YSO Crystals Research Articles

Impact of compositional disorder on electron migration in lutetium–yttrium oxyorthosilicate scintillator

A general description of the dynamics of nonequilibrium carriers in multicomponent activated scintillation materials with a compositional disorder of the crystalline matrix is developed and applied for studying the excitation transfer and timing properties of lutetium–yttrium oxyorthosilicate (LYSO). The energy structure, the density of states, and the effective potential of LSO and YSO crystals have been calculated by using the Quantum Espresso package. An analytical form of the potential fluctuations due to compositional disorder is suggested in the pseudopotential approximation. The spatial distribution of lutetium and yttrium cations in the LYSO crystal has been simulated by the Monte Carlo method using the thermodynamic approach for three qualitatively different cases of cation distribution: uniform, heterogeneous neighboring, and clustered. The impact of the compositional disorder on electron migration is found to be qualitatively different in four typical regions of electron energy. The density of localized states in LYSO calculated using the coherent potential approximation (CPA) and the quasiclassical approach is comparable to the density of secondary carriers expected in an ionization track and might have significant influence on the migration of thermalized carriers. The transport mean free path of nonlocalized electrons limited by elastic scattering on pseudopotential fluctuations is shown to be substantially longer than that due to longitudinal optical phonon emission in the low-energy region (calculated using CPA) and the high-energy region (calculated using the Born approximation). The scattering on pseudopotential fluctuations is important for intermediate-energy electrons due to a substantial influence of the core potential fluctuations on high-energy branches.

Detection of Low-Energy X-rays Using YSO Scintillation Crystal Arrays for GRB Experiments

We developed an X-ray detector using 36 arrays, each consisting of a 64-pixellated yttrium oxyorthosilicate (YSO) scintillation crystal and a 64-channel multi-anode photomultiplier tube. The X-ray detector was designed to detect X-rays with energies lower than 10 keV, primarily with the aim of localizing gamma-ray bursts (GRBs). YSO crystals have no intrinsic background, which is advantageous for increasing low-energy sensitivity. The fabricated detector was integrated into UBAT, the payload of the Ultra-Fast Flash Observatory (UFFO)/Lomonosov for GRB observation. The UFFO was successfully operated in space in a low-Earth orbit. In this paper, we present the responses of the X-ray detector of the UBAT engineering model identical to the flight model, using 241Am and 55Fe radioactive sources and an Amptek X-ray tube. We found that the X-ray detector can measure energies lower than 5 keV. As such, we expect YSO crystals to be good candidates for the X-ray detector materials for future GRB missions.

Highly luminescent cerium-doped YSO/ LSO microcrystals prepared via room temperature sol-gel route

Luminescent properties of cerium-doped yttrium oxyorthosilicate (Y2SiO5; YSO) and lutetium oxyorthosilicate (Lu2SiO5; LSO) prepared by a room-temperature sol-gel route were studied and compared to those of YSO and LSO single crystals. Structural changes in the samples during calcination were also studied by X-ray diffraction and differential thermal analysis. Radioluminescence spectra of the samples show comparable emission intensity with the bulk materials. Photoluminescence decay measurements were used to investigate the decay characteristics of the prepared powders. Changes in luminescent properties after the reduction of particle size by grinding in a ball mill were also demonstrated on the YSO:Ce3+ sample. Acceleration of the faster component of the decay time from 7.1 ns to 6.2 ns was observed. Prepared YSO:Ce3+ and LSO:Ce3+ powders are suggested to be good candidates for fast imaging or other applications.

Measurement of the X-ray Energy by Using YSO and LYSO Scintillation Crystals

Measurement of the X-ray Energy by Using YSO and LYSO Scintillation Crystals

Energy transfer mechanism between Ce and Tb ions in sol–gel synthesized YSO crystals

The luminescence properties of Tb and Ce in Rare Earth Doped crystalline oxides largely depend on their relative concentrations: by increasing the dopant concentration, the luminescence profile changes from blue to green because of the energy transfer among centers. The kinetic properties of the luminescence of optically excited Terbium–Cerium co-doped Y2SiO5 sol–gel synthesized crystal powders have been investigated as a function of the Tb dopant concentration (Ce content fixed at 1% atomic). The interaction among different Tb emitting centers and their relation with Ce centers was explained within the Inokuti-Hirayama model for a dipole–dipole energy transfer mechanism in the low-middle Tb concentration range whilst the Forster–Dexter model was applied in the middle-high Tb concentration range. The kinetic model allows elucidating the role of sensitizer and activator ion as a function of Tb concentration, successfully estimating the energy transfer mechanism efficiency and calculating the critical Tb concentration.

FLUKA studies of hadron-irradiated scintillating crystals for calorimetry at the High-Luminosity LHC

Calorimetry at the High-Luminosity LHC (HL-LHC) will be performed in a harsh radiation environment with high hadron fluences. The upgraded CMS electromagnetic calorimeter design and suitable scintillating materials are a focus of current research. In this paper, first results using the Monte Carlo simulation program FLUKA are compared to measurements performed with proton-irradiated LYSO, YSO and cerium fluoride crystals. Based on these results, an extrapolation to the behavior of an electromagnetic sampling calorimeter, using one of the inorganic scintillators above as an active medium, is performed for the upgraded CMS experiment at the HL-LHC. Characteristic parameters such as the induced ambient dose, fluence spectra for different particle types and the residual nuclei are studied, and the suitability of these materials for a future calorimeter is surveyed. Particular attention is given to the creation of isotopes in an LYSO-tungsten calorimeter that might contribute a prohibitive background to the measured signal.

Comparative analysis of the scintillation and thermoluminescent properties of Ce-doped LSO and YSO crystals and films

The work is devoted to the comparative analysis of the scintillation and thermoluminescent properties of the undoped and Ce-doped A2SiO5 (A=Lu, Y) orthosilicates, prepared in single crystal form by the Czochralski method and in the form of single crystalline film using the liquid phase epitaxy method. We have found that differences in the methods of material preparation resulted in the significant differences in the scintillation and thermoluminescent properties of undoped and Ce doped YSO and LSO crystals and films. Such differences are caused by the presence or absence of main host defects (first of all, oxygen vacancies and flux related impurities) as emission and trapping centers in YSO and LSO crystals and films.

The Calibration and Simulation of the GRB trigger detector of the Ultra Fast Flash Observatory

The UFFO (Ultra-Fast Flash Observatory) is a GRB detector on board the Lomonosov satellite, to be launched in 2013. The GRB trigger is provided by an X-ray detector, called UBAT (UFFO Burst Alarm & Trigger Telescope), which detects X-rays from the GRB and then triggers to determine the direction of the GRB and then alerts the Slewing Mirror Telescope (SMT) to turn in the direction of the GRB and record the optical photon fluxes. This report details the calibration of the two components: the MAPMTs and the YSO crystals and simulations of the UBAT. The results shows that this design can observe a GRB within a field of view of ±35° and can trigger in a time scale as short as 0.2 – 1.0 s after the appearance of a GRB X-ray spike.

Luminescence and Scintillation Properties of Ce-Doped LYSO and YSO Crystals

The luminescence and gamma-ray detection properties of the new cerium-doped rare-earth scintillator lutetium-yttrium oxyorthosilicate ( Lu1.95Y0.05SiO5:Ce, LYSO:Ce) were investigated and compared to those of cerium-doped yttrium oxyorthosilicate ( Y2SiO5:Ce, YSO:Ce) crystal. The light yield and energy resolution were measured using photomultiplier tube (PMT) readout. The non-proportionality of the light yield and energy resolution versus γ-ray energy were measured and the intrinsic resolution of the crystals was calculated. In spite of significant progress in light yield and luminescence properties, the energy resolution of LYSO:Ce appears to still suffer from an excess variance in the number of scintillation photons. The mass attenuation coefficient of LYSO:Ce and YSO:Ce for 662 keV gamma rays was also measured by transmission method and compared with the theoretical values calculated by WinXCom program.

Spectral measurements of runaway electrons by a scanning probe in the TEXTOR tokamak

Energy spectral measurements of runaway electrons were performed by a scanning probe with high spatial and temporal resolution in the TEXTOR tokamak. The probe consists of ten YSO (Y(2)SiO(5):Ce) crystals, which are shielded by tungsten filters. The probe can resolve electrons with different energies between 4 and 30 MeV. An insertion of the probe to the plasma boundary several times during the discharge allowed spectral measurements of runaway electrons at different minor radial positions at the plasma edge as well as the study of runaway production in time. The high temporal resolution of the probe, 0.05 ms, enabled measurements of runaway electrons not only during low density discharges, but also during plasma disruptions.

Understanding the scintillation efficiency of cerium-doped LSO, LYSO, YSO and LPS crystals from microwave study of photoconductivity and trapping

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.

Thermal lensing in diode-pumped ytterbium Lasers-Part I: theoretical analysis and wavefront measurements

A theoretical and experimental study of thermal lensing in Yb-doped crystals is presented. In this first part, we focus on theoretical considerations and we describe an original technique suitable for thermal lensing measurements in end-pumped materials. We first derive an expression of the temperature distribution with account of absorption saturation and pump beam divergence inside the crystal, and we address a more general discussion on the particularities of quasi-three-level lasers, as far as thermal effects and fracture issues are concerned. The thermal lens was then measured using a simple technique based on a Shack-Hartmann wavefront analyzer, under lasing and nonlasing conditions. We demonstrate that the technique allows precise wavefront measurements even on small spots. Thermal lensing measurements are finally presented in Yb-doped YAG, GGG, YCOB, GdCOB, KGW, and YSO crystals.

Thermal lensing measurements in diode-pumped Yb-doped GdCOB, YCOB, YSO, YAG and KGW

A Shack–Hartmann wavefront sensor was used to measure thermal lensing in diode-end-pumped Yb-doped GdCOB, YCOB, YSO, YAG, and KGW crystals, under lasing or nonlasing conditions. Measured thermal lenses are aberration-free, and their focal lengths under lasing action range from 40 to 140 mm for 5 W of absorbed power. When laser action was inhibited the thermal lens dioptric power was increased significantly in most crystals, supplying evidence that nonradiative mechanisms exist. Reduction of thermal effects in a composite YCOB crystal is also investigated, as well as the dependence of thermal lensing on the emission wavelength in YSO. 2002 Elsevier Science B.V. All rights reserved.

YSO, LSO, GSO and LGSO. A study of energy resolution and nonproportionality

We have studied nonproportionality and intrinsic energy resolution of cerium doped YSO, GSO, LSO and LGSO crystals. While LSO and YSO have similar light output, GSO and LGSO have ca. 70% and 20% lower light output than LSO, respectively. YSO, as a compound containing fairly light elements, was expected to be proportional for light output vs. energy scale, like YAP:Ce. Surprisingly it is almost the same nonproportionality as LSO and GSO. Nonproportionality of YSO is followed by large values of intrinsic energy resolution. The comparison of nonproportionality of YSO-LSO and YAP LuAP pairs indicates that the high proportionality of scintillators is connected with the structure of the crystal and not with the presence of light elements. To our knowledge, this is the first study of nonproportionality and intrinsic resolution for LGSO.