Many ultra-dense lutetium or gadolinium based compounds doped with Eu 3+ have been prepared. This paper reports on the major scintillation performances of these compounds. One of them (Lu 2 O 3 :Eu) is particularly promising and have been deposited on a screen. Performances of such a screen are presented.

Ce3+-doped borate crystal fibers of Li6Gd(BO3)(3) (LGBO) and Li6Y(BO3)(3) (LYBO) compositions are grown by the micro-pulling down (mu-PD) method for potential application in developing new neutron detectors. The ternary equilibrium diagram of Li2O-Gd2O3-B2O3 is drawn and the preparations of homogeneous mixed LGBO and LYBO powders and growth conditions for single crystal fibers are discussed. Absorption, excitation and X-ray luminescence spectra are investigated. Absolute light yield derived from energy spectra and kinetic decay curves measured under alpha- and gamma-scintillations of Ce3+-doped LGBO and LYBO single crystal fibers is provided. (c) 2012 Elsevier B.V. All rights reserved.

Single crystals of LuAG:Ce, LuAG:Pr and un-doped LuAG were grown by the vertical Bridgman method and studied for radiation hardness under gamma-rays with doses in the range 10–105 Gy (60Co). A wide absorption band peaking at around 600 nm springs up in all three types of crystals after the irradiations. The second band peaking at around 375 nm appears in both LuAG:Pr and un-doped LuAG. Compositional variations have been done to reveal the spectral behavior of induced color centers in more detail and to understand their origin. Similarities in behavior of Yb2+ centers in as-grown garnets are found, indicating that radiation induced color centers can be associated with residual trace amounts of Yb present in the raw materials. Un-doped LuAG and LuAG:Ce demonstrate moderate radiation hardness (the induced absorption coefficients being equal to 0.05–0.08 cm−1 for accumulated doses of 103–104 Gy), while LuAG:Pr is less radiation hard. The ways to improve the radiation hardness are discussed.

Characterization of spectral effects induced by γ-rays in YAG:Ce single crystals grown by the vertical Bridgman method is done. The influence of Ce3+ ion concentration upon the radiation hardness is shown.

Crystallographic study of the Lu3Al5O12:Sc system shows existence of a wide range of compositions involving substitutions by Sc for both six-fold octahedral and eight-fold dodecahedral sites of the lattice. In comparison to other rare-earth garnets, the total amount of Sc that can be tolerated by this matrix is however much lower. The limited solubility of Sc is associated with the small size difference between the constituent atoms, which approaches a critical value with increasing the Sc content. Preparation of clear single-phase solid solution crystals requires an accurate account of site occupation preferences.

Spectroscopic study of CaNb2O6 single crystals doped with Ce3+, Pr3+ or Tb3+ ions shows existence of a broad excitation band attributed to a rare-earth-trapped exciton, with the hole trapped at the rare earth ion and the electron delocalized over the surrounding of the next-neighbor Nb5+ cations. Their energies are in agreement with the Dorenbos’ model determining the energy level location of trivalent lanthanides in oxides. The rare-earth-trapped excitons occurred in the Pr3+ or Tb3+ doped CaNb2O6 crystals undergo non-radiative relaxation to the 4fn states. In contrast, the exciton trapped on Ce3+ ion was shown to exhibit luminescence from the excitonic state that is the lowest excited state of the system.

Luminescence emission and excitation spectra as well as decay kinetics of Y2O3-Yb nanoparticles elaborated by the polyol mediated synthesis method have been measured at liquid helium temperature under VUV excitation. In nanoparticles the ratio of intensities of Charge Transfer Luminescence (CTL) and exciton excitation peaks differs from that for Y2O3-Yb single crystal, and changes with the particle size variation from 13 to 52 nm. This effect can be connected with the variation of excitation light scattering with particle dimensions. Two types of luminescence decay acceleration are observed. Acceleration of exponential decay time with decrease of nanoparticle size and the appearance of very fast decay component in small nanoparticles that we connected to energy transfer to the surface defects.

LuAG:Ce single crystals with various activator concentrations were grown by the vertical Bridgman technique. Characterization of crystals was done in terms of actual doping level, macroscopic defects and degree of non-equivalent substitutions by Lu for Al in octahedral lattice sites. Scintillation measurements were performed using 2×2×8mm3 shaped samples with Ce concentration in the range 0.05–0.55at%. Essential improvement of performance was demonstrated in samples containing ≥0.2at% of Ce; the light yield measured in LuAG:Ce (0.55at%) was about 26000ph/MeV, or close to that of LSO.

Excellent properties of optical ceramics initially developed as laser materials make them attractive for scintillator applications. The effects of the porosity of ceramics on their luminescence properties is analyzed using a simplified model to account for scattering. The results of the study of optical and luminescence properties of a concentration series of YAG:Yb ceramics fabricated by vacuum sintering and nanocrystalline technology (VSN method) in a wide UV and VUV and X-rays excitation energy range are presented. The results are compared to those obtained for Yb-doped single crystals and undoped YAG ceramics and single crystals.

Luminescence emission and excitation spectra as well as decay kinetics of Y 2 O 3 -Yb nanoparticles elaborated by the polyol mediated synthesis method have been measured at liquid helium temperature under VUV excitation. In nanoparticles the ratio of intensities of Charge Transfer Luminescence (CTL) and exciton excitation peaks differs from that for Y 2 O 3- Yb single crystal, and changes with the particle size variation from 13 to 52 nm. This effect can be connected with the variation of excitation light scattering with particle dimensions. Two types of luminescence decay acceleration are observed. Acceleration of exponential decay time with decrease of nanoparticle size and the appearance of very fast decay component in small nanoparticles that we connected to energy transfer to the surface defects.

Temperature dependence of the charge transfer luminescence (CTL) of Yb-doped yttrium aluminum garnet Y3Al5O12-Yb (YAG–Yb) and Yb-doped lutetium aluminum perovskite LuAlO3-Yb (LuAP–Yb) crystals under X-ray excitation and their thermostimulated luminescence are investigated in the temperature range 30–350K and compared to those of undoped crystals. Simulation using a set of kinetic equations describing the processes of creation of excitons, electron–hole pairs, their trapping and self-trapping, radiative relaxation and quenching is presented for the systems under investigation to analyze qualitatively two different types of experimentally observed temperature dependences: CTL yield decline with the temperature decrease below 110K as in case of YAG–Yb and constant yield in the same temperature range as in case of LuAP–Yb.

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series grown by the Bridgman method were studied under excitation by γ-rays from a 137Cs source. Both series were prepared using the same quality of starting oxides and demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield in the present series of LuAP:Ce crystals measured in 0.2 cm × 0.2 cm × 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at.% 6448±322 ph/MeV and 9911±496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1±0.4% (l=0.2 cm), 9.5±0.5% (l=0.8 cm). Intentional co-doping with Sc3+ ions was tested and resulted in increase of the Ce distribution coefficient from 0.17 in LuAP:Ce to about 0.3 in LuAP:Ce,Sc. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at.%, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on the results of measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.

The microwave resonant cavity technique (MRCT) was used to measure the room-temperature photoconductivity spectrum of a CaF(2):Eu(2+) single crystal between 275 and 450 nm, with the aim of positioning the Eu(2+) levels relatively to the bottom of the host conduction band. A photoconductivity signal was detected at laser wavelengths lambda(l)<= 430 nm (h nu(l)>= 2.9 eV). Its intensity was observed to exhibit a superlinear dependence on the laser mean power for lambda(l)>280 nm and an almost linear one at shorter wavelengths, showing that Eu(2+) photoionization may involve either a one-photon or a two-step two-photon absorption process. The probabilities of both linear and quadratic processes were determined from measurements of the dependences of the photoconductivity signal intensity versus the mean laser power for several laser wavelengths within the spectral range that is under investigation. The Eu(2+) photoionization threshold was estimated at 4.9 eV from the comparison between the MRCT photoconductivity spectrum, the Eu(2+) 4f(6)5d(e(g)) excited-state absorption spectrum, and the calculated density of states of the CaF(2) conduction band. In addition, the photoconduction dynamics in two CaF(2):Eu(2+) samples grown under different experimental conditions was studied. The MRCT signals from the two samples were observed to exhibit different thermal behaviors. This observation is interpreted in terms of differences in trap densities and depths, in connection with thermoluminescence measurements.

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF2 crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ∼10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ɛ1 − iɛ2 induced by the light pulses. The observed signals consisted of short peak at t ∼ 0, approximately 40–70 ns in length, and a long tail with a duration of ∼100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ∼ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF2.

Single crystals of LuAP:Ce and LuYAP(Lu*70%):Ce co-doped with tetravalent (Hf and Zr) and pentavalent (Ta) ions were grown from melts by the Bridgman process. Underlying absorption, slope of the optical edge and transmission in the range of emission were compared to those of LuAP:Ce crystals. Absorption coefficients at 260 nm less than 2 cm−1 have been recorded in LuAP:Ce crystals containing tetravalent ions that are lower than the corresponding figures (5–6 cm−1) measured in undoped LuAP. At high concentrations of added impurities, despite of suppression of the parasitic underlying absorption below 300 nm, the slope of the optical edge and transmission in the range of emission are seriously damaged. Scintillation parameters of crystals with added impurities are compared to those of LuAP:Ce.

The principle of measurements using the "microwave resonant cavity technique" applied to rare-earth-doped insulating materials is reviewed and the physical nature of the expected signals is discussed. Experimental results concerning both single crystal and powdered samples of Lu2SiO5: Ce3+ are presented and discussed as typical examples. From measurements at various temperatures between 300 and 5K and under different pulsed laser beam powers, it is shown that detailed information on the rare earth photoionization process, photoconductivity dynamics and trapping effects may be gained, in addition to photoconductivity spectra. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photoconductivity and trapping effects were studied in various Ce-doped scintillating crystals (LSO, LYSO, YSO and LPS) by means of the microwave resonant cavity technique. In all the investigated crystals, at 300K and under 355nm excitation, transitions from the 4f ground state of Ce3+ ions to low energy levels of the 5d excited configuration give rise to a photoconductivity signal. Temperature- and excitation power-dependent measurements indicate that the photoionization mechanism results from the competition between thermal activation and two-photon (two-step) absorption. However, depending on the crystals, the relative weight of these two possible routes leading to electronic delocalization varies greatly. Moreover, four mixed Lu2−xYxSiO5:Ce (LYSO) crystals of different compositions and with various scintillation yields show different behaviors, in-between Y2SiO5:Ce (YSO) and Lu2SiO5:Ce (LSO). Our measurements concerning trapping and photoconductivity dynamics show that trap-related perturbations affecting the electronic transport during the scintillation process is a major source of losses for the light yield of these scintillating crystals.

Recently, the development of the micro pulling-down method has allowed the growth of single inorganic scintillation crystals with a uniform fiber shape. The dimensions may vary from 0.3 to 3 millimeters in diameter with length up to 1 meter. This paper presents an experimental set up built in order to study the scintillation light propagation within fiber shaped crystals. The interaction position of a gamma photon in the fiber is determined by a coincidence method. Light yields measured by photomultipliers at both sides of the fiber are correlated to the photon interaction position and the scintillating properties of the crystals. X-ray excited luminescence experiments were also performed to study the process of light attenuation along the fibers. X-ray and gamma ray measurements are in good agreement. Two cerium doped lutetium yttrium orthosilicate Lu2(1-x)Y2x SiO5:Ce3+ fibers have been studied. The first was a cylindrical crystal fiber of 6.9 cm in length and of 1 mm in diameter obtained by the micro pulling-down technique. The second was a squared shaped fiber crystal of 1 x 1 x 43 mm(3) obtained from a bulk after cutting and polishing. The interaction localization can be determined with a precision from 4 to 12 mm depending on the fiber shape.