Hot intraband luminescence (IBL) was observed under excitation by high-power (120keV, 10A/cm2) pulsed electron beam, low-energy (10keV) and low-current (2μA) continuous electron beam as well as ...pulsed x-rays. Thus, the fundamental possibility of IBL excitation by x-rays was confirmed, and for most studied materials the absence of the dependence of IBL spectral shape on excitation energy was revealed. For the first time, the intraband luminescence was monitored under low-power excitation, which confirmed the absence of a power threshold of its mechanism and completely ruled out the role of excitation density effects in IBL formation. The data obtained allow the prediction that the IBL can be excited by single photons of 511-keV energy, which is required for enhancing scintillation time resolution in TOF-PET.
Characteristic dimensions and evolution times of regions of secondary electronic excitations created by the interaction of ionizing radiation with matter cannot be measured directly. At the same time ...these are essential parameters both for engineering of nanostructured composite materials defining optimal layer thickness and nanoparticles radii and for the development of optimized scintillators. The paper demonstrates how such spatial and temporal data can be extracted from luminescence decay kinetics excited by vacuum ultraviolet (VUV) and X-ray photons at modern sources of synchrotron radiation MAX IV and PETRA III. Specific features of energy-band structure of self-activated crystal CeF3 are discussed, and its potential for a super-fast detection of ionizing radiation evaluated. Diffusion-controlled dipole–dipole interaction of Frenkel excitons is demonstrated to account well for the luminescence non-exponential decay kinetics providing information on the scales of excited regions created by photons of different energy. For 20 eV photons the radius of excited regions is estimated to be 10 nm, and for 200 eV photons it increases to 18 nm. Effective radius of excited regions of complicated shape created by 19 keV is as large as 80 nm and the diffusion length of Frenkel excitons over radiative time is 14 nm.
Action recognition is a challenging task that plays an important role in many robotic systems, which highly depend on visual input feeds. However, due to privacy concerns, it is important to find a ...method which can recognise actions without using visual feed. In this paper, we propose a concept for detecting actions while preserving the test subject’s privacy. Our proposed method relies only on recording the temporal evolution of light pulses scattered back from the scene. Such data trace to record one action contains a sequence of one-dimensional arrays of voltage values acquired by a single-pixel detector at 1 GHz repetition rate. Information about both the distance to the object and its shape are embedded in the traces. We apply machine learning in the form of recurrent neural networks for data analysis and demonstrate successful action recognition. The experimental results show that our proposed method could achieve on average 96.47 % accuracy on the actions walking forward, walking backwards, sitting down, standing up and waving hand, using recurrent neural network.
Се-doped (Pb,Gd)3(Al,Ga)5O12 single crystalline garnet films were grown using liquid-phase epitaxy from four series of supercooled PbO–B2O3-based melt solutions on Gd3Ga5O12 and Gd3Al2.26Ga2.74O12 ...single crystal substrates. The optical and scintillation properties of the epitaxial garnet films were studied. The 5d-4f emission of Ce3+ ions within 450–650 nm was observed. The highest pulsed cathodoluminescence yield and scintillation yield values under 133Ba excitation for the Pb0.01Ce0.02Gd2.97Al3.13Ga1.87O12 film were 43,100 photons/MeV and 20,000 photons/MeV, respectively. The pulsed cathodoluminescence decay times of the film were 1.8 (1%), 24 (25%), and 60 ns (74%), and the scintillation decay times were 3.9 (7%) and 43.6 ns (93%). Because of the rapid decay and high light yield, Се-doped (Pb,Gd)3(Al,Ga)5O12 garnet films can be used in X-ray scintillators for different applications, such as homeland security.
Cathodoluminescence yield of hot intraband luminescence (IBL) under ∼100 keV pulsed electron excitation was evaluated in several halide and oxide compounds. Its values lie in the range of ...5–35 ph/MeV, which is in accordance with the general assumptions on the mechanism of IBL. The expected inverse correlation between the IBL yield and highest phonon energies was found for binary compounds, therefore the IBL yield dependence on charge carrier thermalisation rate was experimentally confirmed. Consequently, CsI was found to have the highest IBL yield (33 ph/MeV) compared to other studied crystals due to its lowest phonon energy among them. The studied sample of pure CsI shows both cathodoluminescence and scintillation yields significantly higher than the value frequently cited earlier (2000 ph/MeV), pointing out that its intrinsic scintillation properties might be currently underestimated. CsI shows good coincidence time resolution (CTR) up to 110 ps FWHM in a TOF-PET-like geometry, which surpasses all the materials studied so far except the much more expensive scintillators from the lutetium-yttrium silicates family (CTR values up to 73 ps). Therefore, CsI can be considered as a promising material for low-cost TOF-PET machines. For complex (ternary) compounds the phonon energies and IBL yield are not noticeably correlated, and the highest spectral yield (at 2.5 eV) among the studied compounds is shown by Na2Mo2O7.
The paper studies the influence of co-doping with alkali metal ions (Na+, Mg2+) on fast interconfigurational 5d-4f luminescence in LiSrPO4 doped with Pr3+. Na+ and Mg2+ ions were introduced in order ...to provide charge compensation and modify the electronic configuration of the materials. Luminescent properties of the powder samples were investigated using time-resolved pulsed cathodoluminescence and luminescence spectroscopy in the UV-VUV spectral region. The obtained results were compared with those obtained earlier on LiSrPO4:Pr3+ without co-dopants. With introduction of the co-dopants, slight changes in relative luminescence intensity and decay time of Pr3+ ion 4f15d1-4f2 radiative transitions were found. The study revealed a complex interplay between defects of crystalline structure and impurity ions leading to the peculiarities in energy relaxation processes. Energy transfer leading to the delayed recombination processes is responsible for significant afterglow in Pr3+4f15d1-4f2 emission.
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•LiSrPO4:Pr3+,Na+/Mg2+ crystalline powders were synthesized via solid state reaction.•Emission and decay kinetics were studied upon UV-VUV and cathode ray excitation.•5d-4f Pr3+ emission shows fast decay (τ < 20 ns) with secondary slower components.•Defects of unknown nature negatively influence the relaxation processes.•Na+ and Mg2+ as co-dopants almost insignificantly improve luminescence properties.
The diamond composite materials with embedded yttrium-aluminum garnet doped with cerium (YAG:Ce) nanoparticles have been produced by microwave plasma-assisted chemical vapor deposition (CVD). ...Nanoparticles were synthesized by co-precipitation from an aqueous solution while CVD diamond served as the transparent matrix with high thermal conductivity, hardness, and resistivity to radiation damage. The composite films show high-intensity X-ray luminescence (XRL) with a broad band peaking around the wavelength of 550 nm (5d → 4f transition in Ce3+ ion), and a narrow peak of silicon-vacancy (Si–V) centers at 738 nm. The characteristic decay time was measured at τCe < 50 ns for cerium emission and at τSiV∼1 ns for Si–V centers. Excitation spectra of both luminescence types measured near yttrium K-edge (17.1 keV) show a strong correlation, attributed to the excitation of Si–V luminescence by photoelectrons ejected from YAG:Ce nanoparticles. The discovered phenomenon suggests a new way to control X-ray visualization by luminescent diamond composites for fast X-ray detectors and screens.
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Luminescent characteristics of K3Lu(PO4)2:Pr3+ (1 and 5 mol.%) microcrystalline powders, a promising optical material for scintillation applications, were investigated using various experimental ...techniques. The material shows emission features connected with both high intensity interconfigurational 4f15d→4f2 transitions (broad UV emission bands) and intraconfigurational 4f2→4f2 transitions (weak emission lines in the visible range). The output of X-ray excited 4f15d→4f2 emission of Pr3+ increases with a temperature rise from 90 K to room temperature and higher depending on the Pr3+ ions concentration. The high 5% concentration of Pr3+ ions is found to be favourable for the stabilization of a monoclinic phase (P21/m space group) over a trigonal one (P3‾ space group) while emission properties of the material reveal that a phase transition occurs at higher temperatures. Decay kinetics of Pr3+ 4f15d→4f2 emission are recorded upon excitation with high repetition rate X-ray synchrotron excitation and pulse cathode ray excitation. Issues related to a non-exponential decay of luminescence and presence of slow decay components are discussed in terms of energy transfer dynamics. The presence of defects was revealed with thermoluminescence measurements and these are suggested to be the mainly responsible for delayed recombination of charge carriers on the Pr3+ 4f15d states. Some peculiarities of host-to-impurity energy transfer are discussed.
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•K3Lu(PO4)2 doped with 1%Pr3+ and 5%Pr3+ were synthesized by a solid state reaction.•High Pr3+ concentration stabilizes K3Lu(PO4)2:Pr3+ monoclinic phase over trigonal one.•Recombination of host charge carriers on Pr3+ 5 d states is delayed by defects.•High Pr3+ concentration is favourable for efficient host-to-impurity energy transfer.
•Crystalline K2GeF6 particles were synthesized by co-precipitation.•Fast cross- and intraband luminescence with decay times 300–500 ps was observed.•Transitions between valence sub-bands (Ge 4s,4p, F ...2p) cause intraband luminescence.•The decay of K 3p cation excitons results in the formation of Ge 4s,4p holes.•Electronic band structure parameters of K2GeF6 were experimentally determined.
Highly crystalline K2GeF6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra.
CdWO4:Sm single crystals were studied by optical, electron-paramagnetic and thermoluminescence spectroscopy in the temperature range of 4–750 K. A hopping diffusion of self-trapped excitons and ...energy transfer to Sm3+ centres is demonstrated. The mechanism of energy transfer includes the formation of a Sm3+ bound exciton either by direct excitation or in the result of a thermally stimulated hopping diffusion of self-trapped excitons as well as a sequential trapping of charge carriers near the Sm3+ ions. The energy transfer from the perturbed excitons to Sm3+ ions is controlled by an electric dipole-dipole or exchange interaction and it is terminated in the temperature range of 450–650 K due to the thermal destruction of Sm3+-perturbed excitons. In the case of near-surface excitations created in the energy region of high absorption coefficients by 5-eV photons, the Sm3+ emission is additionally quenched by surface-induced quenching centres.
•Energy transfer processes were studied in Sm3+ doped CdWO4 crystals.•Self-trapped excitons are thermally quenched due to hopping diffusion in CdWO4.•Self-trapped excitons transfer energy to Sm3+ ions via Sm3+-bound excitons.•Sm3+-bound excitons are formed by trapping self-trapped excitons or charge carriers.•Energy transfer is terminated due to the destruction of bound excitons at T > 400 K.
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