This review paper provides a description of direct recombination processes between carriers from traps to luminescent centers, involving the occurrence of a tunneling mechanism between their ...space-correlated localized electronic levels.
The experimental evidences that allow to recognize tunneling recombination in phosphorescence time decay and thermoluminescence experiments are first outlined, and compared with those characteristic of recombination processes involving the transfer of carriers in the delocalized bands prior to their recombination. Simple explanations are also proposed for the phenomenological observations in both athermal and thermally assisted tunneling processes.
The importance of such recombinations in three material classes – scintillators, persistent phosphors, and dosimeters, is then outlined and discussed in relation to the material requirements in their distinct application fields. For each application, numerous literature examples are reported. Finally, the paper is complemented by a brief illustration of literature investigations describing experimental evidences of direct trap-center recombination different from tunneling, mostly based on the inspection of emission spectra and thermoluminescence glow curve shapes.
•Decription of localized, tunneling recombinations from an experimental and materials science oriented point of view.•Characteristics of athermal recombinations.•Athermal tunneling versus distribution of trap levels.•The role of temperature: thermally assisted tunneling.•Influence of tunneling processes in several material classes - scintillators, phosphors, dosimeters.
Abstract
Large Stokes shift fast emitters show a negligible reabsorption of their luminescence, a feature highly desirable for several applications such as fluorescence imaging, solar-light managing, ...and fabricating sensitive scintillating detectors for medical imaging and high-rate high-energy physics experiments. Here we obtain high efficiency luminescence with significant Stokes shift by exploiting fluorescent conjugated acene building blocks arranged in nanocrystals. Two ligands of equal molecular length and connectivity, yet complementary electronic properties, are co-assembled by zirconium oxy-hydroxy clusters, generating crystalline hetero-ligand metal-organic framework (MOF) nanocrystals. The diffusion of singlet excitons within the MOF and the matching of ligands absorption and emission properties enables an ultrafast activation of the low energy emission in the 100 ps time scale. The hybrid nanocrystals show a fluorescence quantum efficiency of ~60% and a Stokes shift as large as 750 meV (~6000 cm
−1
), which suppresses the emission reabsorption also in bulk devices. The fabricated prototypal nanocomposite fast scintillator shows benchmark performances which compete with those of some inorganic and organic commercial systems.
•Neutron/γ–ray discrimination via optical light filtration instead of PSD methods.•Study of the CVL in CLYC: an ultra-fast mechanism emitting in the deep-near UV.•Correlation between the time and the ...wavelength domain of the scintillation light.•Correlation between the RL spectra with the TCSPC technique and a fast PMT results.•LET dependence of the quenching effect on the CVL emission in CLYC.
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7Li enriched Cs2LiYCl6:Ce3+ (CLYC) is a promising inorganic scintillator for real-time γ-ray and fast neutron spectrometry. The neutron/γ-ray discrimination is usually accomplished exploiting the different quenching effects of high Linear Energy Transfer (LET) particles on different scintillation mechanisms, usually by means of the time analysis of the pulse shape. In principle, the emission wavelength information could be used to address the same task. However, a systematic study of the correlations between the CLYC decay time, its radio-luminescence spectrum and the LET of the impinging particle has not yet been performed. We therefore investigated the CLYC scintillation process under neutron and γ-ray irradiation, correlating the time response to the scintillation wavelength spectrum using a 1–inch right cylinder>99% 7Li enriched CLYC. We found that the relative intensity of the Core to Valence Luminescence (CVL) is almost constant with photons in the energy range 20–660 keV, i.e. 0.5–5 keV/μm LET, and is only partially quenched by neutrons. Instead, the direct electron-hole capture scintillation mechanism is completely cut under neutron irradiation. The luminescence in between the deep-Ultraviolet and the Near Ultraviolet region (250–350 nm) might be attributed to both the CVL and the host luminescence, also in thick highly Ce3+-doped crystals.
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.
We investigated the influence of the silica-based glass composition and its synthesis conditions on the oxidation states of luminescent rare earths like Ce, Tb, and Eu dopant ions. It was found that ...the repeated melting of the glass led to the stabilization of rare-earth ions in higher oxidation states; in the case of Ce doping, this caused a significant decrease in the scintillation yield, whereas for Tb doping no significant modifications were noticed. Furthermore, it was concluded that the glass matrix influences the rare-earth oxidation state depending on the ions modifying the glass structure. This effect was clearly observed for the Eu-doped series of Ca, Sr and Ba di-silicate glasses, where the conversion of Eu3+ to Eu2+ was found to be dependent on the distortion of the silica network. The influence of glass crystallization on the rare-earth oxidation state stabilization is also discussed.
•The synthesis effect on Ce, Tb, Eu ions valence state in glasses was studied.•The valence state of Ce, Tb, Eu ions was found to be dependent on glass composition.•The possibility of Eu and Ce ions valence control by glass crystallization was shown.
We use various techniques to study optical and scintillation properties of Ce-doped yttrium aluminum garnet, Y
3Al
5O
12 (YAG:Ce), in the form of a high-quality industrial single crystal. This was ...compared to optical ceramics prepared from YAG:Ce nanopowders. We present experimental data in the areas of optical absorption, radioluminescence, scintillation decay, photoelectron yield, thermally stimulated luminescence and radiation-induced absorption. The results point to an interesting feature—the absence of antisite (Y
Al, i.e. Y at the Al site) defects in optical ceramics. The scintillation decay of the ceramics is faster than that of the single crystal, but its photoelectron yield (measured with 1
μs integration time) is about 30–40% lower. Apart from the photoelectron yield value the YAG:Ce optical ceramic is fully comparable to a high quality industrial YAG:Ce single crystal and can become a competitive scintillator material.
The effect of sintering atmosphere on the optical and structural properties of Cerium doped sol–gel silica was investigated. Two sets of xerogels with Ce concentration from 0 to 10
mol% were prepared ...and densified in different conditions. The effect of a post-densification rapid thermal treatment was also considered. Optical absorption measurements evidenced that in oxidizing conditions Cerium is preferably incorporated inside the silica matrix as Ce
4+ while the trivalent state is favoured by reducing conditions. Transmission electron microscopy images show rare-earth clusters formation whose nature was investigated by means of Raman, EDS and microdiffraction measurements. In order to evaluate the efficiency of the glasses as scintillating materials radio-luminescence measurements were carried out. A numerical fit of RL spectra showed the presence of two components peaking at 2.7 and 3.1
eV. After RTT, the relative intensity of the 3.1
eV component increases with Ce concentration.
Participation of shallow and deep traps in the processes of energy transfer and capture is studied by means of time- resolved emission spectroscopy and thermoluminescence in several groups of the Ce ...3+ and Pr 3+ -doped complex oxide single crystal scintillators. Tunnelling-driven recombination processes are distinguished in all the groups of examined materials: closely spaced electron and hole traps give rise to the t -1 phosphorescence decays at low temperatures in the Ce-doped aluminum garnets and perovskites, while thermally assisted tunneling process is proposed to explain temperature independent trap depth in glow curve peaks within 50-250 degC in Ce-doped lutetium orthosilicates.
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