On luminescence stimulated from deep traps using thermally-assisted time-resolved optical stimulation in α-Al2O3:C

We report a study of charge transfer mechanisms of electrons stimulated optically from very deep traps, also known as donor traps, in α-Al₂O₃:C. The investigations were carried out using thermally-assisted time-resolved optical stimulation, thermoluminescence and by way of residual thermoluminescence from the main electron trap. When the charges are optically stimulated from the deep traps, they are redistributed via the conduction band to the main electron trap and the shallow trap from where they are optically or thermally released for recombination at luminescence centres. The luminescence is strongly quenched at high measurement temperature as evident by very short luminescence lifetimes at these temperatures. The main peak due to residual thermoluminescence is located at a higher temperature than the conventional main peak.     

Real time dose rate measurements with fiber optic probes based on the RL and OSL of beryllium oxide

This work covers the examination of fiber optical probes based on the radioluminescence and real time optically stimulated luminescence of beryllium oxide. Experiments are carried out to determine the fundamental dosimetric and temporal properties of the system and evaluate its suitability for dose rate measurements in brachytherapy and other applications using non-pulsed radiation fields. For this purpose the responses of the radioluminescence and optically stimulated luminescence signal have been investigated in the dose rate range of 20 mGy/h to 3.6 Gy/h and for doses of 1 mGy up to 6 Gy. Furthermore, a new, efficient analysis procedure, the double phase reference summing, is introduced, leading to a real time optically stimulated luminescence signal. This method allows a complete compensation of the stem effect during the measurement. In contrast to previous works, the stimulation of the 1 mm cylindrical beryllium oxide detectors is performed with a symmetric function during irradiation. The investigated dose rates range from 0.3 to 3.6 Gy/h. The real time optically stimulated luminescence signal of beryllium oxide shows a dependency on both the dose rate and the applied dose. To overcome the problem of dose dependency, further experiments using higher stimulation intensities have to follow.     

Radioluminescence of LiF:Mg,Ti induced by 4 MeV electrons

This paper reports on the radioluminescence characteristics of LiF:Mg,Ti induced by 4 MeV linear accelerator electrons. The main emission peak of the luminescence is found at 410 nm, but weaker emission occurs around 320, 360, 460, 520 and 560 nm as well. The relative intensities of the emission bands show a strong dependence on the Mg concentration of the sample. The radioluminescence signal reaches its maximum typically 1.5 ms after the irradiation pulse, and decays with a half-life of 4–6 ms. Both the build-up time and the half-life decreases with increasing Mg concentration. The luminescence yield shows a strong dependence on the irradiation dose.     

Fiber-optic-coupled RbMgF3:Eu for remote radiation dosimetry

A portable fiber optic dosimeter has been developed that incorporates RbMgF₃:Eu²⁺ at the end of a multimode polymer fiber. It uses two stimulation sources and takes advantage of the range of stimulable traps in this compound. We observe radioluminescence (RL) during gamma ray irradiation and show that the low-dose RL increases linearly with increasing dose rate where the minimum detectible dose rate is ∼0.015 μSv/s. We show that pulsed infrared-stimulated (940 nm) optically stimulated luminescence (OSL) can be used for real time dose monitoring. The cumulative dose can be readout after irradiation where a linear OSL dose response was observed when stimulating at 505 nm and the minimum detectable dose is ∼50 μSv.     

The dependence of quartz OSL on stimulation energy

Quartz is the mineral most commonly used for sediment dating. In dating practice, the optically stimulated luminescence (OSL) of quartz is measured mainly using the stimulation light whose wavelength is 470 ± 30 nm. The parameters of traps active in the OSL process are also determined for this stimulation band. The zeroing of the OSL is the fundamental condition of applicability of the luminescence dating for specific sediment and takes place in sunlight whose spectrum differs significantly from the band 470 ± 30 nm. In order to be able to know the course of OSL process in nature, a wider knowledge of the dependency of the trap parameters on the stimulation band is needed. Here the results are presented for the OSL measurements carried out with different wavelengths of stimulation light. For each stimulation band the components of the OSL signal are determined by the fitting procedure, and in this way the wavelength dependence of an individual component is found. The experiment has been repeated for two temperatures of OSL detection – the room temperature, which corresponds to natural conditions, and for 125 °C, which is the temperature usually applied for OSL measurement in dating. Four OSL components are presented in both experimental series. The values of their optical cross-section changes along with stimulation energy and temperature, as it is predicted by the model of OSL process including crystal lattice vibrations.     

Luminescence and ESR of the natural alkali feldspars, extracted from sediments, doped by probe impurities

Natural alkali feldspars extracted from sediments were doped by probe impurities (Eu, Ag, Cu) whose behaviour has been studied in other crystal matrices. On the basis of luminescence and ESR characteristics of the doped minerals, and using analogy with other systems, it is concluded that Eu²⁺ and Ag⁺ centres were created in alkali feldspars. It is established that thermal and optical depopulation of traps, actual for luminescence dating, leads to subsequent electron recombination processes.     

Intraband radioluminescence of LiF crystals

Spectra of high-energy electrons are calculated in the electron-hole ionization-passive region of lithium fluoride crystals for the conditions of intense irradiation by short pulses of accelerated electrons. The yield of intraband radioluminescence of these crystals is estimated in comparison with the yield of this kind of luminescence of more extensively studied NaCl crystals. The calculations demonstrate that the yield of radioluminescence determined by electron transitions in the conduction band of LiF crystals is two orders of magnitude weaker than the yield of analogous luminescence of NaCl crystals. This is explained, first, by special features of the energy band structure and, second, by the form of the energy dependence of the density of states in the conduction band of LiF crystals. The yield of hole-type intraband radioluminescence is estimated for various assumed relations between the widths of the valence and the forbidden bands.     

Real-time luminescence from Al2O3 fiber dosimeters

The real-time luminescence signal from Al₂O₃ single crystal fibers, monitored during simultaneous irradiation and optical stimulation, was investigated using computer simulations and experimental measurements. Both radioluminescence (RL) and optically stimulated luminescence (OSL) signals were studied. The simulations were performed initially using a simple one-trap/one-recombination-center energy band model, and then extended to include shallow and deep electron traps as well. Real-time luminescence experiments were performed for different radiation dose rates and optical stimulation powers using periodic laser stimulation of the samples through a fiber optic cable, and the experimental results were compared with the predictions from the computer simulations. The luminescence signal was observed, both theoretically and experimentally, to increase from its initial value to a steady-state level. The steady-state RL and OSL levels were found to be dependent on dose rate, the steady-state level of the real-time OSL being independent of laser power. It was also shown that the total integrated absorbed dose throughout the irradiation period can be determined by correcting the real-time OSL signal for depletion caused by each laser stimulation pulse. The effects of the shallow and deep traps on the time-dependence of the real-time luminescence signal were studied comparing the experimental data from several Al₂O₃ fibers known to have different trapping state concentrations. The additional traps were found to slow the response of the real-time luminescence such that the time to reach steady state was increased as the additional traps were added.     

Modelling the thermal bleaching of OSL signal in the case of a competition between recombination centres

The thermal bleaching of the optically stimulated luminescence (OSL) has been investigated by computer simulations for a model including three traps and two luminescence centres. The deepest trap is active only during the OSL process. Two other traps are active only during the thermal bleaching. The thermal bleaching effects on the OSL intensity as well as on the OSL curve shape are presented for the wide range of trap and luminescence centre parameters and for the different settings of optical detection window. The conventional OSL curve analysis consisting in decomposition of the OSL curve into first order components is applied to the simulation results and the optical cross section spectra obtained as a result of this analysis are compared with the model assumptions. The simulations show that OSL signal can decrease to undetectable level even when the traps related to this signal are not emptied during thermal bleaching. The residual level of the OSL signal after bleaching process, however, depends strongly on centre parameters and concentrations. The modifications of optical detection spectral window lead to significant changes of bleaching effects. The thermal bleaching influences also the optical cross section spectra obtained as a result of the OSL curve decomposition.     

A new mathematical approximation of sunlight attenuation in rocks for surface luminescence dating

The attenuation of sunlight through different rock surfaces and the thermoluminescence (TL) or Optical stimulated luminescence (OSL) residuals clock resetting derived from sunlight induced eviction of electrons from electron traps, is a prerequisite criterion for potential dating. The modeling of change of residual luminescence as a function of two variables, the solar radiation path length (or depth) and exposure time offers further insight into the dating concept. The double exponential function modeling based on the Lambert-Beer law, valid under certain assumptions, constructed by a quasi-manual equation fails to offer a general and statistically sound expression of the best fit for most rock types. A cumulative log-normal distribution fitting provides a most satisfactory mathematical approximation for marbles, marble schists and granites, where absorption coefficient and residual luminescence parameters are defined per each type of rock or marble quarry. The new model is applied on available data and age determination tests.     

An algorithm for unified analysis on the thermoluminescence glow curve

An algorithm was developed to integrally handle excitation by radiation, relaxation and luminescence by thermal or optical stimulation in thermoluminescence (TL) and optically stimulated luminescence (OSL) processes. This algorithm reflects the mutual interaction between traps through a conduction band. Electrons and holes are created by radiation in the beginning, and these electrons move to the trap through the conduction band. These holes move to the recombination center through a valence band. The ratio of the electrons allocated to each trap differs with the recombination probability and these values also relevant to the process of luminescence. Accordingly, the glow curve can be interpreted by taking the rate of electron–hole pairs created by ionizing radiation as a unique initial condition. This method differs from the conventional method of interpreting the measured glow curve with the initial electron concentration allocated to each trap at the end of irradiation. A program using the Visual Studio's C# subsystem was made to realize such a developed algorithm. To verify this algorithm it was applied to LiF:Mg,Cu,Si. The TL glow curve was deconvoluted with a model of five traps, one deep trap and one recombination center (RC).     

The effects of low ambient temperatures on optically stimulated luminescence (OSL) processes: Relevance to OSL dating of martian sediments

Advances in luminescence dosimetry have made geochronological dating of materials from extreme environments possible through the use of optically stimulated luminescence (OSL) single-aliquot techniques. However, these environments present challenges not always encountered in routine OSL dating, such as sediments that have been stored at low, and possibly highly variable, ambient temperatures. In recent years OSL has been proposed as a method for dating recent depositional events on surfaces of other planets, specifically, Mars. As a result it has become necessary to examine the constraints that may be imposed on the OSL method by the extreme environments of extraterrestrial planetary bodies. In this paper we report on investigations of the possible effects a low storage and/or a low OSL measurement temperature could have on the OSL process and the subsequent results. Pertinent OSL properties include the stability of electron traps, the overall luminescence efficiency, and possible thermal assistance processes. The particular focus of the work is on the potential application of the OSL technique for dating surface sediments on Mars. We report the results of OSL experiments on martian simulant materials, and of generalized computer simulations of potential OSL behavior. It is concluded that the stimulation and irradiation/calibration temperatures need to be maintained fixed throughout the experiment – i.e. the dose estimation process – and that the temperature during OSL stimulation needs to be appreciably higher than the highest temperature experienced during natural irradiation. The consequences of these findings for establishing an OSL protocol and instrument package for dating martian regolith material are discussed.     

A systematic study of the radioluminescence properties of single feldspar grains

Spectral radioluminescence properties of a large number of single grains were measured. The main emission in K-feldspar grains occurs in the infrared range with the same dose characteristic as shown by multi-grain samples. For plagioclases, large differences in spectral radioluminescence between single-grains and multi-grain aliquots were detected. Differences in the saturation of the dose characteristics of the individual emissions have been observed. Consequences for dating application are discussed.     

Effects of 60Co gamma-ray irradiation on the optical properties of natural and synthetic quartz from 85 to 300 K

Optical absorption and luminescence measurements have been made between 85 and 300 K on natural and synthetic quartz during and after ⁶⁰Co gamma-ray irradiation. Absorption vs. dose curves can be resolved into the sum of a saturating exponential and a linear component. At 85 K the slope of the linear component is small, while at 300 K the linear term dominates the growth curves. The coloring induced by irradiation at 85 K decreased in a complex manner during anneal at a linear rate to 300 K. During this annealing process multiple-peak thermoluminescent (TL) glow curves were observed and recorded. The TL emission spectra are described accurately by single Gaussian-shaped bands, whereas the gamma-ray induced luminescence is comprised of several poorly resolved bands. All recorded TL glow curves are characterized by a set of four first order glow peaks between 150 and 220 K. The temperature dependence of radioluminescence intensity is described by a classical model in which a temperature-independent radiative transition competes with a thermally activated non-radiative process.     

The role of opacifiers in the luminescence of mosaic glass: Characterization of the optical properties of cassiterite (SnO2)

Thermoluminescence (TL) and radioluminescence (RL) have been exploited to study the optical properties of a few glass tesserae of the baroque mosaics of the vault of San Pietro cathedral in Rome. Recent results gave evidence that the dosimetric properties of mosaic glass depend on the presence of microcrystals in the glass network. For the samples analyzed in the present study, taking into account their chemical composition and presence and type of crystalline inclusions, it was observed that those opacified with tin-based additives (in particular, cassiterite, tin dioxide) showed higher luminescence sensitivity, being therefore more suitable for dating applications. The correlation between the presence of cassiterite and the possibility of successfully evaluating the equivalent dose has been investigated.     

A Tunneling Model for Afterglow Suppression in CsI:Tl,Sm Scintillation Materials

Combined radioluminescence, afterglow and thermoluminescence experiments on single-crystal samples of co-doped CsI:Tl,Sm suggest that samarium electron traps scavenge electrons from thallium traps and that electrons subsequently released by samarium recombine non-radiatively with trapped holes, thus suppressing afterglow. Experiments on single crystals support the inference that electrons tunnel freely between samarium ions and are trapped preferentially as substitutional Sm⁺ near V_KA(Tl⁺) centers where non-radiative recombination is the rate-limiting step. Afterglow in microcolumnar films of CsI:Tl,Sm is enhanced by inhomogeneities which impede tunneling between samarium ions, but is partly suppressed by annealing.     

GaN深中心Zn的低温瞬态研究

本文研究了GaN:Zn的低温瞬态过程,同时测量了衰减中的时间分辨光谱,从两者测量的结果得出:2.89eV的光致发光寿命为300ns(2K).实验证实,Zn作为发光中心的同时也引起一些非辐射陷阱.2.89eV的瞬态曲线可分为两部分,在短时范围(1μs)内基本上是指数形式,可归结为导带中光激发的电子和束缚在ZnGa受主上空穴的复合.在长时范围(t>>1μs)内瞬态曲线则偏离了指数规律,相当好地符合Becqureal经验公式.     

Thermal and non-radiative transitions of electrons at imperfections in ionic crystals

The theory of non-radiative and thermal transitions of electrons at imperfections in ionic crystals is elaborated. The results obtained are used to confirm Mott's and Gurney's theories on the detachment process in two steps of electrons from F centres during the absorption of light in the F band and to explain theoretically the temperature dependence of quantum efficiency. Good agreement with experiment was obtained. The possibility of the luminescence of alkali halide crystals containing F centres at sufficiently low temperatures was also determined.     

紫外线激发后KCl:Cu+的光激励发光

用固相反应法制备了KCl：Cu^＋多晶样品。经紫外光辐照后，用560nm的光照射能产生近紫外蓝光的光激励发光。研究表明，Cu^＋作为发光中心，经紫外光辐照后并没有产生Cu^＋到Cu^＋的变价，即Cu^＋不是空穴俘获中心，而阴离子空位充当电子陷阱。当激发后的样品受到光激励时，电子与空穴中心或Vk心复合后将能量传递给Cu^＋离子，产生d-s跃迁的特征发射。     

Cr+ centres in mixed polytype ZnS single crystal

The glow peaks of mixed polytype ZnS: Cu, Cr, Cl crystals were identified as produced by three Cr⁺ centres observed by electron paramagnetic resonance (EPR): one cubic (C) and two axial (H₁ and H₂). The characteristics of these centres were partly found by the methods of interrupted thermoluminescence, infrared stimulation, “thermo-EPR” (variation of the EPR signal with heating of a previously excited crystal) and also by decay of glow areas and of the three Cr⁺EPR signals after excitation at different temperatures. Moreover, these measurements showed the important part played by the non-thermal emptying of the Cr⁺ traps. This emptying occurs through photoexcitation of the trapped electrons by the reabsorbed luminescence light or through recombination of these electrons with free holes. Both of these processes diminish the efficiency of phosphorescence when the crystal is cooled; so it was necessary to take them into account when analysing the experimental results. The percentage of H₁ and H₂ found by EPR and luminescence is in agreement with the percentage of the hexagonalicity as found from the birefringence measurements. The crystal field calculation gives account of three centres in such a mixed polytype crystal.