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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.

In recent years, the sol-gel technique was proven to allow a good control, at a relatively low densification temperature, of rare earth (RE) ions incorporation and of their dispersion inside the glass matrix. The glass synthesis can be performed by using high purity precursors, reducing the level of unwanted impurities, which is an essential feature for the radiation hardness of such materials. Several studies demonstrated that RE-doped silica glasses prepared by sol-gel route are suitable materials for the realization of scintillating optical fibers, and have application perspectives as wavelength shifters for the collection and transport of scintillation light in High Energy Physics (HEP) experiments. Moreover, the use of RE-doped fibers as scintillators in HEP detectors, possibly in parallel with undoped fibers exploiting Cherenkov light, has been recently proposed. An extremely good radiation resistance and fast response are crucial properties for such application. In this work we present a detailed study of the scintillation properties of SiO2: 0.05 mol% Ce glasses and the results of irradiation tests using gamma-rays from a 60Co source and X-rays up to an integrated dose of 1 kGy. Radio-luminescence investigations have been combined with optical absorption and attenuation length measurements before and after irradiation with X-rays and with 60Co gamma-rays. Comparisons between bulk preforms and fibers have been carried out, in order to disclose the role of the fiber drawing process in the radiation hardness properties. Fibers with a lower (0.0125 mol%) Ce concentration have also been considered, pointing to a reduction of the radiation damage related to the decrease of the dopant concentration. Fibers with fluorinated glass or polymeric cladding have been compared, looking towards a future engineering of the fiber structure to improve the light propagation and the radiation resistance. The evolution of the optical absorption spectra and of the attenuation length as a function of time after irradiation has been investigated in order to understand the room temperature stability of radiation-induced point defects acting as color centers. Moreover, the samples have been subjected to thermal annealing cycles, to check the temperature activated carrier release from radiation-induced defects and the possibility of a complete recovery of the damage. Further analyses on the properties of sol-gel silica fibers have been carried out: the homogeneity of the Ce distribution along the fiber length has been tested by means of the X-ray fluorescence technique. Eventually, we will report the results of the forthcoming test of Ce-doped fibers, assembled in a calorimeter prototype, with GeV electron beams at the CERN test beam facilities, to better investigate the application perspectives of such kind of material as scintillator in High Energy Physics detectors. This work has been supported by the H2020 projects AIDA-2020 (GA no. 654168) and INTELUM (GA no. 644260).

Charge trapping and energy transfer processes are investigated in PbMoO4 single crystals by electron paramagnetic resonance (EPR) and wavelength-resolved thermally stimulated luminescence (TSL) in a correlated manner. New signals produced by two differently perturbed Mo5+ centers (Mo2 and Mo3) were observed in EPR spectra measured in the crystals after 420 nm light irradiation. Two sets of spin-Hamiltonian parameters, g tensor and the 95,97Mo, 207Pb (super) hyperfine tensors, have been determined and analyzed in terms of crystal field and LCAO-MO theories. A significant overlap of the Mo 5d1 and 6s6p ligand Pb orbitals was deduced for the Mo3 whereas the Mo2 center seemed to be not or very slightly affected by the lead orbitals. The obtained TSL results allowed to suppose the existence of at least six glow peaks produced by the de-trapping of charge carrier traps either of intrinsic nature or somehow stabilized by nearby accidental impurities. The peaks having maxima at 51 K, 79 K, and 89 K, in particular, were attributed to the Mo2 and Mo3 centers thermal destruction due to the observed correspondence between the kinetic parameters (trap depths and frequency factors) determined separately for the glow peaks and the EPR intensity thermal decay curves of these centers. The Mo2 EPR decay curve is rather complex experiencing two-step trend probably due to trapped electron recombination with some holes released below 60 K. It was further confirmed by the TSL emission maximum relatively large red shift (~ 100 nm) compared to the much smaller offset measured in radioluminescence. This phenomenon, observed also in samples obtained from extra-pure starting materials, was explained by three-component origin of the spectra, each having its own thermal fading rate. One of them ceased to exist above 60 K. The discussion of the obtained results is provided in comparison with other representatives of the scheelite tungstate and molybdate-based single crystals.

Single crystals of PbMoO 4 were studied by means of light-induced electron paramagnetic resonance (EPR) and wavelength resolved thermally stimulated luminescence (TSL). The work is focused on the electron self-trapped at the (MoO 4 ) 2- complex. Detailed analysis of the EPR spectra of this complex allowed to determine a set of spin-Hamiltonian parameters including g factor and the 95,97 Mo, 207 Pb hyperfine tensors further used in calculations performed in terms of crystal field and LCAO-MO theories. They have shown a significant overlap of the Mo 5 d 1 and 6 s 6 p ligand lead orbitals. The contributions of oxygen and lead to the molecular orbitals with an electron trapped at the (MoO 4 ) 3- complex were about 14% and 38%, respectively. The decay of the molybdenum center EPR intensity with annealing temperature provided information about thermal stability of the center, which thermally destroys already at 40 K. Trap depth and frequency factor, E t = 0.05 eV, and f 0 = 10 4 s −1 , have been determined. The TSL glow curve displays two overlapping peaks in the 40–50 K temperature region, corresponding to the disappearance of the EPR signal from the (MoO 4 ) 3- center. By partial cleaning technique and initial rise method it is shown that the lower temperature peak is related to a trap with thermal parameters similar to those determined for the (MoO 4 ) 3- center. This allows to correlate it to the thermal destruction of the (MoO 4 ) 3- center. The obtained results are discussed in a broader frame of charge trapping mechanism in all the family of tungstate and molybdate-based single crystals.

International audience; The recent trends in scintillator technologies stimulate research efforts toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiations into light. For example, scintillating nanoparticles attract great interest in medical oncological therapies. In this work, the structural and morphological properties of HfO2:Ti nanoparticles with Ti concentrations from 0.03 to 10 mol% and subjected to calcination up to 1000 °C are thoroughly characterized; moreover, X‐ray photoelectron spectroscopy reveals the incorporation of Ti in both Ti (III) and Ti (IV) chemical states in as prepared samples, while the exclusive presence of Ti(IV) is unambiguously identified in calcined nanoparticles. The optical emission under X‐ray excitation evidences an intense Ti (IV)‐related luminescence at 2.5 eV in high temperature calcined samples with a few microseconds scintillation lifetime, and efficiency comparable to that of Bi4Ge3O12 reference scintillator. Finally, the competitive role of defects in charge carriers capture is demonstrated by the monotonic increase of the 2.5 eV band during prolonged X‐ray irradiation, more evident for nanoparticles with titanium concentration below 1 mol%. HfO2:Ti may also find application in X‐ray triggered oncological therapies by using the Ti (IV)‐related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation.

In recent years, oxide materials based on garnet structure are being investigated as very promising candidates in the field of scintillation because of their high density, good chemical stability, optical transparency, and the possibility to easily incorporate luminescent rare-earth ions. Several studies demonstrated that garnet crystals show high light yield and advantageous timing performances, which make them of interest for applications in medical imaging and high energy physics detectors [1]. Among synthetic garnets, Ce-doped gadolinium gallium aluminum garnet (GGAG:Ce) is a relatively new and interesting material. It is a mixed garnet that has displayed very good scintillation and luminescence properties: its high density enhances the interaction with ionizing radiation, and the presence of Gd provides a high cross section for thermal neutron capture [2]. GGAG:Ce preserves a crystalline cubic structure, which allows to produce it in the form of transparent polycrystalline ceramic [3] with favorable characteristics for optical applications such as lasers, LEDs, and scintillators. In this work, ceramic samples were produced by reaction sintering from commercial oxide powders: the mixed powders were pressed into pellets and sintered by a combined process of air sintering and hot isostatic pressing. The sintering process was carefully selected and the use of sintering additives was optimized to eliminate porosity, which is crucial to achieve a good optical transparency. Optical properties were studied by means of optical absorption spectroscopy, steady-state and time resolved photo- and radio- luminescence, and correlated to the fabrication process parameters. Moreover, trapping phenomena caused by the presence of point defects were investigated by wavelength resolved thermally stimulated luminescence in a wide temperature range (10 - 800 K); a significant persistent luminescence signal was also singled out and investigated as a function of temperature. The presence of point defects was also evidenced by the occurrence of a sensitization of the radio-luminescence signal as a function of increasing cumulated X-ray dose, related to a competitive process between traps and Ce recombination centers in free carrier capture. Finally, preliminary results on recently developed layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce (YAG:Pr/GGAG:Ce) ceramics for particle detection and discrimination will be also reported. This work has been supported by H2020 European Institute for Innovation and Technology (EIT) SPARK project (16290) and H2020 Rise project INTELUM (644260). [1] M. T. Lucchini et al., Nucl. Instrum. Methods Phys. Res. A 816 (2016) 176-183 [2] J. Dumazert et al., Nucl. Inst. Methods Phys. Res. A 882 (2018) 53 [3] Y. Ye et al., Opt. Mater. 71 (2017) 23

Oxygen vacancies play an important role in the luminescence processes of inorganic scintillator materials. In order to study the effects of oxygen vacancies on the luminescence properties of Na3LuSi3O9:Ce3+, these phosphors were prepared using a high temperature solid-state reaction method under different atmosphere and raw materials. It was found that oxygen vacancy had great influence on the absorption, photoluminescence and decay curves of Na3LuSi3O9:Ce3+. The luminescence intensity and peak position showed a regular change when synthesizing atmosphere changed. Na3LuSi3O9:Ce3+ with more oxygen vacancies showed much stronger luminescence intensity at high temperature than that without vacancies. And it was also found that the decreasing of oxygen vacancies can quicken the photoluminescence decay of Ce3+ in Na3LuSi3O9. The existence of oxygen vacancies in Na3LuSi3O9:Ce3+ was confirmed by Zr4+ doping and thermoluminescence emission spectra. At last, emission bands of Ce3+ and oxygen vacancies were well distinguished under X-ray excitation and probable cause of the formation of oxygen vacancies was discussed.

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.

Binary alkali-earthsilicate glassesdoped with Ce3+, Tb3+, and Dy3+were obtained by melt-quenching technique and their photo- and radio-luminescence properties were investigated. The disordered structure of the glass is responsible for an inhomogeneous broadening of the electronic radiative 5d1energy level of Ce3+due to their localization in several positions with different surroundings, possessing a slightly different local ligand field. The influence of glass matrix composition on the Stokes shift as a function of ionic radius of the cation was shown. The radio-luminescence spectral shapes were related to the simultaneous occurrence of emissions from all Ce3+configurations with relative weights proportional to their abundance in the glasses. The possibility of excitation of Ce3+in different localizations was demonstrated and confirmed by the systematic shift of emission from excitation. On the contrary, no shift was observed in the Tb3+and Dy3+doped glasses, thanks to the lower effect of ligand field on their radiative electronic levels.

Wavelength-shifting materials in the form ofoptical fibersare requested in order to both convert UVscintillationlight produced by fastscintillatorsinto 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-2SiO2and 3CaF2-2SiO2doped with 0.05, 0.5 and 1at% ofCe3+ions, prepared by an original method consisting in a combination ofco-precipitationandsol-gelapproaches. The glasses have been investigated byRaman scattering, as well as by photo- and radio-luminescence. The glasses are characterized by complex Raman features. Compositephoto-luminescenceexcitationandemission spectraare also detected in all cases, depending uponglass compositionand Ce concentration. Broad excitation spectra extend from approximately 260nm to 360nm, while the compositeCe3+emission is detected in the blue spectral region at around 400nm. A satisfactory matching between the excitation spectra of the glasses and the emission spectrum of CeF3scintillator occurs, confirming their potential application as wavelength shifters of CeF3UV 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 differentsilicateclusters embedded in the glass network, as well as the role ofpoint defectsin the radio-luminescence process.

International audience; The influence of Ca codoping on the optical absorption, photo‐, radio‐, and thermo‐luminescence properties of YAlO3:Ce (YAP:Ce) crystals has been studied for four different calcium concentrations ranging from 0 to 500 ppm. Ca codoping results in a partial oxidation of Ce3+ into Ce4+, The luminescence time response under pulsed X‐ray excitation of the Ce3+/Ce4+ admixure clearly demonstrates the role of hole migration on both the rise time and the generally observed slow components. From an application point of view, Ca codoping significantly improves the timing performances, but the induced presence of Ce4+ ions is also the cause of a reduction in scintillation efficiency.

The problem of Eu incorporation into silica as dispersed dopants, clusters, separate-phase nanoparticles, or nanocrystals, which is of key importance for applications in the fields of lasers and scintillators, is faced by applying to sol−gel silica doped with nine different Eu3+ concentrations (0.001−10 mol % range) various spectroscopic techniques, including crystal field and vibrational mode analysis by means of Fourier transform absorption and microreflectivity (in the 200−6000 cm−1 and 9−300 K ranges), radioluminescence, and Raman scattering studies at 300 K. The variety of methods revealed the following concordant results: (1) amorphous Eu clusters grow when the Eu concentration is increased up to 3 mol % and (2) Si−OH groups are completely removed and ordered phase separation occurs at 10 mol % doping, as suggested by the remarkable narrowing of the spectral lines. Comparison with polycrystalline Eu oxide, Eu silicates, and α- quartz spectra allowed the unequivocal identification of Eu2Si2O7 pyrosilicate and α-quartz as the main components of nanocrystals in 10 mol % Eu-doped silica. Such conclusions were brilliantly confirmed by transmission electron microscopy and electron diffraction analysis. Phonon coupling and anharmonicity were analyzed and are discussed for a few vibrational modes of nanocrystals.

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.

Structural and optical properties of sol–gel silica based glassceramics doped with 0.1 mol% Ce and codoped with Gd at high concentrations, from 5 mol% up to 40 mol%, are investigated and compared to those of analogous samples doped only with Ce. Raman scattering, transmission electron microscopy, and x-ray diffraction reveal the formation of Gd apatite-like silicate (Gd4.67O(SiO4)3) and of Gd pyrosilicate (Gd2Si2O7) nanophases whose morphology and crystallinity depend on the Gd concentration and thermal treatments. Optical absorption measurements demonstrate the role of the densification atmosphere in modifying the charge state of Ce ions. The incorporation of Ce3+ ions in the nanophases is put in evidence by photo- and radio-luminescence results.

Single -crystal growth of Tm3+-doped Y2O3, Sc2O3, and Lu2O3 sesquioxides, with a melting point around 2400 °C, by the micropulling down technique is reported. Crystal quality and composition of the crystals were evaluated by rocking curve and energy-dispersive X-ray (EDX) analyses, respectively. Optical properties including optical transmittance and photoluminescence decay profiles were also examined. Furthermore, scintillation performance of the crystals including pulse height spectra and radio-luminescence spectra under alfa-ray and gamma-ray excitation was evaluated. © 2011 American Chemical Society.

Micro-pulling down (μ-PD) method allows to prepare single crystals quickly and relatively inexpensively. Since it is a melt growth and taking into account segregation phenomena, the μ-PD method allows also to obtain single crystals characterized by dopant concentration gradients. Especially, taking the advantage of the grown crystal high aspect ratio, it allows to prepare crystalline samples with variable and wide range concentrations of dopants. These samples can help in understanding the correlation between dopant concentration and luminescence properties.

A set of terbium-doped silicate glasses densified up to 1050 °C was prepared by the sol-gel method. A broad range of dopant concentrations (0.001-10 mol%) was considered together with an undoped sample as a reference. The effects of a high-temperature (1600-1800 °C) post-densification rapid thermal treatment (RTT) on the luminescence properties of the samples were investigated. RTT enhances the scintillating efficiency of the samples increasing the radio-luminescence intensity up to two orders of magnitude. The relevant effects induced by RTT on radio- and photo-luminescence spectra (RL, PL) and on PL decay curves indicate the occurrence of modifications of the surroundings of Tb3 + luminescence centres. © 2007 Elsevier Ltd. All rights reserved.

Phosphorescence emitted by a new dosimetric system based on a SiO2 optical fibre doped with Ce3+ ions was investigated. The defects in the matrix that, acting as electron traps, originate the phosphorescence signal were studied by means of thermally stimulated luminescence in the temperature interval 313-533 K. A continuous trap distribution with activation energies extending from 0.8 to 1.5 eV was observed. On the basis of these findings, the temperature dependence of the shape of isothermal phosphorescence decay was analysed and the corresponding spectrum was compared with that of radioluminescence. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

LiCaAlF6 (LiCAF) crystals doped with two different ions (europium and lead) have been investigated as potential new dosimetric materials. The stability of thermally stimulated luminescence (TSL) glow peaks in LiCAF:Eu was evaluated by means of the initial rise technique. The decay times at room temperature of the traps related to the dosimetric glow peaks were found to range between 40 and 2 × 104 years confirming the good dosimetric characteristics of this crystal. The glow curve of LiCAF:Pb is dominated by a peak at approximately 300 °C emitting in the UV region ( 3P0,1-1S0 transition of Pb 2+) superimposed to a very broad structure at lower temperature (20-200 °C) featuring recombination at an intrinsic defect centre. The anomalous behavior of the low temperature structure during thermal cleaning procedures prevented any reliable numerical analysis of the TSL glow peak at 300 °C. © 2009 Elsevier Ltd. All rights reserved.

The depth-dose-profile and the transversal beam profile of a 6 MV photon beam are measured by means of Eu-doped silica optical fibres. In both cases, a non-negligible influence of the stem effect was observed as consequence of the change in the relative contribution of the Cerenkov light while varying the vertical and lateral position of the fibre. The approach for removing the spurious luminescence signal based on the radioluminescence (RL) spectral analysis proved to be effective and independent of the irradiation configurations.

We study trapping states of Ce 3+ -doped SrHfO 3 by thermally stimulated luminescence in a wide temperature range (10–730 K). We determine characteristic parameters of the traps by the initial rise technique. We also determine the energy of thermal ionization of the excited state of Ce 3+ in SrHfO 3 host by purely optical method based on the study of UV illumination-induced thermoluminescence. The method provides a value of the thermal ionization energy of about 0.25 eV.

We investigate scintillation properties and energy trapping processes in Pr-doped Lu 3Al 5O 12(Pr:LuAG) optical ceramics, specifically focusing on the effects of sintering aids. Ceramics with good transmittance and scintillation properties comparable with those of single crystals were obtained without the use of sintering aids. Their introduction caused a transparency improvement, accompanied, however, by a deterioration of scintillation properties in terms of light yield and time decay. We interpret the results, discussing the role of aliovalent sintering agents on the formation of point defects acting as traps affecting the transport of charge carriers to the luminescence centers.

We study defect states in Pr3+-doped Lu2Si2O7 crystal by wavelength resolved thermally stimulated luminescence both below and above room temperature. We identify the TSL peaks, analyze them by the initial rise technique and calculate the characteristic parameters of the corresponding traps. The role of tunnelling of charge carriers between traps and recombination centers up to RT is observed and discussed. The TSL spectra show that the charge carrier recombination predominantly occurs at Pr3+ centers. We also provide some comparison with the LPS:Ce3+ crystal prepared by the same method. (C) 2011 Elsevier B.V. All rights reserved.

The optical and structural properties of terbium doped sodium gadolinium phosphate glasses of three different compositions subjected to a crystallization process were studied and compared with those of the parent glassy samples. The structural characteristics of the glassy and crystallized phases were determined by Raman spectroscopy and the results showed a remarkable reduction in the full width at half maximum of the Raman peaks after crystallization. Radio-luminescence measurements revealed the emissions of both Gd3+ and Tb3+ ions. Their intensities strongly increased and their intensity ratio was modified by the crystallization. The luminescence temperature dependence investigated by radio-luminescence measurements in the temperature interval from 10 to 310K became more complicated after crystallization. The role of free carrier trapping phenomena in the modification of the radio-luminescence efficiency was also studied by thermally stimulated luminescence. © 2009 IOP Publishing Ltd.

To develop new and efficient neutron scintillator, Ti-doped LiAlO 2 single crystal was grown by micro-pulling-down method. The photoluminescence characteristics including emission spectra, excitation spectra, and decay curves were measured. This material exhibits relatively high light yield, which exceeds that of the Li-glass standard neutron scintillator. There can be a possibility to further increase the efficiency by increasing the dopant concentration. The Ti4 charge transfer luminescence was found to be the leading luminescence process with 2.5 μs decay time. © 2010 Elsevier B.V. All rights reserved.

We study lutetium hafnate, Lu4Hf3O12, prepared by acetate and citrate combustion in the form of nanometric powders. Optical properties including X-ray excited luminescence, steady-state and time resolved photoluminescence as well as thermally stimulated luminescence are investigated for undoped as well as Eu3+ and Tb3+ doped samples. We identify recombination centers in the host matrix and we tentatively associate the principal emission with a radiative transition of the F + center. We also demonstrate that athermal tunneling of charge carriers between traps and recombination centers is the predominant mechanism of delayed radiative recombination following irradiation by ionizing radiation. © 2011 Elsevier B.V. All rights reserved.

To better understand the scintillation mechanism in SrHfO3-based scintillators we investigate in detail their x-ray excited radioluminescence (RL) emissions. The Gaussian decomposition of the undoped SrHfO3 RL spectra manifests the presence of four emission bands peaking at 3.0, 4.0, 4.6, and 5.1 eV. Based on corresponding photoluminescence emission and excitation spectra we tentatively associate the observed bands with excitonic or defect-related emissions. When doped by Ce3+ or Pb2+ luminescent activators, the RL spectra feature two or three new bands, respectively. The temperature dependence of all RL spectra demonstrates a competition between activators and intrinsic or defect centers in carrier capture. Around room temperature the emission process is completely dominated by recombination at activator centers. In the case of Pb2+ activator we also study in detail the concentration dependence of the RL spectra below room temperature exhibiting an optimal value around 0.3 mol %.

We use acetate–citrate combustion to prepare SrHfO3 powders both undoped and doped with Ce3+ and Pb2+ ions in several concentrations, from 0.01 up to 3 mol%. The evolution of their morphological and structural features during the sintering process is studied by scanning electron microscopy, X-ray diffraction, and Raman experiments. We discuss the dependence of the optical properties, investigated by radioluminescence and time resolved luminescence, on sintering conditions and material structure.

We study defect states in undoped and Eu3+-doped Lu 3GaxAl5-xO12 (x = 0-5) garnet crystals by wavelength resolved thermally stimulated luminescence above room temperature. Eu3+ ions act not only as recombination centres but also as high temperature traps, as testified by the presence of a TSL peak at 450 °C correlated with Eu3+-doping. With increasing gallium content all glow peaks are shifted to lower temperatures. This trend is confirmed also for powder samples prepared by sol-gel technique. The observed low temperature shift of the glow peaks can be considered as a consequence of the band gap reduction following the increase of Ga3+ concentration. The comparison with the glow curve of a Ce3+-doped crystal is also shown. © 2010 Elsevier B.V. All rights reserved.

A mammography system, a conventional radiological equipment and a CT scanner operated at 120 kV were used to investigate the reproducibility, the sensitivity and the response linearity to a variety of X-ray tube current, high-voltage and dose values of a radio-luminescent dosimeter based on a Ce-doped optical fiber. The dosimeter reproducibility was very close to the ionization chamber reproducibility. The detector sensitivity was also calculated in order to evaluate the minimum detectable dose rate and a minimum detectable dose, considering a sampling time of 10 ms. All curves have a very good linear course with a correlation coefficient greater than 0.999. The fiber sensitivity (counts per unit dose) as a function of the X-ray tube voltage obtained with the conventional X-ray equipment decreases by increasing the photon energy, losing about 10% of its value from 60 to 120 kV. Also for CT scanner, the fiber responses are very good and reproducible. The new dosimetric system is complementary to usual dosimetry systems as ionization chambers, since it is mostly suitable for in vivo and on-line dosimetry. Scintillating fibers are very safe because they are not electrically powered and moreover they assure very high spatial resolution due to their reduced dimensions. © 2009 Elsevier B.V. All rights reserved.