Positron Lifetime Research Articles

Upconversion photoluminescence of Er-doped Bi$_{4}$Ti$_{3}$O$_{12}$ ceramics enhanced by vacancy clusters revealed by positron annihilation spectroscopy

Abstract Doping of rare earth elements into Bi$_{4}$Ti$_{3}$O$_{12}$ can significantly enhance the upconversion photoluminescence (UCPL) properties, but its structure-property relationship is still unclear. In this work, Er-doped bismuth titanate Bi$_{4-x}$Er$_{x}$Ti$_{3}$O$_{12}$ ($x$=0, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics were synthesized via solid-state reaction method. The X-ray diffraction analysis confirmed the orthorhombic crystalline structure of the Bi$_{4-x}$Er$_{x}$Ti$_{3}$O$_{12}$ ceramics without any secondary phases. Experiments and calculations of positron annihilation spectroscopy were carried out to characterize their defect structure. The comparison between the experimental and calculated lifetime revealed that vacancy clusters were the main defects in the ceramics. The increase of the intensity of the second positron lifetime component (I$_{2}$) of Bi$_{3.5}$Er$_{0.5}$Ti$_{3}$O$_{12}$ ceramics indicated the presence of a high concentration of vacancy clusters. The UCPL spectra shown the sudden enhanced UCPL performance in Bi$_{3.7}$Er$_{0.3}$Ti$_{3}$O$_{12}$ and Bi$_{3.5}$Er$_{0.5}$Ti$_{3}$O$_{12}$ ceramics, which were consistent with the variation of the second positron lifetime component (I$_{2}$). These results indicate that the enhanced UCPL properties are influenced not only by the concentrations of rare earth ions but also by the concentration of vacancy clusters present within the ceramics.

Positronium image of the human brain in vivo.

Positronium is abundantly produced within the molecular voids of a patient's body during positron emission tomography (PET). Its properties dynamically respond to the submolecular architecture of the tissue and the partial pressure of oxygen. Current PET systems record only two annihilation photons and cannot provide information about the positronium lifetime. This study presents the in vivo images of positronium lifetime in a human, for a patient with a glioblastoma brain tumor, by using the dedicated Jagiellonian PET system enabling simultaneous detection of annihilation photons and prompt gamma emitted by a radionuclide. The prompt gamma provides information on the time of positronium formation. The photons from positronium annihilation are used to reconstruct the place and time of its decay. In the presented case study, the determined positron and positronium lifetimes in glioblastoma cells are shorter than those in salivary glands and those in healthy brain tissues, indicating that positronium imaging could be used to diagnose disease in vivo.

First-principles calculations of optical and positron annihilation properties of NV center in 3C-SiC

We studied the optical and positron annihilation properties of NCVSi in 3C-SiC using first-principles calculations. We found that the charge-state transition and the spin-conserving optical transition levels are very close to each other. In order to distinguish between the NV center and intrinsic defect, we calculate in detail the defect formation energies, temperature-dependent positron lifetimes, and electron-positron momentum distribution of these defects in 3C-SiC. We suggest that using positrons technique alone is insufficiently sensitive in identifying the charge-states of NCVSi, necessitating the use of additional characterization methods to address this issue.

Cooling positronium to ultralow velocities with a chirped laser pulse train

When laser radiation is skilfully applied, atoms and molecules can be cooled1–3, allowing the precise measurements and control of quantum systems. This is essential for the fundamental studies of physics as well as practical applications such as precision spectroscopy4–7, ultracold gases with quantum statistical properties8–10 and quantum computing. In laser cooling, atoms are slowed to otherwise unattainable velocities through repeated cycles of laser photon absorption and spontaneous emission in random directions. Simple systems can serve as rigorous testing grounds for fundamental physics—one such case is the purely leptonic positronium11,12, an exotic atom comprising an electron and its antiparticle, the positron. Laser cooling of positronium, however, has hitherto remained unrealized. Here we demonstrate the one-dimensional laser cooling of positronium. An innovative laser system emitting a train of broadband pulses with successively increasing central frequencies was used to overcome major challenges posed by the short positronium lifetime and the effects of Doppler broadening and recoil. One-dimensional chirp cooling was used to cool a portion of the dilute positronium gas to a velocity distribution of approximately 1 K in 100 ns. A major advancement in the field of low-temperature fundamental physics of antimatter, this study on a purely leptonic system complements work on antihydrogen13, a hadron-containing exotic atom. The successful application of laser cooling to positronium affords unique opportunities to rigorously test bound-state quantum electrodynamics and to potentially realize Bose–Einstein condensation14–18 in this matter–antimatter system.

First-principles calculations for intrinsic defects in MgO: Positron lifetime and momentum distribution of electron–positron pairs

We presented first-principles calculations of the formation energies for various charge states of intrinsic defects in magnesium oxide, including VO, VMg, VMg+O, VO+Mg+O, and VMg+O+Mg. These results can be used to predict their thermodynamic stability and detectability of these defects via positron annihilation spectroscopy. We employed two-component density functional theory to compute the positron annihilation properties, incorporating the effects of temperature and spin polarization. We then performed calculations of the momentum distributions of annihilating electron–positron pairs in these intrinsic defects in MgO. Using one-dimensional momentum distributions (Doppler-broadening spectra), we calculated the line-shape parameters Srel and Wrel. Our positron lifetime calculations were compared with experimental data. These research provides insight into the positron annihilation characteristics of defects in MgO.

Analyzing point defect polarization in tungsten and tungsten carbide under high gamma irradiation for radiation shielding applications

In this study, polycrystalline tungsten and tungsten carbide samples were irradiated with gamma rays at energies of 1.17 MeV and 1.33 MeV, respectively. The irradiation was performed using an “MRX-γ-25 M” radiation camera with an activity of approximately 6.54 Gy/s. Doppler positron spectroscopy (DPS) and positron lifetime spectroscopy (PALS) were utilized to investigate the mechanisms of defect formation in the polycrystalline structure of tungsten samples at different doses of gamma irradiation. In the initial tungsten sample, a significant number of free monovacancy (1 V) clusters were present. An increase in the positron lifetime component τ1 was observed with increasing gamma irradiation, accompanied by a decrease in its intensity. The τ2 value (218 ± 2 ps) suggests the presence of divacancies with a considerable intensity (∼20%). As the gamma dose accumulated, a dynamic evolution of structural defects was evident. The tungsten carbide (WC) samples displayed greater plasticity in response to increasing gamma irradiation doses. Changes in the void volume ratio within the samples were recorded as the gamma dose increased, and movement of these voids towards the surface was observed. Additionally, the operational limits of gamma-irradiated tungsten were assessed, determining a functional threshold of up to 3.378 MGy. This study provides valuable insights into the defect dynamics and structural changes in tungsten and tungsten carbide under gamma irradiation, contributing to the understanding of their behavior in radiation environments.

Defect characterizations of N-rich GaNAs ternary alloys

This paper presents a comprehensive analysis of defects in As-diluted GaN alloys. GaNAs crystal layers with an arsenic content of 1.8, 2.9, 4.1, and 5.5 % are grown on GaN buffer layers using the metalorganic vapour-phase epitaxy (MOVPE) method. Since these alloys have potential in electronics due to the modification of the electronic structure caused by As, it is extremely important to determine their quality. Complementary techniques for the characterization of the alloys are used. Densities of screw and edge dislocations are obtained using X-ray diffraction measurements performed, respectively, from the surface and edge of samples. In turn, a population of vacancies is determined by Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron lifetime positron annihilation spectroscopy (VEPALS), which provide the distribution of vacancies as a function of depth. An increase in dislocation density is observed accompanied by a rise in the concentration of vacancy agglomerations and reduction in Ga-vacancy-hydrogen associates in the function of As-content.

Impact of evolution of structural defects on the ferromagnetic and electrical properties of laser deposited Indium-doped SnO2 thin films

Impact of non-magnetic cationic-substitution on the evolution of structural defects and correlated ferromagnetic and electrical properties are investigated in series of pulsed laser deposited Sn1-xInxO2 (0.0 ≤ x ≤ 0.12) thin films. Beyond the nominal doping concentration of 2 at.% (i.e. x > 0.02), Indium (In)-doped SnO2 film switches to exhibit from n-type to p-type electrical conductivity and simultaneously the magnetization (MS) as well as the Curie temperature (TC) of the films increase significantly. Estimated values of ‘MS’ and ‘TC’ are found to achieve as large as 15.21 emu/cm3 and 540 K respectively when In-doping concentration approaches towards x = 0.08 and afterwards tend to decrease abruptly. Various spectroscopic techniques including Positron Annihilation Lifetime Spectroscopy (PALS) have detected the existence of Sn vacancy (VSn) defects within Sn1-xInxO2 films arise as the effect of In-substitution at Sn site (InSn) under O-rich atmosphere. The estimated positron lifetimes and the increase of line-shape S-parameter confirm the rise of VSn defects which serve as the major source of magnetic moments in non-magnetic host SnO2. Besides, InSn defects introduce excess holes within SnO2 lattice and thereby the magnetic spin-spin RKKY interaction between near-by VSn defects are mediated ferromagetically through the localized holes. For x > 0.08, stabilization of various donor-type defects such as Sn interstitial (Sni), indium interstitial (Ini) actually compensates the acceptors that leads to reduce the effective hole density consequently diminishing the strength of ferromagnetism within SnO2. Hence, tuning of such ferromagnetic and semiconducting properties through non-magnetic cationic substitution in transparent conducting oxides can be very promising in the field of next-generation spintronics.

The role of defect charge, crystal chemistry, and crystal structure on positron lifetimes of vacancies in oxides

Density functional theory based positron lifetime (PL) calculations for cation and oxygen monovacancies in a range of oxides-hematite, magnetite, hercynite, and alumina-have been conducted to compare the impact of defect chemistry and crystal structure on the predicted lifetimes. The role of defect charge state has also been examined. A comparison across the same type of crystalline structure but different composition shows that oxygen vacancies only induce a slight increase in the positron-electron overlap and thus barely modify the PL as compared to the bulk. A much more substantial increase of PL is observed for cation monovacancies, regardless of crystal structure or the elemental nature of the vacancy, which we ascribe to an enhanced localization of charge density around the vacant site. The structural and compositional richness of the oxide leads to longer defect PLs, with defected hercynite exhibiting the longest PLs. The charge state of cation monovacancies modifies only by a small percentage the positron localization, relegating to secondary importance the metal defect's oxidation state in modifying the lifetime of positrons within vacancy traps.

Quantification of radicals in aqueous solution by positronium lifetime: an experiment using a clinical PET scanner

Positrons entered into living organisms can form positronium (Ps), a bound state with electrons. Most of the triplet Ps (ortho-Ps) in insulating materials annihilate with electrons in surrounding molecules, and then the ortho-Ps lifetime varies depending on the surrounding electron density. The ortho-Ps lifetime may add new biological information to positron emission tomography (PET) scan information. In order to discuss the feasibility of quantifying (free) radicals in vivo by the Ps lifetime, we used a clinical PET system to make ortho-Ps lifetime measurements in aqueous solutions containing radicals. The results suggested that differences in radical concentrations in aqueous solutions of the order of a few mM could be quantified by the Ps lifetime if the counting statistic of the detection time difference spectra was more than 108 events. This concentration was much higher than the radical concentration generated in the physiological functions of living organisms. Therefore, we concluded that quantification of radicals generated in vivo by using the Ps lifetime is very difficult employing the current technology.

Annealing behaviors of open spaces and gas desorption in chemical vapor deposited SiO2 studied with monoenergetic positron beams

The annealing properties of open spaces in 90-nm-thick SiO2 deposited from tetraethylorthosilicate (TEOS) using plasma-enhanced chemical vapor deposition (PECVD) were studied with monoenergetic positron beams. From the lifetime of positronium (Ps) and an empirical model assuming a spherical open space, the mean diameter of open spaces was estimated to be 0.45 nm for PECVD-SiO2 before annealing. In the annealing temperature range below 350 °C, the size of the open spaces and their concentration increased as the temperature increased. Because initial water desorption from PECVD-SiO2 occurred in this temperature range, the observed increases in the size and concentration of spaces were attributed to the detrapping of water from such regions. Above 400 °C annealing, Ps formation was suppressed due to carrier traps introduced by the desorption of gas incorporated during TEOS decomposition. The size of the open spaces reached its maximum value (0.61 nm) after 800 °C annealing and started to decrease above 900 °C. After 1000 °C annealing, although the size of the spaces was close to that in thermally grown SiO2, their concentration remained low, which was attributed to residual impurities in the SiO2 network.

Enhanced positronium lifetime imaging through two-component reconstruction in time-of-flight positron emission tomography

Positronium lifetime imaging (PLI) is a newly demonstrated technique possible with time-of-flight (TOF) positron emission tomography (PET), capable of producing an image reflecting the lifetime of the positron, more precisely ortho-positronium (o-Ps), before annihilation, in addition to the traditional uptake image of the PET tracer. Due to the limited time resolution of TOF-PET systems and the added complexities in physics and statistics, lifetime image reconstruction presents a challenge. Recently, we described a maximum-likelihood approach for PLI by considering only o-Ps. In real-world scenarios, other populations of positrons that exhibit different lifetimes also exist. This paper introduces a novel two-component model aimed at enhancing the accuracy of o-Ps lifetime images. Through simulation studies, we compare this new model with the existing single-component model and demonstrate its superior performance in accurately capturing complex lifetime distributions.

Contrasting role of Ca2+ on the Gd2TiO5: Eu3+ light emission under charge transfer and f-f band excitation: Interplay of oxygen vacancies and structural distortion

The possibility of generating new lanthanide ion doped inorganic material that could work efficiently towards solid-state lighting was explored. The material studied in this work is Gd2TiO5 doped with Eu3+ which displays the lowest lattice strain, high solubility, and lacks both charge-compensating defects and structural deformation. In this study, the impacts of exchanging Gd with up to 10 mol% of Ca, an optically inactive ion, on the luminescence characteristics are investigated. Only small variations in the positron lifetimes suggest the Ca2+ doping only enhanced oxygen vacancies in the system.. Time resolved photoluminescence suggested that the emission intensity increases with Ca2+content when excited with 395 nm (f-f band) while it reduced when excited with 270 nm (charge transfer). The host to dopant energy transfer is attributed to the charge transfer from O2− to Eu3+ and the creation of oxygen vacancies due to Ca2+ doping reduced the charge transfer intensity. The intensity with excitation at 395 nm is almost doubled upon 10 % Ca doping and is attributed to enhanced distortions around the dopant favouring the direct excitation of Eu3+. The creation of oxygen vacancies via Ca substitution and the resultant distortions around Eu3+ ion have resulted in opposite variations in the emission intensity under excitation using charge transfer band and direct excitation. The study offers the possibility of exploring more methods of creating vacancies and distortions around Eu3+ to induce the spectral shifts.

Probing of CoO-induced nano-sized defects in Li2SO4–MgO–P2O5 glasses: Insights from positron annihilation lifetime spectroscopy

This study investigates the influence of CoO doping on concentration and characteristics of nano-sized cavities in Li₂SO₄-MgO-P₂O₅ glasses using positron annihilation lifetime spectroscopy (PALS). These defects are known to significantly impact the physical properties of glasses. Glasses with compositions of 20 Li₂SO₄-20MgO-(60-x) P₂O₅: x CoO (0 ≤ x ≤ 1.0 mol%), were prepared using melt-quench technique. PALS analysis revealed a strong dependence of positronium (e⁺- e⁻ pair) lifetime on CoO concentration. The analysis of the results further indicated that the fraction of free volume Δf/f0) and cavity radius (R) reached minimum values between 0.6 and 0.8 mol% CoO, but increased with further CoO doping. This suggests that cobalt ions prefer Oh sites, act as network modifiers, and create a larger concentration of free volume defects when CoO is increased beyond 0.8 mol%. Notably, the PALS-derived free volume defect concentration with CoO concentration is found to follow nearly same trend exhibited by the results of optical absorption and AC conductivity studies.

Investigating the impact of gamma irradiation and temperature on vacancy formation and recombination in ZrB2 ceramics using positron annihilation spectroscopy

In the presented work, the mechanism of evolution of defects in the crystalline structure after irradiation with 1500 kGy and 3000 kGy absorption dose at room temperature in a 60Co gamma source was studied, using 43 nm sized ZrB2 crystals. The crystals were characterized by a purity of 99.9%, a powder density of 0.23 g/cm3, a specific surface area of 80 to 120 m2/g, and a hexagonal P6/mmm spatial structure. The dose rate applied was 6.05 Gy/s, with 1.17 and 1.37 MeV energy lines. The effect of particles resulting from the decay of 22Naradioactive isotope β+ with an activity of 10.5 μCi, as well as gamma quanta with an energy of 1.27 MeV, on the core and valence electrons and vacancies in the crystal structure was studied using Positron Annihilation Lifetime Spectroscopy (PALS) and Doppler broadening analysis methods. Additionally, the changes in the S and W parameters, which characterize the distribution of defects within the volume of the ZrB2 crystal, were studied using the Doppler broadening method. For unannealed material, the positron lifetime τ1 component varied between 174±2 ps and 181±1 ps. After annealing, the positron lifetime component τ2 was decreased from 290±3 ps to 226±3 ps, and the intensity component I2 increased from 17.49% to 40% depending on the radiation dose. The calculated values of the positron lifetime τ for one boron vacancy from ABINIT and MIKA packages were found to be 172 ps and 145 ps, respectively. The material was observed to satisfy the necessary functional conditions at gamma doses not exceeding 3000 kGy.

Positronium lifetime measurement using a clinical PET system for tumor hypoxia identification

Positronium (Ps) imaging has been recently spotlighted for its potential applications in nuclear medicine. Among them, we expect that the Ps lifetime can be a sensor for tumor hypoxia; hence, we are aiming to achieve Ps lifetime imaging, Quantum PET (Q-PET), based on whole gamma imaging (WGI). A linear correlation was previously shown between the Ps lifetime and the oxygen partial pressure (pO2) in water by using a bench-top detector system. However, nobody has ever measured the Ps lifetimes for pO2 values of 10 mmHg and 40 mmHg, corresponding to hypoxic cancer cells and healthy tissue cells, respectively. Therefore, this study aimed at measuring the Ps lifetime of 22Na solutions each having such tiny differences in the pO2 value. For possible extension toward imaging, we used VRAIN, a clinical brain-dedicated time-of-flight (TOF) PET system. The pO2 values of the 22Na solutions were adjusted to 0, 10, 40, and 160 (air) mmHg by adjusting the inflow of nitrogen gas. Each 22Na solution was placed near the center of the detector geometry and measured. Three different energy windows (EWs) for 1275 keV gamma detection were compared (1100–1500 keV, 900–1500 keV and 700–1500 keV). Due to the increased number of events, the error values (σ: standard deviation) of the calculated Ps lifetimes decreased as the EW widened. The optimized EW was determined to be 700–1500 keV, which showed the lowest error value. The Ps lifetime values gradually became shorter as the pO2 values increased. The Ps lifetime values at 0 mmHg and 160 mmHg were 1.9389 ± 0.0025 ns and 1.9104 ± 0.0024 ns at the EW of 700–1500 keV. The difference between the lifetime values was more than three times the errors (±3σ), and we considered it was a statistically significant difference. The Ps lifetime values of 10 mmHg and 40 mmHg were 1.9360 ± 0.0026 ns and 1.9291 ± 0.0024 ns at the same condition, and the difference could be distinguished with an error value of more than ± 1σ.

Characterization of Vacancy Defects Using TEM in Heavy-Ion-Irradiated Tungsten Foils

The nature and type of defects formed due to heavy-ion irradiation in tungsten foils are analyzed using transmission electron microscopy. The recrystallized tungsten foils were irradiated by 80 MeV gold ions at room temperature for a fluence of 1.3 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 1014\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{14}$$\\end{document} ions/cm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^2$$\\end{document} that amounts to a net displacement per atom (dpa) of 0.22. The defect structures were analyzed using bright-field and weak-beam dark field imaging at two different depths to understand the depth profile of the defects. It is found that the defect clusters formed during the irradiation, both at the near-surface and at 2 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu $$\\end{document}m depth are of vacancy type that is confirmed by the local strain analysis and supports the findings of vacancy clusters in positron lifetime measurements. The analysis also shows that the dislocation-lines were of pure edge, pure screw and mixed. The fraction of mixed dislocation is found to increase during irradiation at the expense of pure edge and screw dislocations.

SPLIT: Statistical Positronium Lifetime Image Reconstruction via Time-Thresholding.

Positron emission tomography (PET) is a widely utilized medical imaging modality that uses positron-emitting radiotracers to visualize biochemical processes in a living body. The spatiotemporal distribution of a radiotracer is estimated by detecting the coincidence photon pairs generated through positron annihilations. In human tissue, about 40% of the positrons form positroniums prior to the annihilation. The lifetime of these positroniums is influenced by the microenvironment in the tissue and could provide valuable information for better understanding of disease progression and treatment response. Currently, there are few methods available for reconstructing high-resolution lifetime images in practical applications. This paper presents an efficient statistical image reconstruction method for positronium lifetime imaging (PLI). We also analyze the random triple-coincidence events in PLI and propose a correction method for random events, which is essential for real applications. Both simulation and experimental studies demonstrate that the proposed method can produce lifetime images with high numerical accuracy, low variance, and resolution comparable to that of the activity images generated by a PET scanner with currently available time-of-flight resolution.

Positron annihilation spectroscopy evaluation of using proton irradiation as a surrogate for early neutron radiation damage

A positron annihilation spectroscopy investigation of early stages of radiation-induced damage in Chinese f/m (ferritic/martensitic) steel SIMP and strengthened variant of f/m steel ODS (oxide dispersion strengthened) EUROFER was conducted. Studied materials were irradiated by neutrons at the LVR-15 research reactor to 0.12 dpa at an average temperature of 260 ± 20 °C, and implanted by protons at the 6 MV Tandetron Accelerator of the Slovak University of Technology, to 0.0196 dpa at room temperature (RT) and 300 °C. The potential of using proton irradiation as a surrogate for early neutron radiation damage was investigated. A lifetime spectrometer and Doppler spectrometer was used to obtain simultaneously positron lifetime (PALS) and Doppler broadening spectra (DBS), which provided experimental data used to characterize the vacancy-type defect and their interaction with H atoms in the studied materials. The obtained results show an increase of mean positron lifetime (MLT) in the proton-implanted samples at RT when compared with high temperature irradiation. Similar trend was observed when comparing vacancy concentration of different irradiation conditions. The impact of temperature on mobility and recombination of defects, as well as effect of vacancy stabilization by H atoms, has been addressed. The effect of ODS microstructure on results has been examined. A methodology of comparing various irradiation conditions has been proposed, where the vacancy concentrations have been normalized by the dpa values. The potential application of proton implantation for evaluation of early stages of neutron damage was found to be promising.

Transient positron annihilation spectroscopy under light irradiation at liquid nitrogen temperature: evaluation of defect states in single-crystal ZnO

Abstract A time-dependent measurement system has been developed for positron annihilation spectroscopy to study the effects of light irradiation at liquid nitrogen temperature. The system enables the measurement of positron annihilation lifetimes after pulse light irradiation, utilizing various time windows to investigate the transient changes of photo-excited vacancy-type defects. Additionally, this system facilitates coincidence Doppler broadening measurements during light irradiation at liquid nitrogen temperature. The system was successfully employed to analyze changes in positron lifetime and positron annihilation sites in electron-irradiated single-crystalline ZnO under light irradiation.