Basic principles of technological shaping of optical properties and radiation hardness of PWO crystals

Technological conditions ensuring growth of optically homogeneous lead tungstate (PWO) crystals are reported. It is shown that the basic scintillator characteristics of PWO grown from highly purified raw material and properly doped with lanthanides are mainly determined by inclusions of oxides W_1−yL_yO_3−x (L=Y, La, Gd; 0<x<0.3). Moreover, surface-located inclusions with structure close to tungstenite may also be of importance. It is demonstrated that the scintillation properties can be intentionally designed by varying the oxygen content in the inclusions x and the surface structure. The optimal value for x and the most favorable surface structure are achieved by proper thermal regimes and environment content during the crystal annealing. The results enabled low-cost fabrication of PWO scintillators with steady and reproducible characteristics acceptable for the CERN project ALICE and ensured production of PWO on an industrial scale by “North Crystals” company at a rate of 125–135 crystals per year from every growth apparatus.     

The role of different linear and non-linear channels of relaxation in scintillator non-proportionality

The non-proportional dependence of a scintillator's light yield on primary particle energy is believed to be influenced crucially by the interplay of non-linear kinetic terms in the radiative and non-radiative decay of excitations versus locally deposited excitation density. A calculation of energy deposition, −dE/dx, along the electron track for NaI is presented for an energy range from several electron-volt to 1 MeV. Such results can be used to specify an initial excitation distribution, if diffusion is neglected. An exactly solvable two-channel (exciton and hole(electron)) model containing 1st and 2nd order kinetic terms is constructed and used to illustrate important features seen in non-proportional light-yield curves, including a dependence on pulse shaping (detection gate width).     

Structural characterization of methanol substituted lanthanum halides

The first study into the alcohol solvation of lanthanum halide [LaX₃] derivatives as a means to lower the processing temperature for the production of the LaBr₃ scintillators was undertaken using methanol (MeOH). Initially the de-hydration of {[La(μ-Br)(H₂O)₇](Br)₂}₂ (1) was investigated through the simple room temperature dissolution of 1 in MeOH. The mixed solvate monomeric [La(H₂O)₇(MeOH)₂](Br)₃ (2) compound was isolated where the La metal center retains its original 9-coordination through the binding of two additional MeOH solvents but necessitates the transfer of the innersphere Br to the outersphere. In an attempt to in situ dry the reaction mixture of 1 in MeOH over CaH₂, crystals of [Ca(MeOH)₆](Br)₂ (3) were isolated. Compound 1 dissolved in MeOH at reflux temperatures led to the isolation of an unusual arrangement identified as the salt derivative {[LaBr_2.75·5.25(MeOH)]^+0.25 [LaBr_3.25·4.75(MeOH)]^−0.25} (4). The fully substituted species was ultimately isolated through the dissolution of dried LaBr₃ in MeOH forming the 8-coordinated [LaBr₃(MeOH)₅] (5) complex. It was determined that the concentration of the crystallization solution directed the structure isolated (4 concentrated; 5 dilute) The other LaX₃ derivatives were isolated as [(MeOH)₄(Cl)₂La(μ-Cl)]₂ (6) and [La(MeOH)₉](I)₃·MeOH (7). Beryllium Dome XRD analysis indicated that the bulk material for 5 appear to have multiple solvated species, 6 is consistent with the single crystal, and 7 was too broad to elucidate structural aspects. Multinuclear NMR (¹³⁹La) indicated that these compounds do not retain their structure in MeOD. TGA/DTA data revealed that the de-solvation temperatures of the MeOH derivatives 4–6 were slightly higher in comparison to their hydrated counterparts.     

Crystal growth and luminescent properties of Pr-doped K(Y,Lu)3F10 single crystal for scintillator application

Pr1%:K(Y_1−xLu_x)₃F₁₀ (x=0, 0.2, 0.4) single crystals were grown by the μ-PD method. All the grown crystals were greenish and perfectly transparent without any inclusions or cracks. Radioluminescence spectra and decay kinetics of the Pr1%:K(Y,Lu)₃F₁₀ crystals were measured. Emission from the Pr³⁺ 5d–4f transition, peaking around 260 nm and of the decay time of around 22 ns were observed. The 5d–4f emission intensities of the Pr1%:K(Y,Lu)₃F₁₀ crystals were higher than that of the standard BGO scintillator.     

Quasichanneling of ions in crystals. I. Elastic scattering of ions on atomic “chains” and planes

The scattering of quasichanneled particles on atomic chains and planes is studied by computer simulation for the crystals of chemical compounds. It is shown that quasichanneling is the specific regime of ion motion in the lattice. Its main features are: (1) a sharp increase of the backscattering probability; (2) transition of the ions from the quasichanneled component into a random one; (3) high sensitivity to the structure and the composition of the atomic “chain”; (4) dependence on the value and the direction of the atomic displacements.     

Interest for thin scintillating films

This paper reports on the interest for developing thin scintillating films deposited on various substrates for scintillation purposes. The first part will present some advantages of films for fundamental studies. Mainly they are helpful (when deposited on CaF 2 ) for spectroscopic measurements when absorption coefficient is very high (as for CeF 3 ) or when excitation energies are near the fundamental absorption edge. When deposited on silicon substrate, photoelectron spectroscopy may be performed without any charging effects. The second part describes applications using film shaped scintillators, like high resolution X-ray imaging.     

Trap levels in Y-aluminum garnet scintillating crystals

Point defects acting as trap levels were investigated on undoped, Ce- and (Ce, Si)-doped Y₃Al₅O₁₂ (YAG) crystals by TSL measurements performed over a wide temperature range (10–800 K). Below room temperature, a composite glow curve was observed, whose intensity strongly increased after Ce doping. Moreover, Ce doping introduced new trap levels giving rise to glow peaks in the 100–200 K range. On the other hand, Si co-doping did not influence the low T glow curve in a significant way. The spectral emission of the TSL was found to be governed by the Ce³⁺ 5d–4f radiative transition, while defect related higher energy emission bands were detected only in the undoped crystal. Above RT, the glow curve was found to be much more influenced by Si co-doping since a strong increase of a glow peak at about 250°C was noticed. Scintillation time decays of Ce- and Ce,Si-doped samples are also reported and compared with TSL data. The significance of the results and the potential impact of defect states on the scintillation properties are discussed.     

Physical,structural and luminescence investigation of Eu3+-doped lithium-gadolinium bismuth-borate glasses for LEDs

The aim of the current report is to fabricate Eu³⁺-doped glasses with the chemical composition of 50Li₂O-15Gd₂O₃-5Bi₂O₃-(30-x)B₂O₃-xEu₂O₃ (where x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol%), with the help of conventional melt quenching technique. The fabricated glasses have been studied with help of physical, structural and luminescence properties for application of LEDs. The structural properties were investigated by XRD and FTIR spectra. Physical properties have been measured. Direct and indirect optical energy band gap (E_g) have been calculated and found to be increasing with Eu₂O₃ concentration. Luminescence spectra have been observed from photo and radioluminescence spectra and found in good agreement with each other, however the concentration quenching was not determined for the samples. The high-covalence and asymmetric nature was confirmed from Photoluminescence emission and RL emission transition as well as from the higher values of luminescence intensity ratio. The JO parameters have been found for the better performance of lasing materials. The lifetime's data have been found to be decreasing from 1.64 to 1.50 ms, which is the confirmation of energy transfer in Eu³⁺ ions through cross relaxations. From the calculated properties it has been suggested that the present glass samples might be good for red-light emitting devices.     

Materials doping through sol–gel chemistry: a little something can make a big difference

Several examples of sol–gel preparation of doped materials are taken to illustrate the various situations where the doping elements are responsible for the main function of the material or govern its structure. Other examples are used to illustrate that sometimes unexpected effects can be observed like structural modification and the appearance of new properties. Rare earth doped scintillators demonstrate higher homogeneity for materials prepared via sol–gel chemistry when compared with classical solid state reaction. The XRD study of rare earth doped orthoborates shows that doping can affect the vaterite to calcite phase transition observed in these compounds. A Raman spectroscopic study has been performed on doped silica xerogels and it has been shown that doping ions can modify greatly the densification process in these amorphous materials. Finally, it has been evidenced that sol–gel chemistry allows the preparation of bioactive ceramics with enhanced properties. In particular Zn-doped HAP with anti inflammatory properties has been prepared and Sr-doped bioactive glasses have demonstrated superior in-vitro bioactivity as evidenced by PIXE-RBS study.     

Radioluminescence properties of rare earths doped SrAl2O4 nanopowders

Nanopowders of SrAl₂O₄ pure and doped with rare earths were prepared via a proteic sol-gel methodology. The prepared materials presented a single crystalline phase, confirmed by XRD measurements. AFM results indicate that the average particle size is about 53 nm for SrAl₂O₄ powders. The radioluminescence spectrum of SrAl₂O₄: Eu²⁺, Dy³⁺ is composed by two intense peaks around 520 and 570 nm followed by a weaker emission peaking at 615 nm. It was observed that the intensity of RL emission during irradiation with X-rays decreased as a function of the irradiation time, indicating the build up of radiation damage in the nanopowders. The irradiated samples exhibited a persistent radiation damage that changes the colour of the sample, and also influenced the reduction in the scintillation efficiency. The saturation level of SrAl₂O₄: Eu²⁺ is 96%, exhibiting good resistance to radiation damage.