Stimulated Luminescence Measurements Research Articles

Does charge trapping affect subcritical crack growth behavior of yttria‐stabilized zirconia?

AbstractExposure to ionizing radiation has been known to affect the mechanical properties of solids; however, the exact mechanisms remain debated. In this study, we test the hypothesis that long lived metastable states formed by trapping of charges within defects influence subcritical cracking (SCC). Crack propagation rates were measured in 5 mol% Yttria‐stabilized zirconia samples, with and without prior exposure to Co‐60 gamma radiation (10 kGy absorbed dose). Crack growth was followed in situ by employing a double cantilever beam specimen inside an environmental scanning electron microscope (ESEM). In comparison with the unirradiated samples, an increased energy release rate of ∼10 J/m2 was required to maintain SCC in the irradiated samples conforming to an increase in SCC fracture resistance. Raman and x‐ray studies preclude any phase transformation and volume change due to irradiation; however, there was a significant change in optical absorption characteristics observed as the darkening of the irradiated sample. Thermally and optically stimulated luminescence measurements suggest that sample darkening is caused by metastable states that form due to charge trapping during radiation exposure. A closer examination of the SEM images demonstrates an increased number of microcracks ahead of the main crack in the irradiated specimens. We conclude that charge trapping in defects due to irradiation, and subsequent detrapping during crack propagation by mechanical stresses, initiate the formation of these microcracks. Consequently, energy is consumed during the interactions between the main crack and the developing microcracks, ultimately ensuing an overall increase in fracture resistance in the SCC regime.

Influence of shallow traps on the kinetics of Optically Stimulated Luminescence of the Li and Ce ions co-doped MgB4O7 compound

The presence of point defects associated with shallow traps has been identified as a limiting factor in the design of materials with desirable characteristics for applications such as scintillation and luminescence dosimetry, among others. In this context, this study aims to elucidate the dynamics inherent to the charge carriers in systems containing multiple traps and one single recombination centre. To unravel this complexity, we employed a synergetic approach combining Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) measurements with kinetic models. Specifically, for the Li and Ce co-doped MgB4O7, we verified that the electron population trapped in shallower traps exhibits a slower decay rate under optical stimulation compared to deeper traps. This behaviour led to a delay in the luminescence emission, as a result of charge re-trapping or even competition with other centres. This phenomenon can be attributed to its low photoionization cross-section associated with a high re-trapping rate, providing a rationale for these remarks. In addition, we observed that the shallow traps have a greater optical than thermal probability of being detrapped, indicating that they were not solely detrapped through thermal stimulation. Also, the magnitude of the thermal probability leads us to believe that there is an electron population different from zero in the conduction band at t = 0 (before optical stimulation). Lastly, this study paves the way for a deeper understanding of charge carrier dynamics in systems of multiple traps, improving the performance of existing materials for scintillation and dosimetry.

Pulsed infrared stimulated luminescence measurements for defect pair model studies in feldspars

We present a time-resolved experimental study of feldspar infrared stimulated luminescence (IRSL) as part of a defect pair model investigation of the underlying mechanisms. Within our investigation we recorded ultraviolet emissions that covered a diverse range of alkali feldspars together with a limited selection of plagioclase feldspars. In analysing our measurements, we find that each IRSL pulse response is characterised by a combination of fast and slow components, the relative proportions of which vary across the range of feldspars we tested. We interpret the presence of the components as providing evidence as to the dual roles played by high mobility electrons and by the less mobile alkali ions that are released during the underlying physical processes. Consistent with earlier data, we find further that the luminescent sensitivities vary widely across the range of feldspars we tested.

Zero‐Dimensional Gua3SbCl6 Crystals as Intrinsically Reabsorption‐Free Scintillators for Radiation Detection

AbstractThe search for efficient, re‐absorption‐free scintillators has recently focused the attention on antimony‐based halides, which exhibit largely Stokes shifted luminescence due to radiative recombination of excitons self‐trapped (STE) in strongly Jahn–Teller distorted Sb3+ color centers. Here, the synthesis of a hybrid structure is reported with chemical formula (C13H14N3)3SbCl6 consisting of spatially isolated [SbCl6]3− octahedra separated by organic N,N'‐diphenylguanidinium (Gua) molecules. The optical properties of this material are mainly determined by the inorganic component and are characterized by a pronounced Stokes shift of ≈1.3 eV and a room‐temperature photoluminescence (PL) efficiency of up to 85%. Remarkably, highly efficient radioluminescence (RL) is observed with scintillation light yields of ≈2000 ph MeV−1 using both soft X‐rays and a 124 keV gamma source. Temperature‐dependent PL and RL measurements confirm the minor role of non‐radiative channels, which are completely suppressed below 100 K. Thermally stimulated luminescence measurements suggest that the traps in Gua3SbCl6 crystals have a significantly large energy depth distribution below the absorbing state.

A Bayesian hierarchical age model for optical dating of single grains of quartz

In optical dating, the last time that a sample of sediment was exposed to sunlight is determined by dividing its equivalent dose (De) by the dose rate. For single-grain dating, the sample De is based on the statistical analysis of the distribution of De values estimated for individual grains, whereas the dose rate is usually determined from measurements of the environmental radioactivity of the bulk sample, together with allowances for radiation sources internal to the grains and cosmic rays. Conventionally, the De and dose rate are measured and analysed separately to produce an estimate of the depositional age of a sample, but this approach may result in loss of information because distributions of single-grain De values are influenced by several factors. Existing statistical models do not incorporate all the key information contributing to age estimation, such as the pattern and scale of dispersion of single-grain De values and dose rates, the associated measurement uncertainties, the effect of natural variability among grains, and the outlier probabilities of De and dose rate estimates. Here we propose an empirical Bayesian hierarchical age model (BHAM) for optical dating of quartz samples that incorporates the above information to estimate their depositional ages. The BHAM is based on the implementation of standardised growth curve and LnTn methods to integrate information on the full distribution of single-grain De values, sources of measurement uncertainty, beta-dose heterogeneity (observed or modelled), and detection of outliers. We present the results of validation tests using data sets of optically stimulated luminescence measurements and dose rates obtained for quartz samples dated previously from Denisova Cave (Russia) and for simulated samples. We conclude that the BHAM represents a robust and flexible approach to dealing with data for single grains of quartz within a Bayesian hierarchical framework and is suitable for application to sediments deposited in a variety of depositional settings.

Thermoluminescence and Optically Stimulated Luminescence properties of in-vitro aged dental porcelain

Among the materials applied in dentistry, porcelain is the most predominant one, as it has been used for dental restorations for decades. Towards the Thermoluminescence (TL) investigation of dental porcelain, and its capability to act as a personal accident dosimeter, the present work further examines -through a series of measurement protocols-the physical mechanisms responsible for the isothermal TL signal of its main dosimetric TL peak. Moreover, the influence of light on the TL of the same main peak is studied by means of Optically Stimulated Luminescence (OSL) measurements, while the origin of the OSL signal is further investigated. Finally, the existence of very deep traps of the material is examined and discussed via specific protocols.

Red persistent luminescent Sr3Al2O5₋xSxCl2:Eu2+, Tm3+ phosphors based on lattice engineering

Research and development of persistent luminous phosphors for longer wavelengths, especially for red, is relatively limited, despite significant achievements in persistent luminescent materials for blue and green afterglow. This study uses high-temperature solid phase reactions to successfully prepare S2− doped Sr3Al2O5Cl2:Eu2+, Tm3+ red phosphors. After incorporating S2− into the original O lattice site, Rietveld's refinement of the prepared samples revealed that the occupancy of Tm3+ in the host lattice was optimized. The photoluminescence spectra of the samples showed that the emission is significantly redshifted from 610 to 630 nm as S2− content increases. The main reason for the redshift behavior is that incorporating S2− into the lattice increases crystal field splitting, positively affecting the centroid shift. Additionally, the afterglow of the S2− doped samples are enhanced, with stronger initial brightness and increased duration. This can be attributed to deeper trap energy levels and higher trap concentrations created after incorporating S2−, as confirmed by thermally stimulated luminescence measurements. Consequently, long-lasting red phosphors with brighter afterglow and longer duration were obtained by S2−doping. Possible mechanisms and detailed processes are presented and explained.

Optical and Scintillation Properties of Record-Efficiency CdTe Nanoplatelets toward Radiation Detection Applications.

Colloidal CdTe nanoplatelets featuring a large absorption coefficient and ultrafast tunable luminescence coupled with heavy-metal-based composition present themselves as highly desirable candidates for radiation detection technologies. Historically, however, these nanoplatelets have suffered from poor emission efficiency, hindering progress in exploring their technological potential. Here, we report the synthesis of CdTe nanoplatelets possessing a record emission efficiency of 9%. This enables us to investigate their fundamental photophysics using ultrafast transient absorption, temperature-controlled photoluminescence, and radioluminescence measurements, elucidating the origins of exciton- and defect-related phenomena under both optical and ionizing excitation. For the first time in CdTe nanoplatelets, we report the cumulative effects of a giant oscillator strength transition and exciton fine structure. Simultaneously, thermally stimulated luminescence measurements reveal the presence of both shallow and deep trap states and allow us to disclose the trapping and detrapping dynamics and their influence on the scintillation properties.

The pathways of electronic excitation back energy transfer processes (BET) in novel Eu3+ heterocyclic 1,3-diketonates bearing a perfluorinated moiety

A series of novel brightly emitting Eu3+ coordination compounds with 1,3-diketones bearing a thiophene moiety and a linear CnF2n+1 chain (n = 1–8) were synthesized and fully characterized. The dependence of back energy transfer (BET) processes on the fluorinated chain length of the ligand environment was investigated in detail. The rates of radiative, non-radiative, and BET processes were estimated by a complete set of spectroscopic methods, including temperature stimulated luminescence measurements. It was demonstrated that fluorination provided a remarkable increase in the photoluminescence quantum yield up to 58% along with an increase in sensitization efficiency from 45 to 86%. It was explained by the reduction of non-radiative relaxation on molecular oscillators. The probability of the BET process has a trend to decrease from 500 to 100 s−1 upon an extension of the diketone fluorinated chain. Moreover, the back energy transfer process occurs from the resonant level of Eu3+ ion to ligand triplet state T1 through activation energy barrier ΔEA between potential energy surfaces of T1 and 5D0 states. It was found that extension of the fluorinated chain in ligand alter the activation energy and frequency factor of BET. The influence of the both parameters on BET was studied. The results obtained give a fundamental basis for further rational design of highly efficient luminescent compounds of Eu3+ ions.

Molecular library of OLED host materials—Evaluating the multiscale simulation workflow

Amorphous small-molecule organic materials are utilized in organic light emitting diodes (OLEDs), with device performance relying on appropriate chemical design. Due to the vast number of contending materials, a symbiotic experimental and simulation approach would be greatly beneficial in linking chemical structure to macroscopic material properties. We review simulation approaches proposed for predicting macroscopic properties. We then present a library of OLED hosts, containing input files, results of simulations, and experimentally measured references of quantities relevant to OLED materials. We find that there is a linear proportionality between simulated and measured glass transition temperatures, despite a quantitative disagreement. Computed ionization energies are in excellent agreement with the ultraviolet photoelectron and photoemission spectroscopy in air measurements. We also observe a linear correlation between calculated electron affinities and ionization energies and cyclic voltammetry measurements. Computed energetic disorder correlates well with thermally stimulated luminescence measurements and charge mobilities agree remarkably well with space charge–limited current measurements. For the studied host materials, we find that the energetic disorder has the greatest impact on the charge carrier mobility. Our library helps to swiftly evaluate properties of new OLED materials by providing well-defined structural building blocks. The library is public and open for improvements. We envision the library expanding and the workflow providing guidance for future OLED material design.

Uncertainty evaluation for organ dose assessment with optically stimulated luminescence measurements on mobile phone resistors after a radiological incident

A radiological incident may result in members of the public being exposed to high doses of ionizing radiation. Rapid triage and a rough dose assessment is then an important component of the process of identifying people requiring immediate medical attention, supporting clinical observations and widening the scope to categorize those individuals who are potentially exposed but are pre-symptomatic. Optically stimulated luminescence on mobile phone resistors is well established in this context. Previous studies have shown good dosimetric properties and dose reconstruction protocols were optimized in the MULTIBIODOSE project (Bassinet et al., 2014).Evaluation of the full uncertainty budget and the detection limit is of fundamental importance for any personal dosimetry technique. In this work it was investigated how different factors influence the organ dose assessment with optically stimulated luminescence on mobile phone resistors, how one can correct for these, and how much they contribute to the uncertainty. The investigated factors include calibration, zero dose signal, reproducibility, dose response, fading, red light exposure, storage temperature, energy response and conversion from resistor dose to organ dose depending also on exposure scenario and mobile phone position.The uncertainty and detection limit were found to depend strongly on the delay between incident exposure and measurement and knowledge of the incident scenario. The uncertainty also depends strongly on the dose level. Detection limits of the order of 100 mGy were found, which is more than sufficient for triage. For doses around the detection limit the uncertainties related to the zero dose signal and reproducibility are dominant, while for higher doses of the order of grays the uncertainties related to fading and conversion from resistor to organ doses are dominant. For the higher doses the uncertainties are around 50% (k = 2) for known incident scenarios and up to more than 100% (k = 2) for unknown scenarios.Based on the results from this work recommendations are given to further decrease the uncertainty and detection limit for this technique.

Thermal annealing and transformation of dimer F centers in neutron-irradiated Al2O3 single crystals

The precise study of the thermal annealing of the F2-type dimer defects, being under discussion in the literature for a long time and responsible for the number of absorption bands below 4.5 eV, has been performed in corundum single crystals irradiated by fast neutrons with a fluence of 6.9 × 1018n/cm2. The Gaussian components of the radiation-induced optical absorption with the maxima at 4.08, 3.45 and 2.75 eV have been considered as a measure of the F2, F2+and F22+centers, respectively. In contrast to the F and F+ centers, the concentration of which continuously decreases at the sample heating up to 1100 K, the concentration of dimer defects with different charge states passes the increasing stages above 500 K starting from the F22+centers. The tentative mechanisms of such rise of the F22+centers as well as of the subsequent transformation/rise of dimer centers, F22+→F2+→F2at 650-800 K are considered. The possible sources of carriers needed for the recharging of dimer centers are also analysed on the basis of thermally stimulated luminescence measurements up to ~850 K.

Fire Suppression Impacts on Fuels and Fire Intensity in the Western U.S.: Insights from Archaeological Luminescence Dating in Northern New Mexico

Here, we show that the last century of fire suppression in the western U.S. has resulted in fire intensities that are unique over more than 900 years of record in ponderosa pine forests (Pinus ponderosa). Specifically, we use the heat-sensitive luminescence signal of archaeological ceramics and tree-ring fire histories to show that a recent fire during mild weather conditions was more intense than anything experienced in centuries of frequent wildfires. We support this with a particularly robust set of optically stimulated luminescence measurements on pottery from an archaeological site in northern New Mexico. The heating effects of an October 2012 CE prescribed fire reset the luminescence signal in all 12 surface samples of archaeological ceramics, whereas none of the 10 samples exposed to at least 14 previous fires (1696–1893 CE) revealed any evidence of past thermal impact. This was true regardless of the fire behavior contexts of the 2012 CE samples (crown, surface, and smoldering fires). It suggests that the fuel characteristics from fire suppression at this site have no analog during the 550 years since the depopulation of this site or the 350 years of preceding occupation of the forested landscape of this region.

TL -OSL properties of nanosized BaMg2Al6Si9O30:Ce3+/ Dy3+ phosphor

In this article we have reported Ce3+ / Dy3+ activated BaMg2Al6Si9O30 nanosized phosphor synthesized by the combustion method. Prepared phosphors was initially irradiated by Sr-Y90β-emitting source and then it was used for the Thermoluminescence- Optically Stimulated Luminescence (TL-OSL) measurement. Here, BaMg2Al6Si9O30:Ce3+/ Dy3+ phosphor exhibits an isolated broad TL glow curve at around 2010 C. And further, it was well resolved into two sub-bands to estimate the trap depth and TL parameters of the individual glow curve. Along with this, we have carried out the OSL measurement of BaMg2Al6Si9O30:0.1Ce3+,yDy3+ phosphor which is well fitted into third order exponential. The micro and nanostructure of the phosphors product has been confirmed from SEM, TEM analysis. Again the phase purity of the materials was confirmed by XRD, which has hexagonal structure. Also diffraction pattern of the prepared phosphor is indexed well using theoretical tools of XRD. Hence from the investigation of TL-OSL properties, it is seen that synthesized thermoluminescent materials may be a promising candidate for the irradiation of foods and seeds.

A method for routinely monitoring the reproducibility of thermal pretreatment prior to optically stimulated luminescence measurements

Thermal pretreatments are used prior to optically stimulated luminescence (OSL) measurements in methods of dose determination, but the reproducibility of such heating appears to be poor within laboratories, and between laboratories. Monitoring the position of the 110 °C thermoluminescence (TL) peak in quartz can be used to routinely assess the reproducibility of heating within a repeated set of measurements on an aliquot (e.g. during the single aliquot regenerative dose protocol) providing feedback on whether changes in sample preparation can improve reproducibility. Poor reproducibility between measurements from a single aliquot are likely to arise from grains between the hotplate and sample holder. Variability between aliquots may also be affected by deformed sample holders. Differences between readers may also be caused by differences in the calibration of the hotplate. Reproducibility of heating can be improved within runs, and between instruments, by taking care to avoid loose grains, by removing deformed sample holders, and if necessary by calibrating hotplates. It is suggested that individual aliquots should only be accepted if the apparent 110 °C TL peak temperature within a sequence of measurements varies by 5 °C or less. Improving the reproducibility of thermal treatment is important in reducing scatter in equivalent dose determinations based on OSL from quartz, but also for signals such as TT-OSL from quartz, and the various feldspar luminescence signals. Temperature control is also critical for accurate kinetic analyses.

Evaluation of thermoluminescence trapping parameters in Li2B4O7 co-doped with Ag+ and Gd3+ using various experimental techniques

The aim of this study is to determine the thermoluminescence (TL) kinetic parameters, including the activation energy (E), frequency factor (s) and order of kinetics (b), for the traps of silver and gadolinium co-doped lithium tetraborate (Li2B4O7:Ag,Gd) phosphor. Four different experimental techniques were applied, including Fractional Glow Technique (FGT), Computerized Glow Curve Deconvolution (CGCD), as well as both Isothermal TL (ITL) and Optically Stimulated Luminescence (OSL) measurements at various stimulation temperatures. The TL glow curve of the phosphor was determined to be composed of two single peaks with delocalization temperatures around 80 °C and 360 °C with a heating rate of 2 °C/s after 10 Gy beta dose. Moreover, a composite TL signal is observed within the temperature region between 110 °C and 320 °C. Both the deconvolution analysis as well as the FGT partially indicates that this composite signal consists of two overlapping TL glow peaks along with a satellite TL peak of suppressed intensity. The reusability of the phosphor was studied up to 20 experimental cycles using beta dose values of 0.5, 5 and 10 Gy to support the FGT procedure. The plot of activation energy vs measured temperature yielded a smooth continuum, with the presence of three not so prominent plateau regions. On the contrary, the CGCD analysis indicated the presence of five TL peaks with increasing activation energy values ranging between 0.63 and 1.56 eV. The ITL results indicate that for the constituents of the composite TL signal within 110 °C and 320 °C, the activation energy is less than 0.5 eV, while the corresponding OSL results of the same signal indicate an activation energy value of 0.83 eV. Based on the results of the present study, (a) the continuum shape of the E vs Tfin plot of the FGT analysis and (b) the behavior of the normalized ITL decay curves provide adequate arguments for the existence of a tunneling recombination pathway in the phosphor. The argumentation is also supported by recent related literature.

Luminescence and Scintillation Properties of Novel Disodium Dimolybdate (Na2Mo2O7) Single Crystal

The luminescence and scintillation properties of novel Na2Mo2O7 (NMO) crystals grown by two different techniques, the conventional Czochralski and the low-thermal-gradient Czochralski techniques, were studied. The transmittances of the NMO crystals were measured at room temperature. The luminescence properties and decay times of the crystals from room temperature 300 to 10 K were measured by using an LED diode emitting at the wavelength of 280 nm. Very weak luminescence was observed at the room temperature. The luminescence, however, was enhanced significantly at lower temperatures. The crystals showed a red emission peak of 663 nm at 10 K. The scintillation decay time was $2~\mu \text{s}$ at room temperature and was increased to $\sim 750~\mu \text{s}$ at 10 K. Low-temperature thermally stimulated luminescence measurements were carried out to investigate the presence of trap centers in the grown crystal. The scintillation light yield of the scintillator was measured by a photon counting technique using a 90Sr beta source in the same temperature range from 300 to 10 K. The light yields of the NMO crystals grown at two different institutes were also compared with a standard CaMoO4 crystal.

Photoluminescence, scintillation properties and trap states of La2Si2O7Ce single crystal

Single crystal of La2Si2O7:Ce (LaPS:Ce) was successfully grown by floating zone method for the first time. The structure of as grown sample is confirmed to be monoclinic with space group P21/c by x-ray diffraction and energy dispersive spectra. Its photoluminescence (PL) and scintillation properties of LaPS:Ce crystal were characterized through vacuum ultra-violet excitation and emission spectra, PL decay curve, x-ray excitation luminescence spectrum, γ-ray multi-channel spectrum and afterglow spectrum. Emission peaks of this material are around 3.5 eV and 3.3 eV, corresponding to the typical transition 5d → 4f of Ce3+. The PL decay time of LaPS:Ce is 29.3 ns. The light yield of the sample is determined to be about 5400 ph MeV−1 with a low afterglow level. The trap states in LaPS:Ce are also studied through the thermally stimulated luminescence measurement.

Characterization and some fundamental features of Optically Stimulated Luminescence measurements of silver activated lithium tetraborate

A new lithium tetraborate (Li2B4O7 or abbreviated as LTB) material was produced by adding various concentrations of Ag impurities to allow better luminescent properties using the solution combustion synthesis (SCS) method. The formation of single phase LTB was confirmed using X-ray Diffraction (XRD) data and Scanning Electron Microscopy (SEM) analysis indicated the existence of a tetragonal crystalline domain. Two broad band emissions located at ∼ 272 nm (near UV region) and 526 nm (green region) were observed from room temperature photoluminescence (PL) under 205 nm excitation The synthesized material consisted of polycrystalline LTB with 1 wt% Ag (abbreviated herein as LTB:Ag) exhibits considerable thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) which is several times more sensitive to beta radiation than the other concentrations attempted. It was determined that the OSL signal has been a collection of three component signals. A step-preheating procedure to investigate the depth of the trapping centers associated with the OSL signal was carried out. We suggest that the TL peak at 200 °C mainly contributes to the OSL signal. It was observed that the total OSL area shows a linear dose response for beta doses ranging from 1 to 100 Gy. The minimum detectable dose (MDD) value was found to be around 3 mGy using the total OSL area. Under optimum conditions (irradiation with beta-rays), the reproducibility of total OSL area was determined with a −3% deviation at the end of the 9th irradiation-blue light stimulation-readout cycle. The dark storage stability of the total OSL signals was investigated and fading of the total OSL area was found to be approximately 25% after one week. The trap depth corresponding to the OSL signal was found to be 0.99 eV and 0.94 eV using various heating rate and isothermal annealing methods, respectively. Finally, silver doped lithium tetraborate is shown to have promise as an optically stimulated luminescent dosimeter, particularly in medical and personal applications.

Cryogenic scintillation properties of n-type GaAs for the direct detection of MeV/c2 dark matter

This paper is the first report of n-type GaAs as a cryogenic scintillation radiation detector for the detection of electron recoils from interacting dark matter (DM) particles in the poorly explored MeV/c2 mass range. Seven GaAs samples from two commercial suppliers and with different silicon and boron concentrations were studied for their low temperature optical and scintillation properties. All samples are n-type even at low temperatures and exhibit emission between silicon donors and boron acceptors that peaks at 1.33 eV (930 nm). The lowest excitation band peaks at 1.44 eV (860 nm), and the overlap between the emission and excitation bands is small. The X-ray excited luminosities range from 7 to 43 photons/keV. Thermally stimulated luminescence measurements show that n-type GaAs does not accumulate metastable radiative states that could cause afterglow. Further development and use with cryogenic photodetectors promises a remarkable combination of large target size, ultra-low backgrounds, and a sensitivity to electron recoils of a few eV that would be produced by DM particles as light as a few MeV/c2.