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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. [ABSTRACT FROM AUTHOR]

Substantially faster scintillators for radiation detectors are currently in demand for future high-luminosity high energy physics experiments and medical imaging devices. To meet this demand, the excitation transfer through Gd-sublattice in Ce-activated garnet-type scintillators is studied by time-resolved cathodoluminescence (CL) and photoluminescence (PL) techniques. The transfer is evidenced in the different decay rate in CL experiments and after resonant photoexcitation of Ce ions and is confirmed by the simulation of excitation transport through gadolinium subsystem and from Gd3+ to Ce3+ ions using the Monte Carlo technique. Energy levels of Gd3+ involved in the transfer are identified. The comparison of the results of time-resolved PL and CL measurements and the simulations of carrier dynamics revealed that the emission decay in Ce-doped Gd-containing garnets is accelerated by aliovalent codoping due to blocking the excitation transfer via gadolinium subsystem. • Matrix-building Gd in Ce-doped garnet scintillators delays excitation transfer. • Both Gd multiplets 6I and 6P are important for excitation transfer in LuGAGG:Ce. • Aliovalent codoping eliminates slow excitation transfer via the Gd subsystem. [ABSTRACT FROM AUTHOR]

Numerical approach to account for the influence of randomly fluctuating potential on carrier mobility in compound semiconductors with compositional disorder is developed and exploited for calculating the heavy hole-defined ambipolar diffusion coefficient in Al x Ga 1- x N layers. The rates of inelastic heavy hole-phonon scattering were obtained by integrating the quasi-classical hole-phonon scattering rates for the virtual crystal over the semiclassical confining potential obtained by using the local landscape method. The calculated dependence of the heavy-hole diffusion coefficient on the aluminum content in Al x Ga 1- x N crystal in a wide compositional range (x = 0 – 1) evidences a substantial influence of localization on the diffusivity of nonequilibrium carriers and is in a good quantitative correspondence with the experimental data obtained in this work by using the light-induced transient grating technique. The adopted Monte Carlo simulation technique also enables diffusivity calculations in other multicomponent compounds (e.g., high-In-content InGaN) and their structures, in particular, those for LEDs and µ-LEDs. [Display omitted] • New approach to calculate carrier diffusivity in disordered compounds is developed. • Dependence of hole diffusion coefficient on Al content in AlGaN is calculated. • The dependence fits well that measured by light-induced transient grating technique. • A modified description of hole-phonon scattering in disordered crystal is suggested. • The simulation technique is suitable for other multicomponent disordered compounds. [ABSTRACT FROM AUTHOR]

Cerium-doped mixed garnet-type single crystals (Gd x Y 1- x) 3 Al 2 Ga 3 O 12 with different yttrium content have been fabricated and studied as a prospective scintillating material enabling improvement of scintillation properties by tuning the composition of the matrix-building crystal. The influence of the matrix composition on the emitting Ce ion is studied using linear and time-resolved nonlinear optical absorption and time-resolved photoluminescence spectroscopy. The study of photoluminescence at resonant excitation revealed a composite origin of Ce3+ emission band. This behavior is interpreted by the contribution of Ce3+ ions located in inequivalent positions expected due to the disorder caused in the garnet-type lattice of a mixed crystal by compositional fluctuations. The substitution of gadolinium by yttrium in the lattice results not only in an emission blue shift and in decreased splitting of the lowest doublet 5d state of Ce3+ but also in an increased separation between the lowest doublet level 5d 1 and the lowest triplet 5d 3 level, as well as in changing the rates of intracenter and extracenter energy relaxation. [Display omitted] • Photoluminescence spectrum is composite due to inequivalent positions of Ce3+ ions. • Decay time of Ce3+ emission decreases from 55 ns at 1.9 eV to 40 ns at 2.5 eV. • Substitution of Gd by Y decreases 5d 1 -5d 2 splitting of Ce3+ and increases 5d 1 -5d 3 separation. • Introduction of Y in GAGG lattice increases Ce3+ intracenter relaxation time. • Capture of trapped electrons to nonradiative recombination centers is enhanced in GYAGG. [ABSTRACT FROM AUTHOR]

A novel technique for testing the timing properties of scintillators is presented. The technique is based on transient absorption (TA) induced in a scintillating material by a selective excitation of the activator ion. A figure of merit to assess the timing properties of scintillators is suggested. This parameter was estimated for a set of cerium doped lutetium–yttrium oxyorthosilicate (LYSO:Ce) bars, which have been fabricated for Barrel Timing Layer sensor of Compact Muon Solenoid detector (CMS BTL) and exhibited different timing properties, and compared with the results obtained by conventional coincidence time resolution (CTR) measurements. The figure of merit applied for the tested bars shows a strong correlation (Pearson's correlation coefficient R = 0.95) with the CTR. These results suggest that the TA technique could be used as an experimental method to expand in a complementary way the extensive qualification procedure of LYSO:Ce crystals that will be performed for the production of the CMS BTL detector. • Coincidence time resolution correlates with activator population rise time. • Population rise time can be probed by transient absorption in femtosecond domain. • Transient absorption can be exploited for testing of scintillator timing properties. [ABSTRACT FROM AUTHOR]

The scintillation properties of compositionally disordered self-activated scintillation materials: (Pb, Ca)WO 4 , (Pb, Sr)WO 4 and, (Pb, Ba)WO 4 are described for a fist time. New family of the scintillation materials has a density more than 7 g/cm3, an effective charge Z eff >70. The scintillation kinetics occurs faster and the light yield (LY) is close to the LY of Bi 4 Ge 3 O 12 (BGO). New materials have a good prospect for application in nuclear instrumentation and medical imaging devises. • Scintillation properties of (Pb,Ca)WO 4 , (Pb,Sr)WO 4 and (Pb,Ba)WO 4 investigated for the first time. • Mixed tungstates possess a higher light yield of scintillation than lead tungstate. • Scintillation kinetics are good enough to obtain high coincidence time resolution. [ABSTRACT FROM AUTHOR]

The development of lead tungstate scintillator with the scintillation decay decreased down to the subnanosecond domain is reported. The material exhibits a scintillation decay with a time constant of 640 ps at a light yield of 7 phe/MeV and a high radiation tolerance to the electromagnetic component of ionizing radiation. It is shown to be an outstanding candidate for dual readout of scintillation and Cherenkov photons in electromagnetic calorimetry at future collider experiments. [ABSTRACT FROM AUTHOR]

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 < 230 ps FWHM for 511 keV 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. [ABSTRACT FROM AUTHOR]

Performance parameters of a composite detection module of Spaghetti type Calorimeter (SPACAL), which is considered for the inner part of the future LHCb Electromagnetic Calorimeter (ECAL) in Upgrade Phase 2, are simulated using GEANT4 software supported by analytical calculations. The accuracy of the timestamp is estimated in both the front and back sections of the composite detector, whereas the energy resolution is estimated for the total signal from the front and back sections of the detector. The rise time of the scintillation response used in the calculations is reconstructed using the results of the coincidence time resolution measurements with annihilation γ -quanta. The time and energy resolutions with PMT readout are compared for Ce-doped garnet- and oxyorthosilicate-type single crystal scintillators without codoping and codoped with aliovalent impurities. [ABSTRACT FROM AUTHOR]

The aim of this work has been to improve the time resolution of radiation detectors for future high-energy physics experiments and medical imaging applications. Ce-doped oxyorthosilicate Lu 2 SiO 5 :Ce (LSO) and mixed oxyorthosilicate Lu 1.6 Y 0.4 SiO 5 :Ce (LYSO) have been investigated as prospective scintillators for such high-time-resolution applications. A differential optical absorption technique with sub-picosecond time resolution upon selective excitation of Ce3+ ions to different excited states has been adopted to study carrier dynamics in these scintillators, and coincidence time resolution measured using 511 keV γ-quanta has been exploited to test their timing properties. A delay in population of the emitting level of Ce3+ has been observed, and is interpreted in terms of electron trapping, which is more pronounced in mixed yttrium-containing LYSO crystals due to composition fluctuations. It is shown that the delay, which affects the luminescence response time, can be eliminated by co-doping of LYSO:Ce with calcium at concentrations as low as 5 ppm. The faster kinetics of electron transfer correlates with a better coincidence time resolution. Thermalization and spatial distribution of non-equilibrium carriers has been studied theoretically to link the results obtained by the time-resolved differential optical absorption technique with the behavior of the non-equilibrium carriers generated by irradiation. • The time resolution of LYSO is more affected by carrier trapping than that of LSO. • Carrier trapping in LYSO is enhanced by fluctuations in the compound composition. • Ca-co-doping, even at a concentration of 5 ppm, improves the response time of LYSO:Ce. • Femtosecond pump-probe spectroscopy is a useful tool for improving Ce-doped scintillators. [ABSTRACT FROM AUTHOR]