SrI₂:Eu²⁺ and CaI₂:Eu²⁺ are two of the brightest known scintillators, but they both suffer from self-absorption. Their respective undoped isostructural compounds EuI₂ and YbI₂ are not suitable for scintillation due to the additional problem of concentration quenching. These compounds can however be doped with Sm²⁺ to turn them into near-infrared emitting scintillators, with the additional benefit that the self-absorption probability of the Sm²⁺ emission is low. Here, the scintillation properties of SrI₂:1%Sm²⁺, EuI₂:4%Sm²⁺, and YbI₂:1%Sm²⁺ single crystals are assessed which were grown by the vertical Bridgman technique. SrI₂:1%Sm²⁺ and EuI₂:4%Sm²⁺ fall within the ideal wavelength range for detection with silicon based photodetectors and are spectroscopically very similar to each other. However, the key difference is that the scintillation decay time of EuI₂:4%Sm²⁺ is 1.1μs, much shorter than the 1.8μs of SrI₂:1%Sm²⁺. Both SrI₂:Sm²⁺ and EuI₂:Sm²⁺ are identified as interesting candidates for further optimisation in the development of near-infrared emitting scintillators.

X-ray photon-counting detectors (PCDs) are a rapidly developing technology. Current PCDs used in medical imaging are based on CdTe, CZT, or Si semiconductor detectors, which directly convert X-ray photons into electrical pulses. An alternative approach is to combine ultrafast scintillators with silicon photomultipliers (SiPMs). Here, an overview is presented of different classes of scintillators, with the aim of assessing their potential application in scintillator-SiPM based indirect X-ray PCDs. To this end, three figures of merit (FOMs) are defined: the pulse intensity, the pulse duration, and the pulse quality. These FOMs quantify how characteristics such as light yield, pulse shape, and energy resolution affect the suitability of scintillators for application in indirect PCDs. These FOMs are based on emissive characteristics; a fourth FOM (ρZ_eff^3.5) is used to also take stopping power into account. Other important properties for the selection process include low self-absorption, low after-glow, possibility to produce sub-mm pitch pixel arrays, and cost-effectiveness. It is shown that material classes with promising emission properties are Ce³⁺- or Pr³⁺-doped materials, near band gap exciton emitters, plastics, and core-valence materials. Possible shortcomings of each of these groups, e.g., suboptimal emission wavelength, nonproportionality, and density, are discussed. Additionally, the engineering approach of quenching the scintillator emission, resulting in a targeted shortening of the decay time, and the possibility of codoping are explored. When selecting and/or engineering a material, it is important to consider not only the characteristics of the scintillator but also relevant SiPM properties, such as recharge time and photodetection efficiency.

Lead halide perovskites are reportedly a very promising group of materials for scintillation due to their fast sub-nanosecond exciton luminescence, small band-gaps, and high theoretical light yield. Unfortunately, they only show emission at cryogenic temperatures. In this work single crystals of CsPbBr₃ and CsPbCl₃ are studied at cryogenic temperatures. Upon comparing the 10 K emission spectra measured under X-ray and UV–vis excitation, a new near-infrared emission was found for both CsPbBr₃ and CsPbCl₃ only present under X-ray excitation. The integral light yields of CsPbBr₃ and CsPbCl₃ at 10 K are estimated to be 34,000 and 2,200 photons/MeV under 40 keV X-ray excitation, respectively. The main components of the light yield of CsPbBr₃ at 10 K are the near band-gap free exciton emission that suffers from self-absorption and the broad near-infrared emission that falls outside the typical detection range of a photo-multiplier tube. Due to the combination of the two aforementioned effects it was not possible to measure a γ-ray pulse height spectrum for CsPbBr₃ at 10 K. Despite all the suitable properties, like the fast decay, a small band-gap, and the positive prospects of 3D perovskite based scintillators, we conclude that these materials perform poorly as scintillation crystals.

Discovering energy storage materials with rationally controlled trapping and de-trapping of electrons and holes upon x-rays, UV-light, or mechanical force stimulation is challenging. Such materials enable promising applications in various fields, for instance in multimode anti-counterfeiting, x-ray imaging, and non-real-time force recording. In this work, photoluminescence spectroscopy, the refined chemical shift model, and thermoluminescence studies will be combined to establish the vacuum referred binding energy (VRBE) diagrams for the LiSc_1-xLu_xGeO₄ family of compounds containing the energy level locations of Bi²⁺, Bi³⁺, and the lanthanides. The established VRBE diagrams are used to rationally develop Bi³⁺ and lanthanides doped LiSc_1-xLu_xGeO₄ storage phosphors and to understand trapping and de-trapping processes of charge carriers with various physical excitation means. The thermoluminescence intensity of x-ray irradiated LiSc_0.25Lu_0.75GeO₄:0.001Bi³⁺,0.001Eu³⁺ is about two times higher than that of the state-of-the-art x-ray storage phosphor BaFBr(I):Eu²⁺. Particularly, a force induced charge carrier storage phenomenon appears in Eu³⁺ co-doped LiSc_1-xLu_xGeO₄. Proof-of-concept non-real-time force recording, anti-counterfeiting, and x-ray imaging applications will be demonstrated. This work not only deepens our understanding of the capturing and de-trapping processes of electrons and holes with various physical excitation sources, but can also trigger scientists to rationally discover new storage phosphors by exploiting the VRBEs of bismuth and lanthanide levels.

Discovering bismuth based smart materials that can respond to thermal, mechanical, and wide range X-ray to infrared photon excitation remains a challenge. Such materials have various uses like in advanced information encryption. In this work, valence state change between Bi²⁺, Bi³⁺, and Bi⁴⁺, and the dual role of Bi³⁺ in trapping electrons and holes have been studied in Bi³⁺ or/and Ln³⁺ (Ln=Tb or Pr) doped LiScGeO₄ family of compounds by vacuum referred binding energy (VRBE) diagram construction, thermoluminescence, and spectroscopy. Electron release from Bi²⁺ has been evidenced. It can be used to experimentally determine the VRBE in the Bi^{2+ 2}P_1/2 ground state and to realize Bi³⁺ negative quenching luminescence. Particularly, a new force induced charge carrier storage phenomenon has been discovered for non-real-time force recording. Wide range of emission tailorable afterglow, unique Bi³⁺ ultraviolet-A, white, and infrared afterglow have been demonstrated by using Bi³⁺ as a hole trapping and recombination center and using energy transfer processes from Bi³⁺ to Tb³⁺, Pr³⁺, Dy³⁺, or Cr³⁺. Proof-of-concept advanced anti-counterfeiting, information encryption, and X-ray imaging will be demonstrated. This work not only develops smart storage phosphors, but more importantly unravels the valence change between Bi²⁺, Bi³⁺, or Bi⁴⁺ and how it can affect the trapping and release of charge carriers with thermal, optical, or mechanical excitation. This work therefore can promote the discovery and development of Bi³⁺ based smart materials for various applications.

Thermoluminescence (TL) often involves the liberation of a charge carrier (an electron or a hole) from a charge carrier trapping centre into the conduction band (CB) or the valence band (VB) with subsequent recombination with a counter charge carrier at a luminescence centre. TL glow peak analysis can provide the energy ΔE_t needed to liberate such charge carrier which then defines the location of the charge transition levels (CTL) of the carrier trapping centres below the CB-bottom or above the VB-top. The temperature at the maximum of the TL glow peak changes 3–4 K per 0.01 eV change in ΔE_t thus providing an extremely sensitive probe of energy changes in CTLs. This work collects and reviews data on glow peaks due to electron or hole release from lanthanide dopants in 36 different inorganic compounds. To compare results from different literature sources, data were always re-analysed using the same method that is solely based on the temperature at the maximum of the glow peak. The changes in ΔE_t along the lanthanides series provides insight at the sub 0.1 eV level on the changes in CTL energies. We will use a compound-dependent parameter to account for the nephelauxetic effect and a compound dependent parameter to account for lattice relaxation around the lanthanide. Together with information from lanthanide luminescence spectroscopy, the vacuum referred binding energy (VRBE) diagram will be constructed for each compound. The lanthanide electron or hole trap depth read from the VRBE scheme will be compared with that derived from the TL glow peak. Surprisingly good agreement will be demonstrated.

The U-value defined as the energy difference between the Eu^4+/3+ and Eu^3+/2+ charge transition levels (CTLs) is the most important parameter in constructing vacuum referred binding energy diagrams (VRBEs) with all the lanthanide CTLs with respect to the vacuum level of energy. The parameter is difficult to determine from experiment and the aim of this work is to establish a method to estimate the U-value from the average electronegativity of the cations in the compound. Since the U-value is controlled by the same physical processes, i.e., covalence and anion polarizability, as the centroid shift ϵ_c of the Ce³⁺ 5d configuration, one may estimate the U-value from that centroid shift. That method provides already good values for U for about 175 different compounds. Those U-values are compared with the average cation electronegativity χ_av, and relations will be established from which the U-value can be estimated with about ±0.1 eV accuracy from just the composition of the compound. It can be applied to all types of stoichiometric inorganic compounds like the halides (F, Cl, Br, I), chalcogenides (O, S, Se), and nitrides (N). The U-value complemented with the bandgap and the energy needed for electron transfer from the valence band top to a trivalent lanthanide dopant is then sufficient to construct a VRBE diagram with all lanthanide levels with respect to the vacuum level and the host valence and conduction bands.

Recent research activity on Sm²⁺-doped compounds has significantly increased the amount of available data on 4f⁵5d → 4f⁶ decay times. This enabled the systematic comparison of spectroscopic and time resolved luminescence data to theoretical models describing the interplay between the 4f⁵5d and 4f⁶[⁵D₀] excited states on the observed decay time. A Boltzmann distribution between the population of the excited states is assumed, introducing a dependence of the observed 4f⁵5d → 4f⁶ decay time on the energy gap between the 4f⁵5d and 4f⁶[⁵D₀] levels and temperature. The model is used to interpret the origin of the large variation in reported 4f⁵5d → 4f⁶ decay times through literature, and links their temperature dependence to applications such as luminescence thermometry and near-infrared scintillation. The model is further applied to the analogous situation of close lying 4f^n-15d and 4fⁿ states in Eu²⁺ (⁶P_7/2) and Pr³⁺ (¹S₀).

Location of lanthanide levels in the bandgap, vacuum referred binding energy (VRBE) in the lanthanide ground state and energy of lanthanide charge transition levels (CTLs) are just three different namings for the same concept. A concept of importance for the performance of lanthanide activated compounds. Energy differences of CTLs with the conduction band bottom and valence band top are important when it concerns e.g. lanthanide luminescence, charge carrier trapping, and valence stability. Effect of temperature on CTL energy or VRBE has so far never been addressed despite that luminescence application and thermoluminescence studies may span a temperature range from 10 K to 1000 K. In this work information on the bandgap (or energy of host exciton creation) around 10 K and at RT in compounds is gathered to demonstrate that bandgap decreases by 0.1 eV to 0.3 eV when temperature increases to RT. A similar decrease will be demonstrated for the energy of electron transfer from the VB to a trivalent lanthanide. The findings have consequences for VRBE-diagram construction, i.e. the experimental parameters for such construction should all apply to the same temperature. They also have consequences on how to relate luminescence thermal quenching energy barriers and TL derived electron and hole trap depths with a VRBE diagram. By proper evaluating the effects of temperature, accuracy of VRBE diagrams and consistency with luminescence and thermoluminescence data can be improved.

Due to recent development in detector technology, photon-counting computed tomography (PCCT) has become a rapidly emerging medical imaging technology. Current PCCT systems rely on the direct conversion of X-ray photons into charge pulses, using CdTe, CZT, or Si semiconductor detectors. Indirect detection using ultrafast scintillators coupled to silicon photomultipliers (SiPM) offers a potentially more straightforward and cost-effective alternative. In this work a new 2D perovskite scintillator, benzylamonium lead bromide (BZA)₂PbBr₄, is experimentally characterised as function of temperature. The material exhibits a 4.2 ns decay time under X-ray excitation at room temperature and a light yield of 3700 photons/MeV. The simulation tool developed by Van der Sar et al. was used to model the pulse trains produced by a SiPM-based (BZA)₂PbBr₄ detector. The fast decay time of (BZA)₂PbBr₄ results in outstanding count-rate performance as well as very low statistical fluctuations in the simulated pulses. These features of (BZA)₂PbBr₄, combined with its cost-effective synthesis make (BZA)₂PbBr₄ very promising for PCCT.

Developing X-ray charged dosimeters with excellent charge carrier storage capacity and stability is challenging. Such energy storage dosimeters have fascinating use in developing novel applications, for instance, in radiation detection, advanced multimode anti-counterfeiting, and flexible X-ray imaging of curved objects. Herein, novel LiTaO₃:Ln³⁺,Eu³⁺ (Ln = Tb or Pr) perovskite dosimeters are reported by combining the vacuum referred binding energy (VRBE) diagram of LiTaO₃ and the optimization of dopant's concentration and compound synthesis condition. Based on the VRBE diagram prediction, charge carrier capturing and de-trapping processes in Eu³⁺ and/or Ln³⁺ (Ln = Tb or Pr) doped LiTaO₃ will be studied to unravel the role of Eu³⁺ as a good electron trapping centre and to discover a record storage phosphor. The ratios of the thermoluminescence intensity of the optimized LiTaO₃:0.005Tb³⁺,0.001Eu³⁺ to that of the state-of-the-art BaFBr(I):Eu²⁺, Al₂O₃:C, or NaLuF₄:Tb³⁺ are 5.2, 8.8, or 2.8, respectively. The charge carriers can be stored more than 1000 h in LiTaO₃:0.005Tb³⁺,0.001Eu³⁺. Proof-of-concept anti-counterfeiting application will be demonstrated by combining the colour-tailorable photoluminescence, afterglow, thermally, or optically stimulated luminescence in LiTaO₃:0.005Tb³⁺,xEu³⁺ and LiTaO₃:0.005Pr³⁺,0.001Eu³⁺. Multimode anti-counterfeiting application will be proposed by combining a high absolute X-ray scintillation light yield of 19000 ± 1800 ph/MeV of LiTaO₃:0.005Tb³⁺,0.001Eu³⁺. Proof-of-concept flexible X-ray imaging application will be demonstrated by using the optimized LiTaO₃:0.005Tb³⁺, 0.001Eu³⁺ dispersed in a silicone gel film.

LaBr₃:Ce³⁺ is a compound with excellent scintillation properties, but its ultraviolet emission does not match well with the detection efficiency curves of silicon based photodetectors. In this work, Sm²⁺ is studied as an activator for LaBr₃ as its near-infrared emission can be detected with close to 100% efficiency by such photodetectors. LaBr₃:Sm²⁺ single crystals were grown with and without co-doping of Ce³⁺ or Pr³⁺. The samples were studied by means of X-ray excited and photoluminescence spectroscopy at temperatures between 10 K and 300 K. Their spectroscopic properties are compared to LaBr₃:Ce³⁺ and LaBr₃:Eu²⁺. The effect of using Ce³⁺ or Pr³⁺ as scintillation sensitiser for Sm²⁺ is assessed. It is found that energy transfer from host to Sm²⁺ greatly improves upon Ce³⁺ co-doping, but the quenching temperature of the Sm²⁺ emission decreases. The quenching mechanism of both the Ce³⁺ and Sm²⁺ emission in LaBr₃ is elaborated on. Furthermore, the effect of charge compensating defects on the light yield and spectroscopic properties is discussed.