The quest for new materials for thermoluminescence (TL) and optically stimulated luminescence (OSL) dosimetry continues to be a central line of research in luminescence dosimetry, occupying many groups and investigators, and is the topic of many publications. Nevertheless, it has also been a research area with many pitfalls, slow advances in our understanding of the luminescence processes, and rare improvements over existing materials. Therefore, this paper reviews the status of the field with the goal of addressing some fundamental questions: Is there a need for new luminescence materials for TL/OSL dosimetry? Can these materials be designed and, if so, are there strategies or rules that can be followed? What are the common pitfalls and how can they be avoided? By discussing these questions, we hope to contribute to a more guided approach to the development of new luminescent materials for dosimetry applications.

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This work presents a fresh insight into the excited charges trapping in the Lu₂O₃:Tb,M (M= Ti, Hf) ceramics and their characteristics as storage and/or persistent luminescence phosphors. The results were obtained by applying an exceedingly versatile set of experiments based on thermoluminescence and thermoluminescence excitation spectroscopy and exposed a dual-nature of these materials. In the contrary to the previous research, here we found that at least some of these materials can generate efficient persistent luminescence due to the presence of shallow traps which can be charged only upon specific irradiation conditions – by the spin-forbidden 4f→5d transition of Tb³⁺ around 360 nm and, possibly, the ⁷F₆→⁵D₃ intra-configurational transition of the activator at just slightly longer wavelengths. Besides that, changing the sample charging temperature the efficiency of filling the traps – both deep and shallow – with the 360 nm radiation varied greatly and exposed a very broad distribution of trap energies. Charging with 360 nm radiation at room temperature fills only the shallow traps giving, never reported in Lu₂O₃:Tb,Ti and Lu₂O₃:Tb,Hf, intense persistent luminescence, while at higher temperatures the deep traps are filled. At any temperature, radiation of wavelengths < 320 nm fills almost exclusively deep traps responsible for TL at high temperatures, 230 °C in Lu₂O₃:Tb,Hf and 355 °C in Lu₂O₃:Tb,Ti.

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Luminescence dosimetry is the process of quantifying the absorbed dose of ionizing radiation using detectors that exhibit luminescence. The luminescence intensity scales with energy absorbed from the radiation field. Calibration enables conversion of the luminescence intensity to the quantity of interest, for example the absorbed dose, kerma and personal dose equivalent. The different techniques available — thermoluminescence (TL), optically stimulated luminescence (OSL) and radiophotoluminescence (RPL) — share a common theoretical framework. Alongside applications in radiation protection, including personal dosimetry and area monitoring, luminescence dosimetry is also used in industry, research and medicine. Examples include quality assurance in radiation therapy, mapping of radiation levels in new accelerators, the estimation of ionizing radiation dose to organs in medicine and accidents, and the characterization of the radiation environment in space. The objective of this Primer is to summarize the fundamental concepts of luminescence dosimetry, the main experimental considerations, analysis procedures, typical results, applications and limitations, with an outlook into potential future advances.

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BaAl₂O₄:Eu²⁺,Dy³⁺ is related, both by structure and luminescence, to one of the best persistent luminescent phosphors, SrAl₂O₄:Eu²⁺,Dy³⁺. At room temperature (RT), the green persistent emission of BaAl₂O₄:Eu²⁺,Dy³⁺ remains visible for hours after ceasing irradiation. Similar to SrAl₂O₄, BaAl₂O₄ with hexagonal P6₃ structure, has two M²⁺ sites, but, limited optical activity from the 2nd site is observed in the emission of BaAl₂O₄:Eu²⁺,Dy³⁺ - even at 77 K. Using combined approach of photoluminescence, thermoluminescence (TL), and persistent (excitation) luminescence measurements, the origin and properties of persistent luminescence of BaAl₂O₄:Eu²⁺,Dy³⁺ were studied in detail. Ultraviolet (UV) excited and persistent emission are identical and no contribution from the Eu²⁺ in the high-symmetry Ba site was observed. TL excitation spectra clarified the unstructured conventional excitation spectrum; now it is evident that defects or the Dy³⁺ co-dopant do not contribute to persistent luminescence via direct energy absorption. Mechanisms for persistent luminescence should thus be revised.

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LiLuSiO₄:Ce and LiLuSiO₄:Ce, Tm show very efficient charge carrier storage properties upon beta irradiation after samples have received treatment in vacuum. They outperform the commercial storage phosphor BaFBr(I):Eu²⁺ in many aspects. The influence of the synthesis conditions, Ce and Tm concentration, nonstoichiometry and codoping with Ca, Hf, Al and Ge are reported. Based on the results of the synthesis optimization, thermoluminescence (TL) emission and TL excitation spectra a mechanism of charge carrier transfer, storage, and recombination during irradiation and thermal or optical readout is proposed.

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Background: While alpha microdosimetry dates back a couple of decades, the effects of localized energy deposition of alpha particles are often still unclear since few comparative studies have been performed. Most modern alpha microdosimetry studies rely for large parts on simulations, which negatively impacts both the simplicity of the calculations and the reliability of the results. A novel microdosimetry method based on the Fluorescent Nuclear Track Detector, a versatile tool that can measure individual alpha particles at sub-micron resolution, yielding accurate energy, fluence and dose rate measurements, was introduced to address these issues. Methods: Both the detectors and U87 glioblastoma cell cultures were irradiated using an external Am241 alpha source. The alpha particle tracks measured with a Fluorescent Nuclear Track Detector were used together with high resolution 3D cell geometries images to calculate the nucleus dose distribution in the U87 glioblastoma cells. The experimentally obtained microdosimetry parameters were thereafter applied to simulations of 3D U87 cells cultures (spheroids) with various spatial distributions of isotopes to evaluate the effect of the nucleus dose distribution on the expected cell survival. Results: The new experimental method showed good agreement with the analytically derived nucleus dose distributions. Small differences (<5%) in the relative effectiveness were found for isotopes in the cytoplasm and on the cell membrane versus external irradiation, while isotopes located in the nucleus or on the nuclear membrane showed a substantial increase in relative effectiveness (33 - 51%). Conclusions: The ease-of-use, good accuracy and use of experimentally derived characteristics of the radiation field make this method superior to conventional simulation-based microdosimetry studies. Considering the uncertainties found in alpha radionuclide carriers in-vivo and in-vitro, together with the large contributions from the relative biological effectiveness and the oxygen enhancement ratio, it is expected that only carriers penetrating or surrounding the cell nucleus will substantially benefit from microdosimetry.@en

Lu₂O₃:Pr,Ti storage phosphors were prepared by means of high temperature (1700 °C) sintering both in a reducing atmosphere of the N₂-H₂ mixture (3:1 by volume) and in ambient air. Their thermoluminescent (TL) properties were presented and discussed. Pr singly-doped material showed only very inefficient TL. Ti co-doping boosted the TL efficacy, and the most potent TL was observed for ceramics containing 0.05 mol% of Pr and 0.007 mol% of Ti and made in the reducing atmosphere. Samples prepared in air produced noticeably less intense TL. The glow curves of both materials consisted of one broad asymmetric band with the maximum around 357 °C for the heating rate of 4.7 °C/s. The glow peaks could be fitted with three (reduced samples) or two (air-sintered) components. The latter lacked the high-temperature part of TL compared to the former. T_max-T_stop experiments indicated that the TL is connected with continuous distribution of trap depths, which were estimated to cover the range of ~ 1.7 to 2.3 eV, and their specific values were slightly dependent on the methodology. Anomalous dependence of the TL intensity on the heating rate made the semi-localized transition the likely mechanism affecting the TL properties of Lu₂O₃:Pr,Ti ceramics. The collected data allowed to construct vacuum referred binding energy (VRBE) level scheme with Pr³⁺ and Ti^3+/4+ energy levels in the band gap of Lu₂O₃ host that could explain the TL mechanism in Lu₂O₃:Pr,Ti ceramics.

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It is demonstrate that LiLuSiO₄:Ce,Tm has excellent charge carrier storage properties. Comparative studies are performed with the state-of-the art X-ray storage phosphor BaFBr(I):Eu. The thermoluminescence (TL) intensity of the optimized LiLuSiO₄:Ce,Tm is four times higher than the TL of BaFBr(I):Eu. After 3 h of storage at room temperature (RT) there is still 94% of initial thermoluminescence and only 48% for BaFBr(I):Eu which demonstrates insignificant thermal fading. The signal can be read out by a blue LED and less efficiently by a red laser. It does require higher stimulation energy than in the case of BaFBr(I):Eu. The self-irradiation with a dose rate of 13 µGy h⁻¹ due to ¹⁷⁶Lu isotopes is not a serious drawback for application.

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Structured illumination microscopy (SIM) for the imaging of alpha particle tracks in fluorescent nuclear track detectors (FNTD) was evaluated and compared to confocal laser scanning microscopy (CLSM). FNTDs were irradiated with an external alpha source and imaged using both methodologies. SIM imaging resulted in improved resolution, without increase in scan time. Alpha particle energy estimation based on the track length, direction and intensity produced results in good agreement with the expected alpha particle energy distribution. A pronounced difference was seen in the spatial scattering of alpha particles in the detectors, where SIM showed an almost 50% reduction compared to CLSM. The improved resolution of SIM allows for more detailed studies of the tracks induced by ionising particles. The combination of SIM and FNTDs for alpha radiation paves the way for affordable and fast alpha spectroscopy and dosimetry. Journal compilation

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Thermoluminescent properties and energy storage characteristics of Lu₂O₃:Tb,M (M = Hf, Ti, Nb) sintered ceramics induced by ionizing radiation are presented and discussed. Dose-response dependence, radiation hardness and fading are studied. A linearity of the former exceeding seven orders of magnitude is confirmed for Lu₂O₃:Tb,Hf and Lu₂O₃:Tb,Nb ceramics. Lu₂O₃:Tb,Hf shows the best TL performance and also its fading is the lowest reaching 15% over 7 h and shows tendency to saturate. During the same period of time the Lu₂O₃:Tb,Ti, despite having TL at higher temperatures, losses about 25% of the stored energy and the TL signal of Lu₂O₃:Tb,Nb fades by almost 40% over 7 h. First order TL kinetics is confirmed for all three compositions. A self-dose effect in Lu₂O₃:Tb,Hf due to a natural content of the radioactive isotope (2.6%) is proved to be important for long-time reading of low doses.

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To answer the need for better tools for alpha radiation radiobiology and microdosimetry research, a novel irradiation setup based on a honeycomb collimator, in combination with Fluorescent Nuclear Track Detectors (FNTD) for alpha radiation dosimetry and spectroscopy, was introduced. FNTDs are a novel type of small, crystalline detector that can visualize individual alpha particles and simultaneously measure their location, velocity direction and energy with good accuracy. The performance of FNTDs for alpha radiation dosimetry was evaluated for the first time and the results were compared to extrapolation chamber measurements and simulations. The surface dose rate to water of the irradiation setup for two different honeycomb collimators, measured using FNTDs, agreed with the extrapolation chamber measurements within 6%. The simulations underestimated the surface dose rate to water for the first collimator and overestimated the dose for the second collimator, indicating the sensitivity to manufacturing errors in the collimators of this irradiation setup. The dose homogeneity in the setup was measured using radiochromic film and showed variations of less than 5%, making this setup, in combination with the rich information obtained regarding the spatial, angular and energy distributions of the alpha particles, obtained using the FNTDs, ideal for microdosimetry and radiobiology experiments. The accuracy and ease-of-use of FNTDs in addition to the surface or absorbed dose and fluence of the radiation field indicate that these detectors are prime candidates for research applications in the field of alpha radionuclide therapy.

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We investigated thermally activated ionization and thermally activated crossover as the two possibilities of quenching of 5d luminescence in Pr3+-doped Y3Al5-xGaxO12. Varying the Ga content x gives the control over the relative energy level location of the 5d and 4f2:PJ3 states of Pr3+ and the host conduction band (CB). Temperature-dependent luminescence lifetime measurements show that the 5d luminescence quenching temperature T50% increases up to x=2 and decreases with further increasing Ga content. This peculiar behavior is explained by a unique transition between the two quenching mechanisms which have an opposite dependence of thermal quenching on Ga content. For low Ga content, thermally activated crossover from the 4f5d state to the 4f2(PJ3) states is the operative quenching mechanism. With increasing Ga content, the activation energy for thermally activated crossover becomes larger, as derived from the configuration coordinate diagram, while from the vacuum referred binding energy diagram the activation energy of thermal ionization becomes smaller. Based on these results, we demonstrated that the thermal quenching of Pr3+:5d1-4f luminescence in Y3Al5-xGaxO12 with x=0,1,2 is a thermally activated crossover while for x=3,4,5 it results from the thermal ionization.

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Thermally stimulated luminescence (TSL) is known as a technique used in radiation dosimetry and dating. However, since the luminescence is very sensitive to the defects in a solid, it can also be used in material research. In this review, it is shown how TSL can be used as a research tool to investigate luminescent characteristics and underlying luminescent mechanisms. First, some basic characteristics and a theoretical background of the phenomenon are given. Next, methods and difficulties in extracting trapping parameters are addressed. Then, the instrumentation needed to measure the luminescence, both as a function of temperature and wavelength, is described. Finally, a series of very diverse examples is given to illustrate how TSL has been used in the determination of energy levels of defects, in the research of persistent luminescence phosphors, and in phenomena like band gap engineering, tunnelling, photosynthesis, and thermal quenching. It is concluded that in the field of luminescence spectroscopy, thermally stimulated luminescence has proven to be an experimental technique with unique properties to study defects in solids.@en

Two different charge carrier trapping processes have been investigated in RE2O2S:Ln3+ (RE = La, Gd, Y, and Lu; Ln = Ce, Pr, and Tb) and RE2O2S:M (M = Ti4+ and Eu3+). Cerium, praseodymium and terbium act as recombination centers and hole trapping centers while host intrinsic defects provide the electron trap. The captured electrons released from the intrinsic defects recombine at Ce4+, Pr4+, or Tb4+ via the conduction band. On the other hand, Ti4+ and Eu3+ act as recombination centers and electron trapping centers while host intrinsic defects act as hole trapping centers. For these codopants we find evidence that recombination is by means of hole release instead of electron release. The released holes recombine with the trapped electrons on Ti3+ or Eu2+ and yield broad Ti4+ yellow-red charge transfer (CT) emission or characteristic Eu3+ 4f–4f emission. We will conclude that the afterglow in Y2O2S:Ti4+, Eu3+ is due to hole release instead of more common electron release.@en

Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ as well as Eu³⁺- and Ce³⁺-doped samples have been investigated. The PL spectra show that Eu³⁺ enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce³⁺ gives two broad excitation bands in the range of 200−450 nm. First-principle calculations for Ce³⁺ on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce³⁺ 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce³⁺, Sm³⁺)-codoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce³⁺ and the absence of afterglow in (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂.

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Two different trapping and detrapping processes of chargecarriers have been investigated in GdAlO3:Ce3+,Ln3+(Ln = Pr, Er, Nd, Ho, Dy,Tm, Eu, and Yb) and GdAlO3:Ln3+,RE3+(Ln = Sm, Eu, and Yb; RE = Ce, Pr,and Tb). Cerium is the recombination center and lanthanide codopants act aselectron-trapping centers in GdAlO3:Ce3+,Ln3+.Different lanthanide codopantsgenerate different trap depths. The captured electrons released from thelanthanide recombine at cerium via the conduction band, eventually producingthe broad 5d−4f emission centered at∼360 nm from Ce3+. On the other hand,Sm3+,Eu3+, and Yb3+act as recombination centers, while Ce3+,Pr3+, and Tb3+actas hole-trapping centers in GdAlO3:Ln3+,RE3+. In this situation, wefindevidence that recombination is by means of hole release instead of the morecommonly reported electron release. The trapped holes are released from Pr4+or Tb4+and recombine with the trapped electrons on Sm2+,Eu2+,orYb2+andyield characteristic trivalent emission from Sm3+,Eu3+,orYb3+at∼600,∼617, or∼980 nm, respectively. Lanthanum wasintroduced to engineer the valence band energy and change the trap depth in Gd1−xLaxAlO3:Eu3+,Pr3+andGd1−xLaxAlO3:Eu3+,Tb3+. The results show that the valence band moves upward and the trap depth related to Pr3+or Tb3+decreases.@en

Ceramics of Lu2O3 activated with either Tb3+ or Pr3+ and co-doped with one of the transition metal ions – Ti, Hf or Nb – were investigated for their energy storage properties. Photoluminescence, thermoluminescence (TL) and optically stimulated luminescence spectra were recorded and discussed together with thermoluminescent glow curves. The Pr3+ and Tb3+ ions constitute luminescent centers and participate in the energy storage. However, as the thermoluminescence glow curves showed, the Ti, Hf and Nb co-dopants have a significant impact on glow curve pattern, in particular on the temperature at which the maximum thermoluminescence occurred. For the Tb,Ti and Pr,Ti as well as for the Pr,Nb one main TL peak appeared around 360–370 °C. For other compositions it varied from 210 °C for Tb,Nb, through 230 °C for Tb,Hf to 250 °C for Pr,Hf. In either case additional TL components of lower intensities were observed. They spread over 50–450 °C. The TL data led to the conclusion that the co-dopant – a necessary component to induce a significant energy storage capacity in the materials – may not be directly involved in trapping the excited carriers. Instead, it may rather generate other point defects which, individually or as clusters, would do the work. Energy storage was found to be connected with an appearance of an induced excitation/absorption band spreading over around 350–500 nm and peaking near 400 nm. A prolonged stimulation into this band caused green (for Tb-series) or red (for Pr-series) optically stimulated emission. Such a stimulation allows entirely bleaching out the induced absorption. The optically stimulated luminescence for all members of the Tb-series led to luminescence from both C2 and C3i sites offered by the host lattice. All compositions of the Pr-series produced optically stimulated emissions upon 400 nm radiation only by means of the Pr3+ occupying the C2 site.@en

Y₃Al₅O₁₂(YAG):Ce³⁺ is the most widely applied phosphor in white LEDs (w-LEDs) because of strong blue absorption and efficient yellow luminescence combined with a high stability and thermal quenching temperature, required for the extreme operating conditions in high-power w-LEDs. The high luminescence quenching temperature (∼600 K) has been well established, but surprisingly, the mechanism for temperature quenching has not been elucidated yet. In this report we investigate the possibility of thermal ionization as a cause of this quenching process by measuring thermoluminescence (TL) excitation spectra at various temperatures. In the TL excitation (TLE) spectrum at room temperature there is no Ce³⁺:5d₁ band (the lowest excited 5d level). However, in the TLE spectrum at 573 K, which corresponds to the onset temperature of luminescence quenching, a TLE band due to the Ce³⁺:5d₁ excitation was observed at around 450 nm. On the basis of our observations we conclude that the luminescence quenching of YAG:Ce³⁺ at high temperatures is caused by the thermal ionization and not by the thermally activated cross over to the 4f ground state. The conclusion is confirmed by analysis of the positions of the 5d states of Ce³⁺ relative to the conduction band in the energy band diagram of YAG:Ce³⁺.

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