Cerium-doped mixed garnet-type single crystals (GdxY1-x)3Al2Ga3O12 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 5d1 and the lowest triplet 5d3 level, as well as in changing the rates of intracenter and extracenter energy relaxation.
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•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 5d1-5d2 splitting of Ce3+ and increases 5d1-5d3 separation.•Introduction of Y in GAGG lattice increases Ce3+ intracenter relaxation time.•Capture of trapped electrons to nonradiative recombination centers is enhanced in GYAGG.
Coincidence time resolution (CTR) of scintillation detectors based on Ce- and Mg-codoped Gd 3 Al 2 Ga 3 O 12 (GAGG) scintillation crystals and high-density silicon photomultipliers (SiPMs) is shown ...to be 165 ps (full width at half maximum) for 511-keV <inline-formula> <tex-math notation="LaTeX">\gamma </tex-math></inline-formula>-quanta, approaching that achieved by using LSO scintillators. To study the prospective for further improvement of the time resolution, the population of the emitting Ce centers was investigated by optical pump and probe technique using selective photoexcitation and probing by a white light continuum with subpicosecond time resolution. The importance of free electron trapping for excitation transfer to emitting Ce ions was revealed. The influence of transfer delay on the scintillation response time is described, and the dynamics of electron relaxation to the lowest excited level of Ce ion is studied experimentally and analyzed by taking into account intracenter relaxation and relaxation via conduction band. The influence of electron diffusivity on the rise time of the population of the emitting level is described. It is shown that codoping of GAGG:Ce by magnesium even at the level as low as 10 ppm efficiently decreases the scintillator response time by enhancing the electron diffusivity.
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.
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 Lu2SiO5:Ce ...(LSO) and mixed oxyorthosilicate Lu1.6Y0.4SiO5: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.
The influence of co-doping of Gd3Al2GA3O12:Ce (GAGG:Ce) scintillator with magnesium on the rise time of luminescence response was studied in two GAGG:Ce crystals grown in nominally identical ...conditions except of Mg co-doping in one of them. Time-resolved photoluminescence spectroscopy and free carrier absorption techniques were exploited. It is evidenced that the Mg co-doping decreases the rise time down to sub-picosecond domain. Meanwhile, the light yield decreases by ∼20%. Thus, the feasibility of exploitation of the fast rise edge in luminescence response for ultrafast timing in scintillation detectors is demonstrated. The role of Mg impurities in facilitating the excitation transfer to radiative recombination centers is discussed.
•Photoluminescence rise time is compared in GAGG:Ce and GAGG: Ce,Mg.•Sub-picosecond photoluminescence rise time is revealed in GAGG:Ce,Mg.•The role of Mg in facilitating the excitation transfer is discussed.
Dynamics of two distinct bands present in the photoluminescence spectra of non-polar m-plane InGaN/GaN multiple quantum wells (MQWs) was studied by using photoluminescence (PL), cathodoluminescence ...(CL) and differential transmission (DT) spectroscopy. The competition between these bands caused the large shift of peak emission wavelength with increasing excitation. DT measurements allowed attributing the high-energy PL band to the optical transitions between the ground QW states, while the low-energy PL band was assigned to the recombination of localized carriers. CL measurements confirmed the wide spot-to-spot dispersion of deep localized states and suggested that this disorder occurs on a small spatial scale. PL measurements showed that the distribution of localized states is very sensitive both to indium content and to structure parameters. Temperature-dependent PL studies revealed strong carrier-phonon interaction.
•Double-peaked emission spectra in non-polar m-plane InGaN MQWs.•Large energy difference (up to 300 meV) between the two PL bands.•High-energy band attributed to transitions between ground QW states.•Low-energy band attributed to recombination of localized carriers.•Wide and asymmetric PL spectra shape due to strong carrier-phonon interaction.
•SE threshold increases with layer thickness despite reduction of defect density.•Photon dynamics is the main factor controlling SE threshold.•Carrier lifetime weakly affects SE threshold.•Layer ...thickness is a compromise between higher overlap and lower optical losses.
Room-temperature luminescence properties of a dense electron-hole plasma were studied in a set of GaN epilayers with thickness varying from 2 to 25 µm. The stimulated emission threshold was measured by photoluminescence and light-induced transient grating techniques under short-pulse excitation. Both techniques revealed the stimulated emission threshold increase with layer thickness despite the reduction of defect density. Numerical modeling of photoexcited carrier dynamics showed the different roles of carrier and photon populations. The stimulated emission threshold is mainly determined by the photon dynamics with weak influence of nonradiative carrier recombination. Increasing layer thickness results in the decreasing overlap of the gain layer with the optical mode, which reduces the transfer of energy from carriers to photons.
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.
Non-linear absorption spectroscopy in pump and probe configuration has been used to test the population of non-equilibrium carriers in Ce-doped Y3Al5O12 (YAG), Lu3Al5O12 (LuAG), and Gd3AlxGa(5-x)O12 ...(GAGG) crystals with and without codoping by Mg2+ ions. A faster rise time of the induced optical density has been observed in all crystals codoped with Mg with respect to that in Mg-free samples. A significant difference in the time evolution of the differential optical density in GAGG with respect to YAG and LuAG crystals has also been measured. In both GAGG:Ce and GAGG:Ce,Mg an absorption band with maximum in the blue-green range and a decay time of 1.4ps is present. This band is due to the absorption by free electrons before they are trapped or re-captured by Ce3+ ions. A broad absorption band in the yellow-red region with very short rise time and a decay time longer than 150ps has been observed in all the Ce-doped garnets under study and can be attributed to the absorption from the Ce3+ excited states.
We provide evidences that multicomponent garnet-type Ce-doped crystal GAGG (Gd3Al2Ga3O12) is a promising scintillator to be applied in harsh irradiation environments, particularly, in high-energy ...physics experiments and reactor research facilities, where long-term operation is mandatory. Applicability of this scintillator for the upgrade of the detectors at future accelerators with high luminosity like High luminosity LHC is considered and GAGG:Ce with different codopings is compared with Ce-doped oxyorthosilicate crystals, which are currently also strong candidates for such applications. It is shown that the irradiation with 24 GeV protons at a fluence of 5x1014 p/cm2 has no significant effect on optical absorption in the spectral range of the scintillator emission. The contribution of radioisotopes formed in the material by irradiation with protons to the noise pedestal and the noise energy equivalent due to harmful radio-luminescence excited by the radionuclides remains negligible at short gates in collider experiments. Moreover, we show that the irradiation-generated color centers absorb outside the spectral range of Ce luminescence. These centers do not significantly affect the dynamics of nonequilibrium carriers, which is responsible for the timing properties of the scintillator. The density of free carriers decays with a characteristic time of 2 ps, while the decay constant for trapped carriers is ∼50 ns both before and after irradiation.
•Luminescence intensity due to hadron-induced isotopes is similar in GAGG and LSO.•Proton fluence of up to 5x1014 p/cm2 does not affect timing properties of GAGG:Ce.•GAGG is prospective for radiation detectors in harsh radiation environments.
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