significantly suppressed by Mg co-doping. Possible explanations, i.e., trap concentration reduction or increased competition in free electron capture between thermoluminescence traps and Ce4+ centers, were discussed.

A beam profiler called the Universal Beam Monitor (UniBEaM) has been developed by D-Pace Inc. (Canada) and the Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, University of Bern (Switzerland). The device is based on passing 100 to 600 micron cerium-doped optical fibers through a particle beam. Visible scintillation light from the sensor fibers is transmitted over distances of tens of meters to the light sensors with minimal signal loss and no susceptibility to electromagnetic fields. The probe has an insertion length of only 70 mm. The software plots the beam intensity distribution in the horizontal and vertical planes, and calculates the beam location and integrated profile area, which correlates well with total beam current. UniBEaM has a large dynamic range, operating with beam currents of ∼pA to mA, and a large range of particle kinetic energies of ∼keV to GeV, depending on the absorbed power density. Test data are presented for H - beams at 25keV for 500 μA, and H + beams at 18MeV for 50pA to 10 μA. Maximum absorbed power density of the optical fiber before thermal damage is discussed in relation to dE/dx energy deposition as a function of particle type and kinetic energy. UniBEaM is well suited for a wide variety of beamlines including discovery science applications, radio-pharmaceutical production, hadron therapy, industrial ion beam applications including ion implantation, industrial electron beams, and ion source testing.

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for scintillator devices combining efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation and mitigated reabsorption losses in large-volume/high-density detectors. To date, the simultaneous achievement of all such features is still an open challenge. Here we realize this regime with poly(methyl methacrylate) nanocomposites embedding CsPbBr3 perovskite nanocrystals as sensitizers for a conjugated organic dye featuring a large Stokes shift and a fast emission lifetime in the red spectral region. Complete energy transfer from the nanocrystals to the dye under both X-rays and α-particle excitation results in highly stable radioluminescence with an efficiency comparable to that of commercial-grade inorganic and plastic scintillators; an ~3.4 ns emission lifetime, competitive with fast lanthanide scintillators; and reabsorption-free waveguiding for long optical distances. Poly(methyl methacrylate) nanocomposites embedding CsPbBr3 perovskite nanocrystals can be used to simultaneously achieve optimized parameters in scintillator devices.

Ce-doped garnets find their use as scintillators as well as phosphors. Among synthetic garnets, gallium gadolinium aluminium garnet (GGAG) is a relatively new and interesting material, a mixed garnet that has displayed very good scintillation and luminescence properties. In the presented work Ce-doped GGAG ceramic samples were produced by reaction sintering from commercial oxide powders. The mixed powders were pressed into pellets and sintered. In order to achieve transparency of a ceramic material, it is crucial to eliminate porosity. Therefore, a number of process parameters and their influence on the microstructure and optical quality of the ceramics were studied: starting powders, sintering additives and sintering conditions. The sintering process consisted of two steps, viz. pressureless sintering in air followed by hot isostatic pressing. Luminescence properties of the sintered ceramics were studied by means of photo-luminescence and correlated to the fabrication process parameters.

Current research in High Energy Cosmic Ray Physics touches on fundamental questions regarding the origin of cosmic rays, their composition, the acceleration mechanisms, and their production. Unambiguous measurements of the energy spectra and of the composition of cosmic rays at the "knee" region could provide some of the answers to the above questions. So far only ground based observations, which rely on sophisticated models describing high energy interactions in the Earth's atmosphere, have been possible due to the extremely low particle rates at these energies. A calorimetry based space experiment that could provide not only flux measurements but also energy spectra and particle identification, would certainly overcome some of the uncertainties of ground based experiments. Given the expected particle fluxes, a very large acceptance is needed to collect a sufficient quantity of data, in a time compatible with the duration of a space mission. This in turn, contrasts with the lightness and compactness requirements for space based experiments. We present a novel idea in calorimetry which addresses these issues whilst limiting the mass and volume of the detector. In this paper we report on a four year R&D program where we investigated materials, coatings, photo-sensors, Front End electronics, and mechanical structures with the aim of designing a high performance, high granularity calorimeter with the largest possible acceptance. Details are given of the design choices, component characterisation, and of the construction of a sizeable prototype (Calocube) which has been used in various tests with particle beams.

International audience; In this work, we investigate the optical properties of hafnium dioxide nanocrystals, upon X-ray irradiation, looking for spectral evolution following thermal treatments in air up to 1000 °C that modify the crystal size as well as their point defect concentrations. Radio-luminescence measurements from 10 K up to room temperature reveal a rich and evolving picture of the optical features. A complete spectral analysis of the broad luminescence spectra reveals the presence of several emission components in the visible and UV regions. The lower energy components peaking at 2.1, 2.5, and 2.9 eV are characterized by a thermal quenching energy of 0.08 eV, while the corresponding value for the UV bands at 4.1 and 4.7 eV is close to 0.23 eV. We tentatively assign the components ranging from 2 to 3 eV to the presence of optically active defects of an intrinsic nature, together with the occurrence of titanium impurities; conversely, the bands at higher energies are likely to be of an excitonic nature. The comparison with previous photo-luminescence studies allows evidencing characteristic differences between the features of luminescence emissions caused by intra-centre excitation and those occurring under ionizing irradiation. Finally, scintillation measurements in the visible range reveal the existence of a fast decay in the nanosecond time scale for the smallest hafnia nanocrystals. This study offers a clear description of HfO2 luminescence characteristics upon excitation by X-rays and can lead to a better comprehension of the structure–property relationship at the nanoscale in metal oxides.

Wavelength-shifting materials in the form of optical fibers are requested in order to both convert UV scintillation light produced by fast scintillators into a more easily detectable visible light, and to efficiently transport it to the detectors. In the present investigation we have considered glasses with composition 3CaO-2SiO 2 and 3CaF 2 -2SiO 2 doped with 0.05, 0.5 and 1 at% of Ce 3 + ions, prepared by an original method consisting in a combination of co-precipitation and sol-gel approaches. The glasses have been investigated by Raman scattering, as well as by photo- and radio-luminescence. The glasses are characterized by complex Raman features. Composite photo-luminescence excitation and emission spectra are also detected in all cases, depending upon glass composition and Ce concentration. Broad excitation spectra extend from approximately 260 nm to 360 nm, while the composite Ce 3 + emission is detected in the blue spectral region at around 400 nm. A satisfactory matching between the excitation spectra of the glasses and the emission spectrum of CeF 3 scintillator occurs, confirming their potential application as wavelength shifters of CeF 3 UV scintillation light. Moreover, bright photo-luminescence signals are accompanied by a very poor radio-luminescence. The results are discussed by taking into account the presence of different silicate clusters embedded in the glass network, as well as the role of point defects in the radio-luminescence process.

Ce-doped garnets find their use as scintillators as well as phosphors. Among synthetic garnets, gallium gadolinium aluminum garnet (GGAG) is a relatively new and interesting material, a mixed garnet that has displayed very good scintillation and luminescent properties [1]. In the presented work GGAG ceramic samples were produced by reaction sintering from commercial oxide powders. The mixed powders were pressed into pellets and sintered. In order to achieve transparency of a ceramic material, it is crucial to eliminate porosity. Therefore, the sintering process was carefully selected and the use of sintering additives was studied. The sintering process consisted of two steps, viz. sintering in air followed by hot isostatic pressing. The influence of starting powders, sintering additives and sintering conditions on the microstructure and optical quality of the ceramics was studied. Fig. 1 shows one of the ceramic samples and its microstructure. Luminescence properties were studied by means of photo- and radio-luminescence and correlated to the fabrication process parameters. In some of the samples, significant persistent luminescence was observed and studied. Moreover, point defects were investigated by thermally stimulated luminescence, also in comparison with a single crystal with the same composition. [1] Y. Wu, F. Meng, Q. Li, M. Koschan, C. L. Melcher. Role of Ce4+ in the scintillation mechanism of codoped Gd3Ga3Al2O12:Ce. Phys. Rev. Appl. 2 (2014) 044009.

The process of hole and electron localization in LuAG:Ce,Mg ceramics is studied by electron spin resonance (ESR) and thermally stimulated luminescence (TSL). Hole traps, which are created by UV irradiation, are detected in the form of O− centers. Mg-perturbed variants of O− centers are proposed to exist. The thermal stability of such defects is studied, proving that they are stable up to room temperature. The interaction between O− centers and shallow electron traps is studied by thermally stimulated luminescence (TSL) and phosphorescence experiments, which reveal the occurrence of an a-thermal tunneling process between trapped electrons and randomly distributed Ce centers. By correlating the TSL-derived trap parameters and temperature dependent ESR intensities, it is found that O− centers compete with Ce centers in free electron capture.

Fiber-optic-coupled scintillation dosimeters are characterized by their small active volume if compared to other existing systems, and are therefore particularly suited for internal in vivo dosimetry. Due to possible differences between calibration conditions (i.e., room temperature) and conditions of clinical application (i.e., body temperature), their temperature dependence should be accurately studied. In this work, the temperature dependence of a Ce3+ doped SiO2 scintillation detector coupled to a SiO2 optical fibre was investigated for high dose rate brachytherapy applications. To this aim, two sets of irradiations with two different Ir-192 sources were performed in a water bath phantom at water temperatures ranging between 17°C and 40.4°C (Experiment 1). The relative response of the dosimeter was collected and analyzed. The same experiment was repeated with a second optical fibre which was designed without the active Ce3+ doped part at its end (Experiment 2) as well as by changing the length of the passive fibre inserted in water (Experiment 3). The two sets of measurements of experiment 1 were in accordance, indicating a linear increase with temperature of the scintillator sensitivity, with an average increase of 0.27 ± 0.2%/°C. In experiment 2, a 0.5%/°C increase of the collected signal resulted for the passive fibre. No significant difference of the temperature coefficient was found by changing the length of the fibre inserted in water (experiment 3). The obtained results show that a temperature-specific correction factor should be adopted at temperatures different than room temperature (e.g. for in vivo internal dosimetry). Further studies are required to understand the observations. © 2014 Elsevier Ltd. All rights reserved.

Over the years, many efforts have been made to develop radiation detectors to handle the complex issues of small field dosimetry and achieve the increasing accuracy, precision and in vivo dose monitoring required by the new advanced treatment modalities. In this context, interest has surged in the development of sensors based on scintillating optical fibres. In this paper, the near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibres, coupled with a laboratory prototype based on an avalanche photodiode, were studied by irradiating the fibres with photons and electron beams generated by a Varian Trilogy accelerator. The performance of the system in standard and small field sizes has also been investigated, comparing the output factor, percentage depth dose and off-axis ratio measurements of the prototypal detector with other commercial sensors, including the Exradin W1 scintillator. The results of this study demonstrate that the drawback due to the stem effect in Yb-doped silica optical fibres can be managed in a simple but effective way by optical filtering. The robustness of the system in complex dosimetric scenarios and the accuracy and precision achieved by Yb-doped fibres in relative dose assessments suggest an effective use of the system for real-time in vivo dosimetry applications.

In the last decade, many efforts have been devoted to the development of smart multifunctional materials. Among them, inorganic nanostructures have gained importance because of their outstanding luminescence properties and their potential applications as new building block materials for the next generation electronics and in a variety of lighting applications. In particular, many recent researches have been addressed on the achievement of progress in the synthesis of nanosized metal oxides. In this field, a significant attention is paid to hafnium dioxide (hafnia or HfO2), which can be employed in optical protective and thermal barrier coatings. Thanks to its mechanical resistance, hafnia finds applications as ceramic, super hard materials and catalysts, or as component in gas sensors and fuel cell electrolytes. Lastly, HfO2 is now evaluated as potential alternative gate dielectric to replace SiO2 in the future generation of electronic nanodevices. Regarding the scintillating properties, the high atomic number Z=72 and the quite high density (9.6 g cm-3) make HfO2 nanocrystals good hosts for phosphor and scintillating applications where a large stopping power for ionizing radiation (X-rays, γ-rays) is required (1). Bulk hafnia is very difficult to grow due to its very high melting point (2774 °C). Actually, HfO2 can be synthetized also in the nanocrystals form to fabricate thin films, optical ceramics and nanocomposite materials (2). Indeed, besides the well-known luminescence of HfO2 nanocrystals activated by the incorporation of rare earth ions, some recent studies evidenced the occurrence of a blue fluorescence from undoped nanocrystals upon UV excitation; remarkably, a broad bluish luminescence appears upon X-ray illumination (RL). These findings suggest the potential of HfO2 nanocrystals as radiation detectors, but the lack of a detailed model that relates their structural and RL properties still hinders the development of efficient nanoscintillators with optimized structure and chemical composition. We studied the RL features of undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale in order to elucidate their origin. Upon X-ray irradiation, the nanocrystals show six emission bands in the near UV/visible spectral range, detectable between 10 K and 300 K. The visible luminescence bands at 2.2 eV, 2.5 eV and 2.8 eV are similar to those detected in our previous PL studies (3), while the UV emission at 4.2 eV and 4.6 eV have been observed for the first time. The excitonic behavior of the UV luminescence is evidenced. The strong increase of the 2.5 eV blue luminescence in annealed samples is likely related to the presence of titanium and it might be used for the design of highly efficient blue scintillating materials. 1. LeLuyer C. et al. (2008), HfO2:X (X = Eu3+, Ce3+, Y3+) Sol Gel Powders for Ultradense Scintillating Materials. J. Phys. Chem. A, 112, pp. 10152-10155. 2. Lange S. et al. (2006), Luminescence of Re-Ions in HfO2 Thin Films and Some Possible Applications. Opt. Mater., 28, pp. 1238-1242. 3. Villa I. et al. (2016), Size-Dependent Luminescence in HfO2 Nanocrystals: Toward White Emission from Intrinsic Surface Defects. Chem. Mater., 28(10), pp. 3245–3253.

Particle discrimination in a mixed radiation field using scintillators is a challenging topic in radiation detection research. We propose a novel approach relying on the possibility of identifying particles interacting in a phoswich detector from the emission spectrum of the produced scintillation signal. As a proof of concept, we focus on the discrimination between neutrons and gamma rays. Lu 3 Al 5 O 12 :Pr and Gd 3 Al 2 Ga 3 O 12 :Ce thin single crystal scintillators, coupled to two different silicon photomultipliers equipped with optical filters, are simultaneously used in the same phoswich detector. Their optical emissions peak at approximately 310 nm and 545 nm respectively, and thus they can easily be distinguished by optical filtering. While both crystals are sensitive to gamma rays, neutrons interact only with the Gd 3 Al 2 Ga 3 O 12 :Ce thanks to the presence of Gd acting as neutron converter. Optical filtration and an anti-coincidence algorithm are therefore used to perform particle discrimination, rejecting coincidence signals arising from gamma rays, which simultaneously deposit energy in both crystals, and counting anti-coincidence signals due to neutrons, which deposit energy only in the Gd 3 Al 2 Ga 3 O 12 :Ce. The simple neutron counter developed here is intended to be a proof of the principle of the feasibility of the color-based particle discrimination technique.

The use of aliovalent sintering aids, especially TEOS, for scintillation ceramics preparation caused serious problems due to the need for charge compensation. Isovalent ions, on the other hand, do not cause such a problem and remain a new field to be explored. By using La 2 O 3 and Sc 2 O 3 isovalent sintering aids, we succeeded to produce Pr:LuAG ceramics with pure LuAG phase, good micro-structure, and characterized by high optical quality with 80% transmittance in the visible range. Luminescence and scintillation properties were also investigated and turned out to be similar to those of previously studied transparent ceramics obtained with aliovalent sintering agents. Thermoluminescence data show the presence of several trapping states experiencing a disordered surrounding. It can be concluded that sintering aids, regardless of their valence, introduce local disorder due to their different radii with respect to that of the lattice ions resulting in the appearance of a UV emission and decrease of Pr 3+ fast 5d–4f luminescence intensity. The aspects of energy transfer and trapping due to sintering aids in LuAG lattices are discussed.

Electron traps are known to a have great influence on the carriers transport process in Ce based scintillators. On the other hand, the role of hole traps in the scintillation process has been less considered. By means of electron spin resonance (ESR), we detected hole traps in highly Ce-doped LuAG: Ce,Mg ceramics in the form of sigma-type O- centers. The g-tensor components turn out to be g(1) = 2.0103, and g parallel to = 2.0023, revealing the axial symmetry of these defects. Mg-perturbed variants of O- centers are proposed to exist in LuAG: Ce,Mg. Their occurrence is related to an elongated Mg-O bond as evidenced by DFT calculations. Finally, the multiple role of O-centers in the scintillation process is discussed. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The search for new materials which could be used as scintillating bolometers is a subject of great interest in view of next-generation experiments dedicated to the study of neutrinoless double beta decay (DBD). Preliminary measurements performed on ZnSe bolometric and scintillation characteristics gave very promising results for what concerns the amplitude of the scintillation signal and the capacity to discriminate between á and â particles. The current work studies the possibility to develop a scintillating ZnSe crystal adapted for the use as hybrid (thermal and scintillation) detector operating at very low temperatures (< 20 mK), able to discriminate between á and â induced events. The results of optical transmission, X-ray excited steady-state luminescence and thermo-luminescence measurements performed on ZnSe crystals produced in different conditions are presented. Based on these measurements, the scintillation properties of undoped ZnSe crystals are carefully described. The correlation between these measurements and bolometric/scintillation tests is used in order to fix the production conditions of ZnSe crystals which best guarantee scintillation efficiency at very low temperatures for possible use in a DBD experiment. © 2010 IEEE.

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. © 2010 IEEE.

To improve particle detectors in physics research and PET scanners in medicine, for example, better scintillators are sought. In this study, researchers succeed in creating a ceramic that is free of defect-induced host luminescence. In a combined ``defect engineering'' and ``band-gap engineering'' approach, the detrimental influence of lattice defects is significantly reduced, by simultaneously lowering the concentration of point defects and shifting their energy levels. Thanks to this approach, efficient scintillator material with high density, high light yield, and short decay time is obtained.

This paper deals with some aspects of scintillation response of Ce-doped Y-Lu aluminum garnets or perovskites in comparison with Ce-doped orthosilicates. An essential difference between the Ce-doped aluminum garnets/perovskites and orthosilicates consists in larger fluctuations of photoelectron yield of the latter scintillators where differences can reach up to 50% and measured values critically depend on the recent history of the sample (illumination by day light, thermal annealing, etc.). This behavior of Ce-doped silicates seems to be due to the vicinity of the 5d1 level of Ce3+ from the conduction band, which makes the material more sensitive to the presence of deep electron traps monitored by thermo-luminescence above room temperature. The presence of such electron trapping sites thus appears more critical than in aluminum garnets/perovskites.

Effect of Ga substitution in the (Lu, Pr) 3Al 5O 12(Pr : LuAG) scintillator was examined at the crystals grown by the micropulling down (μ-PD) method. Host luminescence due to an exciten localized around the Lu-Al antisite defect was suppressed by Ga admixture. Concentration depenendence was investigated and suppression was observed even at the lowest Ga concentration of 5 mol%. Faster decay time as well as less intense slower components were obtained upon increasing the amount of Ga. In the thermostimulated luminescence measurement, the glow curve peaks are gradually shifted to lower temperatures with the gallium admixture, and noticeable intensity decrease was observed only for the (Lu, Pr) 3(Ga 0.2Al 0.8) 5O 12. The undesired quenching effect of Ga on the Pr 3+ luminescence seems to be reasonably low up to (Lu, Pr) 3(Ga 0.2Al 0.8) 5O 12 sample. It is estimated to introduce nonradiative losses of less than 10% with respect to Ga-free Pr:LuAG. Bulk crystals were also grown by the Czochralski (Cz) method and these effects were confirmed. © 2008 IEEE.

Luminescence and scintillation properties of single crystalline films (SCF) of Ce- and Pr-doped Y 3 Al 5 O 12 (YAG) and Lu 3 Al 5 O 12 (LuAG) garnets were analysed and compared with their bulk single crystal (SC) analogues. It was shown that the main peculiarities of luminescent properties of Ce- and Pr-doped YAG and LuAG SCF are due to the absence of Y Al and Lu Al antisite defects (AD) and to the extremely low concentration of vacancy-type defects in SCF with respect to SC. In SCF the absence of the AD-related traps results in faster energy transfer to the emission centres and accelerated scintillation response.

Single crystals of PbWO 4 and Lu 3 Al 5 O 12 -based scintillators were studied by correlated electron paramagnetic resonance and thermally stimulated luminescence experiments. The nature of some of the intrinsic trapping centers and their role in the energy transfer and storage processes were explained. Examples of material doping and co-doping are given, which succeeded in suppression of specific point defects and improvement of scintillation performance.

Time development of a Nphels/MeV photoelectron yield of various Ce and Pr-doped inorganic crystals (mainly garnets, perovskites and silicates) was studied in detail. Slow scintillation decay components influence time dependence, especially at those scintillators containing Lu as lattice ions where an increase of Nphels per 1 MeV in the time span 0.25–5 µs is more than 50%. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Modern applications of scintillator materials demand cutting-edge performances and require often a response speed in the nanosecond time scale. Slow light emission causing an "afterglow" is, therefore, of considerable concern in the development of fast scintillators. The mechanism of afterglow emission in mixed Ce-doped oxyorthosilicate scintillators is investigated by means of time-resolved scintillation, thermally stimulated luminescence (TSL), and radio-luminescence measurements. Various Ce-doped Lu2xGd2-2xSiO5 oxyorthosilicate crystals (with x ranging from 0 to1) and Lu1.8Y0.2SiO5 grown by the Czochralski technique are considered. The detailed TSL analysis reveals that thermally assisted tunneling recombination of electrons trapped by oxygen vacancies with holes trapped by Ce luminescence centers occurs for all compositions. The reduction of the afterglow intensity by adding gadolinium or yttrium into the host is accompanied by a lowering of the traps concentration, as deduced by the TSL intensity. Such lowering of the oxygen vacancy concentrations is found to be correlated with the decrease of the melting temperature induced by gadolinium or yttrium content increase, which governs the oxygen vapor pressure. The occurrence of a similar mechanism also in other scintillators and its influence on carrier trapping is discussed.

International audience; Point defects in the host lattice of a scintillator material can trap carriers, slowing down their migration or even preventing their transfer to luminescent centers. Such competition schemes between defects and luminescent centers may explain also the hysteresis effect, which consists of a progressive enhancement of scintillation efficiency with accumulated dose. We propose a comparison between the scintillation hysteresis effect of Yb-doped sol–gel silica glasses in bulk and fiber forms, and we correlate them with traps monitored by wavelength-resolved thermally stimulated luminescence in both materials. The results demonstrate that the fiber-drawing process is responsible for modifications of the defectiveness of the glass network, with a change of the local distribution of the traps surrounding the luminescent center. The consequence of such modifications is the removal, in the fiber samples, of the thermally stimulated luminescence peak ascribed to traps closer to Yb ions and unstable at room temperature. We highlight that suitable postdensification thermal treatments can significantly modify the concentration and spatial distribution of defects around a luminescent center and can therefore be used as a tool for the engineering of scintillating glasses.

Yb3+-doped YAlO3 single crystals were grown by the Czochralski method with a radio frequency heating system. The starting melt compositions of Y1-xYbxAlO3 were varied as x = 0.02, 0.1, 0.2, 0.3 and 0.45. Photo- and thermo-luminescence characteristics including emission decays were measured within 4-300 K. Very fast charge transfer luminescence of Yb3+ was found, with the typical double peak spectrum at 345 and 525 nm. At room temperature the photoluminescence and scintillation decay curves show a leading subnanosecond decay component without any additional slower processes. The high density of Yb-rich YAlO3 makes this material a promising candidate for fast scintillators. (C) 2004 Elsevier Ltd. All rights reserved.

The sol-gel technique has been considered for the preparation of Ce3+-doped silicate scintillating glasses. Ce3+, concentrations in the range 0.1-1 mol% have been studied. The structural and optical properties of the glasses have been investigated by several techniques like Raman spectroscopy, radio-luminescence, time resolved luminescence, and wavelength resolved thermally stimulated luminescence. Particular attention has been paid to the influence of the concentration of dopant ions and to the different phases of the xerogel densification process governing the glass formation. The results achieved indicate that Ce3+ activators possess promising optical features in the considered glass matrix both for what concerns the luminescence efficiency and the scintillation decay time (of approximately 50 ns under Na-22 excitation). (C) 2002 Elsevier Science B.V. All rights reserved.

Vibrational modes of OH, as stretching, bending, combination, and overtones, were monitored by means of FTIR spectroscopy in Ce3 - and Gd3 -doped silicate glasses, prepared by means of the sol-gel technique, submitted to different thermal treatments, and aimed to applications as scintillators. The dopant concentration was varied in a wide range (10 -3 -8% m.f.) and co-doped samples were also investigated. In this framework, the effects of OH and of rare-earth (RE) clustering, i.e. two possible non-radiative decay channels, on the IR spectra in the 500-14 000 cm(-1) were examined. OH was detected as belonging either to water and to silanol in samples densified at 450 and 750 degreesC, while in those treated at 1050 degreesC only Si-OH groups were present in a concentration of the order of 1% m.f., nearly regardless of the RE amount. An absorption peaking at 4250 cm(-1) was identified as related to an OH combination mode (bending???) perturbed by a Gd cluster on the basis of its amplitude dependence on the Gd concentration and rapid thermal treatment.

Radiation induced sensitization in scintillator materials (also commonly called “bright burn” or “memory effect”) represents an issue for those applications, like medical imaging and real-time radioluminescence dosimetry, which strongly rely on high reproducibility and dose linearity of the luminescence output. This effect is caused by defects which act as traps for radiation-induced free carriers and compete with the recombination centres along the recombination path, thus lowering the luminescence efficiency of the scintillation materials. Considering that the competition between these two processes is driven by the capture cross sections and by the relative concentration of empty traps with respect to that of the recombination centres, the progressive trap filling during ionizing irradiation results in an increase in the radiative recombination probability and, thus, in an enhancement in the RL intensity. Despite such effect might have profound consequences on scintillator performance, it has rarely been studied and only few reports are present in the literature [1-3]. Indeed, the use of standard scintillators (like LSO, YAG, …) may prove difficult for a more thorough investigation on this effect mainly because of the critical dependence on material reproducibility in term of traps which are related to uncontrolled impurities and defects.We recently proposed YPO4:Ce,Nd as a model material for such a study [4], since the main electronic trap is represented by Nd3+ ions [5] whose content can be easily selected during the sample synthesis. The obtained results clearly showed that RL efficiency increase is strongly dependent on the Nd content and on measurement temperature. On the basis of the experimental results, a model based on thermoluminescence trap filling and able to accurately describe the memory effect was proposed. In this communication, we extend our work on the memory effect performed on YPO4:Ce,Nd by considering two other ions (namelyPr andDy) in place of the Nd ones. The various codopants gives rise to electronic traps with different characteristics. The results obtained with the Dy and Pr-codoped crystals are compared with those on the Nd-codoped ones in order to put in evidence the role of trap thermal stability, and doping ions in determining the memory effect amplitude and shape. The obtained experimental results are the base for the further refinement of the trap filling model recently proposed for YPO4:Ce,Nd. The model validity and generality will also be discussed.

Optical ceramics are finding a strong technological interest in the field of laser and scintillation materials as an alternative to single crystals. The most favourable structural composition is the cubic one; in fact, due to the isotropy of the optical parameters occurring in this case, it is possible to obtain highly transparent bulk materials made of compacted micro - or nano - metric grains. Presently the mostly studied optical ceramics are rare-earth doped Y- and Lu- Aluminium garnets [1, and references therein]. In several cases sintering agents like Si, Mg, Sc, or La are also used. Indeed, it was found that their introduction causes a transparency improvement, accompanied, however, by a deterioration of the scintillation properties in terms of light yield and time decay. It was suggested that such ions, irrespective of their valence, introduce local disorder and cause the formation of point defects acting as traps and affecting the migration of charge carriers to the luminescence centres [1-3]. As a matter of fact, irrespective of sintering agents local disorder is inherently present in optical ceramics due to the existence of grain boundaries. Really, such ceramics are intriguing systems characterized by the simultaneous presence of relatively ordered regions within the grains, which constitute the major part of the material volume, and of defect-rich interface regions between grains. In these boundary regions segregation of rare-earth doping ions was also found to occur [4]. Thermally stimulated luminescence (TSL) experiments have been performed on Lu3Al5O12 (LuAG) ceramics doped with Ce3+ or Pr3+ in order to characterize defects in such materials and to compare the results with the defect properties in single crystals [1-3]. In this contribution, we focus on the information that can be obtained from such measurements specifically concerning the occurrence of local disorder, which causes a broadening of glow curves related to inhomogeneous broadening of defect levels. Due to the high concentration of defects and luminescent centres at grain boundaries, the shape of TSL peaks is expected to be particularly sensitive to the quality of such interfaces so that it can be used as a qualitative probe of local disorder and provide a useful tool during material optimization. Our characterization is completed by Raman scattering data providing complementary structural information.

The fundamental knowledge of defects in a material is of crucial importance in order to understand the energy transfer and trapping processes occurring during irradiation with ionizing radiations, especially if application for radiation detection as scintillator or dosimeter is foreseen. Point defects can give rise to localized energy levels in the forbidden gap trapping free carriers during irradiation; in a scintillator, carrier trapping by defects can compete with the prompt scintillation process. In the research field of scintillators such defects are qualitatively separated in two broad families, usually named “deep” and “shallow” traps. Deep traps are those with a lifetime much longer with respect to the scintillation decay time. They trap carriers in a stable way and really lower the scintillation light. On the other hand, “shallow” traps have a lifetime longer but comparable with respect to the scintillation decay time, so that carrier de-trapping from such states and subsequent radiative recombination causes slow scintillation tails and thus alters the timing properties of the scintillator. If slow tails extend in the milli-second time scale and longer, the phenomenon is usually called “afterglow” and it is due to traps with characteristics lying between those of shallow and of deep ones. Thermally stimulated luminescence (TSL) studies are frequently employed for the investigation of traps; wavelength resolved measurements, where the amplitude of the TSL emitted light is measured both as a function of temperature and wavelength, allow to find simultaneous information about traps and centres and are particularly useful for a satisfactory understanding of the trapping-recombination processes. The lecture is devoted to a description of the investigation of traps by wavelength resolved TSL measurements, with emphasis on experimental aspects as well as on data analysis. Methods and experimental procedures aimed at the evaluation of trap parameters are presented and compared, in order to give a picture of the potential and limits of this high sensitivity technique. The discussion is also grounded on useful examples concerning TSL studies in selected scintillator materials featuring different trapping-recombination mechanisms, like crystalline silicates, perovskites, and tungstates.

An overview of recognized structural defects, impurities and related trapping levels and their role in the scintillation mechanism is provided and discussed in single crystal materials belonging to tungstate, Ce- or Pr-doped aluminum perovskite, garnet and finally to Ce-doped silicate scintillators. New achievements and open problems in deeper understanding of electron and hole self-trapping phenomena and of the nature of defects in the crystal structure and their ability to localize migrating charge carriers are indicated. Fast optical ceramics and nanocomposite materials are pointed out as possible future advanced scintillators. Such novel technologies can in principle explore materials which are not available in the bulk single crystal form, but their figure-of-merit is dramatically dependent on the surface-interface defect states and related trapping and nonradiative recombination phenomena. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The cover picture of the current issue refers to the Rapid Research Letter by Martin Nikl et al. [1]. It depicts the structure of a Lu3Al5O12:Ce single crystal, a possible high performance scintillator material. Its scintillation performance is, however, degraded by the occurrence of antisite defects. Such defects arise in the material structure sketched in the figure due to the occurrence of Lu3+ ions at the Al3+ octahedral sites as seen in the middle left octahedron. This defect constitutes a shallow electron trap, which delays energy delivery to the Ce3+ emission centers, and thus slows down the scintillation response of the material. Thermoluminescence measurements appear as sensitive diagnostic tool to evidence these defects in the single crystals grown. Martin Nikl is senior scientist and head of the Laboratory of Luminescence and Scintillation Materials at the Institute of Physics of the Czech Academy of Sciences. His main research activities include the characterization of the luminescence and scintillation properties of wide band-gap materials and the influence of material defects on them.

Significant improvements have continuously been made in radiation therapy technologies, and therefore innovative detectors must also be developed to ensure the beam quality of these new irradiation systems and to allow in vivo dosimetry measurements. Optical fiber based radioluminescence (RL) dosimeters are a promising option for these purposes. They can enable real time dose measurement and their small size can be exploited in small radiation field dosimetry. A major difficulty involved in marketing these systems is the spurious luminescence, generally known as stem effect, that is intrinsically present in this type of detector. The possible mechanisms causing the stem effect during irradiation are fluorescence phenomena and especially Cerenkov light, produced in the undoped fiber portion. In this study we have faced the problem by making use of a spectral discrimination method exploiting the sharp 5D0-7F2 line emission of Eu3+ [1]. First, the incorporation of Eu3+ ions in sol-gel silica was investigated as a function of dopant concentration and synthesis parameters. Preliminary structural (TEM), vibrational (Raman, FTIR) and optical absorption studies [2, 3] have allowed to find the most suitable rare-earth (RE) concentrations and synthesis conditions for optimizing both RL efficiency and RE dispersion, avoiding the formation of aggregates. Indeed, clusters of few tens of nanometers were observed already for SiO2:3 mol% Eu3+. The 10 mol% doped sample contains also aggregates of much larger dimensions, whose diffraction patterns are in agreement with the presence of rare earth crystalline oxide silicates. Eu3+-doped fibers were realized by drawing cluster-free silica with 600 ppm doping level. The spectral emission of composite fibers made of a small portion (1 cm) of doped silica fiber coupled to a long (15 m) undoped fiber for remote signal transport has then been investigated under irradiation with photons and electrons of different energies, field sizes and orientations, in order to discover the origin of the stem effect and to evaluate its influence on the RL spectral shape. The possibility of an efficient discrimination between the RL dosimetric signal and the spurious one is proved and discussed.

A prompt transport of radiation induced free carriers toward luminescence centers is a key factor for an efficient conversion of high energy radiation into light in scintillator materials. However, the transport stage of the scintillation process can be hampered by the presence of lattice imperfections. In this study, we investigated the increase of radioluminescence (RL) efficiency after prolonged material irradiation. The general character of the phenomenon was revealed by its occurrence in several scintillator crystals like Lu2Si2O7:Pr, Bi4Ge3O12, Lu3Al5O12:Ce, and Lu0.3Y0.7AlO3:Ce. A detailed investigation was carried out for Lu2Si2O7:Pr. We demonstrated that the RL efficiency increase is due to the progressive filling of traps responsible for the thermoluminescence glow peaks at 460 and 515 K, which compete with Pr3+ centers in free carrier trapping during irradiation. Correspondingly, their emptying results in the recovery of the initial lowest RL efficiency. Spatial correlation between traps and Pr3+ ions was evidenced by the detection of an a-thermal tunneling afterglow between trap levels and 4f excited states of Pr3+. Such hysteresis phenomenon represents a memory of previous irradiations that remains stored in the crystal. It is a further effect caused by traps, which deserves attention like other manifestations of defects in scintillators like the reduction of light yield and the occurrence of slow tails in the scintillation time decay.

A prompt transport of radiation induced free carriers towards luminescence centers is a key factor for an efficient conversion of high energy radiation into light in scintillator materials. However the transport stage of the scintillation process can be hampered by the presence of lattice imperfections. Spatial correlation between defects and luminescent centers can occur due to the need of local charge compensation, or due to the presence of slight lattice distortions induced by a different ionic radius of the luminescent species with respect to lattice constituents. Such a spatial correlation between defects acting as traps and centers causes a strong competition between them in the capture of free carriers during the very final stage of their relaxation. The occurrence of center-trap aggregates can be put in evidence by the analysis of thermo- , radio-luminescence, and afterglow phenomena. Several examples will be shown concerning bulk oxide scintillators like lutetium ortho- and pyro-silicates, perovskites, and tungstates. The case of rare earth doped nanoparticles will also be presented as an example of particular spatial confinement between traps and centers. The influence of such aggregations at the atomic scale in the scintillation efficiency, timing, and radiation damage will be discussed.

We used combustion synthesis to produce phase pure GdOBr:Ce with varying Ce concentration. Several combinations of NH4Br and fuel concentration were tested before phase pure GdOBr:Ce was produced by using a 50% excess of NH4Br and a 50% lean concentration of fuel. In addition to the expected emissions from Ce and Gd, we also detected emissions from Eu3+ and Tb3+. Further research indicated that these extra rare earth emissions were due to impurities in the Gd2O3 starting material. While not the desired effect, we were able to monitor both Eu and Ce intensities as a function of Ce concentration in GdOBr. The Ce emission is seen to reach a maximum between 0.1 and 0.5 mol%. At higher Ce content, the Ce luminescence is quenched because of the concentration quenching effect and the higher concentration of O-H and N-H groups as revealed by IR spectra. Eu emission progressively decreases as Ce content increases from a combination of competition with Ce for charge capture in the RL process and the effect of OH and NH groups. As Ce concentration increases the RL intensity ratio of the bands of the Ce doublet changes even though the energy difference between the two bands (0.24 ± 0.1 eV) is compatible with the spin-orbit splitting of the ground level of Ce3+ (0.25 eV). Fitting results indicate that this relative change is not due to changes in self-absorption, and the effect may be due to changes in local symmetry of the Ce ion. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)