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 used in medical imaging. Current PCDs are based on room-temperature semiconductors, such as CdTe and CZT, directly converting incident X-ray photons into electrical pulses. An alternative to this approach is the use of ultrafast scintillators in combination with silicon photomultipliers. A very interesting class of materials potentially suitable for this application is scintillators exhibiting core−valence luminescence (CVL), which typically has a decay time between 0.5 and 2 ns. In this work, two families of Cs−Cl-based compounds, Cs−Zn−Cl and Cs− Mg−Cl, are investigated for their potential application in PCDs. These families of compounds are especially interesting because most members exclusively show CVL at room temperature, resulting in a fast scintillation pulse containing no slow components. Additionally, several approaches to tailor the scintillation properties of these materials, i.e., doping with Br− and Zn²⁺, are studied. Unfortunately, all compounds show a strong drop in the CVL response in the diagnostic energy range (25−150 keV), the operational range of a PCD. PCDs based on these materials will thus be able to handle the high X-ray fluence rate of an imaging task but will not be able to sufficiently discriminate the energies of incident X-ray photons. In addition to the Cs−Zn−Cl and Cs−Mg− Cl compounds, the nonproportional response of the CVL component of BaF₂ is studied utilizing fast digitization of individual scintillation pulses in order to discriminate between processes related to the CVL and self-trapped exciton emission of BaF₂.

Recently, joint replacement surgery is facing significant challenges of patient dissatisfaction and the need for revision procedures. In-situ monitoring of stress stability at the site of artificial joint replacement during postoperative evaluation is important. Mechanoluminescence (ML), a novel “force to light” conversion technology, may be used to monitor such bio-stress within tissues. However, this is hindered by ultraviolet–visible ML emission wavelength, low ML intensity, and high strain response sensitivity. Here, by incorporating Sb3+ ions into Sr3Sn2O7 crystals, a highly strain-responsive material, with ML originating from intrinsic defect emissions is obtained. The Sr3Sn1.98Sb0.02O6.99 film produces detectable ML signals under compressive strain as low as 50 µst in the absence of biological tissue. After pre-irradiating with red light through 15 mm of porcine tissue, ML signals can still be detected through the same tissue thickness. Notably, this material enabled real-time stress imaging through 4 mm of porcine skin during mild finger joint bending. This work presents a novel methodological framework and proposes a new mechanism to defect ML. It offers a fresh perspective for designing high-performance ML materials and lays the foundation for innovative research to enhance the functionality of artificial tissues and joints in living organism.

The defect levels of the 3dq transition metals (TM) within the bandgap of compounds provide compounds with properties that are utilized in e.g. luminescence, lasers, photochromism, batteries, catalysis, semiconductors, biochemistry. Knowledge of the ground-state level locations, or equivalently the charge transition level (CTL) energies, or equivalently the vacuum-referred binding energies (VRBE), is important to understand or engineer performance. Despite 70 years of interest in the topic, understanding and controlling TM defect levels remains elusive. In this work, experimental data, theories developed, progress over time, and current status are reviewed, and new insights are presented. We will start with the classic theory, first for free-ion 3dq TMs and then for TMs in inorganic compounds and organic complexes. The Slater–Condon Fk, the Racah A, B, and C parameters, the crystal field interaction, and the Tanabe–Sugano diagrams will be treated on a tutorial level. An expression reproducing the CTL energies relative to the vacuum level as a function of the number q of electrons in the 3dq TMs will be derived. The expression contains five essential parameters related to the chemical shift, Racah parameters, the nephelauxetic effect, and the crystal field. Data on TMs of different valences in 18 chemical environments are collected from the literature. These are inorganic compounds ranging from wide-band-gap halides (F, Cl, Br), chalcogenides (O, S, Se), small-band-gap II–VI and III–V semiconductors, and two TM organic complexes. All provide octahedral or tetrahedral coordinated sites for the TM. Data from luminescence and absorption spectroscopy, deep-level transient spectroscopy, photocurrents, thermoluminescence, and electrochemistry are translated into CTL energies. Next, the derived expression is used to reproduce the CTL energies, providing the values of the five parameters for each compound. The parameters appear strongly related to each other and change predictably with the valence of the TM and the properties of the environment.

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.

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).

Discovering light dosimeters that can function effectively from liquid nitrogen temperature to 700 K presents significant challenges. Such dosimeters facilitate a range of cutting-edge applications, including anti-counterfeiting measures at low temperature for cryo-preservation. To facilitate such discovery, stacked vacuum referred binding energy diagrams for the LiYGeO4 cluster of crystals have been first constructed. They offer a robust method for controlling both electron and hole trapping depth in the LiYGeO4 cluster of crystals. Wide temperature shifting of Bi2+ and Eu2+ thermoluminescence (TL) glow bands emerges from 200 to 500 K for LiYxLu1-xGeO4:0.01Bi3+ and LiYxLu1-xGeO4:0.01Bi3+, 0.001Eu3+, by changing x, facilitating conduction band tailoring. Wide temperature shifting of Bi4+ TL glow bands emerges from 300 to 700 K for LiYGezSi1-zO4:0.01Bi3+, by tuning z, facilitating valence band tailoring. TL glow band peaks near 135, 185, 232, and 311 K emerge in LiyNa1-yYGeO4: 0.001Bi3+. Particularly, the discovered Bi3+ or/and lanthanide modified LiYGeO4 cluster of crystals exhibit superior charge carrier storage capacity and minimal TL fading properties. For instance, the ratio of TL intensity of the optimized LiYGe0.75Si0.25O4:0.001Bi3+ to that of industrial BaFBr(I):Eu2+ is as high as ∼4. Interestingly, imaging of intense optically driven Bi3+ ultraviolet-A (UVA) luminescence has been validated in 254 nm energized LiY0.25Lu0.75GeO4:0.01Bi3+ with a 100 lux white LED illumination. Together with ZnS:Mn2+, LiTaO3:Bi3+, Sm3+, and Cs2ZrCl6:Sb3+ perovskites, the realization of wide range liquid nitrogen temperature to 700 K Bi3+ thermoluminescence in Bi3+ or/and lanthanide modified LiYGeO4 cluster of crystals with superior charge carrier storage capacity offers promising use for versatile anti-counterfeiting, information storage, and delayed x-ray imaging purposes.

NaI is the most commonly used host lattice for scintillators, which makes it interesting to further improve its scintillation properties. Many alternative activators have been tried instead of the conventionally used Tl+. In this work, Sm2+ is used as an near-infrared emitting activator for NaI to study whether it is suitable for readout with silicon based photodetectors. NaI single crystals (co-)doped with 0-0.2% Tl+ and 0.2%–2% Sm2+ were grown by the vertical Bridgman technique. The emission of the samples was studied under optical and X-ray excitation. It is shown by photoluminescence decay studies that Tl+ works as a sensitiser for Sm2+. The samples indicate the formation of multiple (at least 5) different Sm2+ emission sites. Annealing the samples changes their emission intensity and scintillation properties. NaI:Sm2+ shows great similarities with its Eu2+-doped counterpart. Finally, it is demonstrated that NaI:Sm2+ can be read out with silicon photomultipliers and an energy resolution of 11% has been attained.

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.

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.

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.

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.

Small bandgap scintillators have gained significant attention in recent years. Especially Cs4PbBr6 is an interesting material, mitigating the small Stokes shift-related problem of perovskites like CsPbBr3. In this work, optical and scintillation properties of Cs4PbBr6 single crystals are investigated as a function of temperature, with a detailed focus at 10 K. The Cs4PbBr6 single crystals were grown using the vertical Bridgman method. Due to incongruent melting, CsPbBr3 inclusions are formed, generating a 540 nm emission band. Prepairing Cs4PbBr6 via solid-state synthesis yields CsPbBr3-inclusion-free material, showing no green 540 nm emission band. In Cs4PbBr6 samples with and without CsPbBr3 inclusions, a new emission band at 610 nm ascribed to an unknown defect was found. Based on the presented experiments, an emission mechanism is proposed for Cs4PbBr6. This shows that both defects and CsPbBr3 inclusions play a role in the emission behavior of Cs4PbBr6 but only the CsPbBr3 inclusions are responsible for the 540 nm emission.

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.

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.

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.

Background: Mainly because of its long half-life and despite its scientific relevance, spectroscopic measurements of Lu176 forbidden β decays are very limited and lack formulation of shape factors. A direct precise measurement of its Q value is also presently unreported. In addition, the description of forbidden decays provides interesting challenges for nuclear theory. The comparison of precise experimental results with theoretical calculations for these decays can help to test underlying models and can aid the interpretation of data from other experiments. Purpose: Perform the first precision measurements of Lu176β-decay spectra and attempt the observation of its electron capture decays, as well as perform the first precision direct measurement of the Lu176β-decay Q value. Compare the shape of the precisely determined experimental β spectra to theoretical calculations, and compare the end point energy to that obtained from an independent Q value measurement. Method: The Lu176β-decay spectra measurements and the search for electron capture decays were performed with an experimental setup that employed lutetium-containing scintillator crystals and a NaI(Tl) spectrometer for coincidence counting. The β decay Q value was determined via high-precision Penning trap mass spectrometry (PTMS) with the LEBIT facility at the National Superconducting Cyclotron Laboratory. The β-spectrum calculations were performed within the Fermi theory formalism with nuclear structure effects calculated using a shell model approach. Results: Both β transitions of Lu176 were experimentally observed and corresponding shape factors formulated in their entire energy ranges. The search for electron capture decay branches led to an experimental upper limit of 6.3×10-6 relative to its β decays. The Lu176β-decay and electron capture Q values were measured using PTMS to be 1193.0(6) and 108.9(8) keV, respectively. This enabled precise β end point energies of 596.2(6) and 195.3(6) keV to be determined for the primary and secondary β decays, respectively. The conserved vector current hypothesis was applied to calculate the relativistic vector matrix elements. The β-spectrum shape was shown to significantly depend on the Coulomb displacement energy and on the value of the axial vector coupling constant gA, which was extracted according to different assumptions. Conclusion: The implemented self-scintillation method has provided unmatched observations of Lu176, independently validated by the first direct measurements of its β-decay Q value by Penning trap mass spectrometry. Theoretical study of the main β transition led to the extraction of very different effective gA and log10f values, showing that a high-precision description of this transition would require a realistic nuclear structure with nucleus deformation.

RE₂O₂S:Eu³⁺-Ln³⁺ (RE = La, Gd, Y, Lu; Ln = Pr, Tb) samples were prepared by a solid-state reaction method to develop new red persistent phosphors and to demonstrate the hole detrapping mechanism. All Eu³⁺-singly doped RE₂O₂S show very weak thermoluminescence (TL) glow peaks, while by codoping Pr³⁺ or Tb³⁺ ions, additional strong TL peaks were observed. In the TL spectra and persistent luminescence (PersL) spectra, only Eu³⁺ luminescence lines were observed, but there was no Pr³⁺ and Tb³⁺ luminescence. From the PersL excitation spectra, it is found that PersL is caused after excitation to the charge transfer state of Eu²⁺-S^- in which the hole is in the valence band. These results show that Eu³⁺ acts as a recombination center and Pr³⁺ and Tb³⁺ ions act as hole trap centers. The deeper hole trap depth of Pr³⁺ than that of Tb³⁺ and the RE dependence of hole trap depth are explained using a vacuum referred binding energy diagram considering the nephelauxetic effect. La₂O₂S:Eu³⁺-Pr³⁺ was the best composition among the samples as a persistent phosphor at ambient temperature, showing strong red persistent luminescence in a short time range (>100 mcd/m² for a few seconds).

The benefits of doping Cs₄EuBr₆ and Cs₄EuI₆ with Sm²⁺ are studied for near-infrared scintillator applications. It is shown that undoped Cs₄EuI₆ suffers from a high probability of self-absorption, which is almost completely absent in Cs₄EuI₆:2% Sm. Sm²⁺ doping is also used to gain insight in the migration rate of Eu²⁺ excitations in Cs₄EuBr₆ and Cs₄EuI₆, which shows that concentration quenching is weak, but still significant in the undoped compounds. Both self-absorption and concentration quenching are linked to the spectral overlap of the Eu²⁺ excitation and emission spectra which were studied between 10 K and 300 K. The scintillation characteristics of Cs₄EuI₆:2% Sm is compared to that of the undoped samples. An improvement of energy resolution from 11% to 7.5% is found upon doping Cs₄EuI₆ with 2% Sm and the scintillation decay time shortens from 4.8 s to 3.5 s in samples of around 3 mm in size.

The luminescence properties of Tm2+-doped BaCl2 with an orthorhombic structure have been studied as a function of temperature and compared to other Tm2+-doped chlorides. In addition to the 2F5/2 → 2F7/2 (4f13 → 4f13) line emission, two 4f125d1 → 4f13 band emissions are observed at 20 K that can be ascribed to the spin-allowed (3H6,5d1)S=1/2 → 2F7/2 and spin-forbidden (3H6,5d1)S=3/2 → 2F7/2 transitions. So far, the Tm2+ spin-allowed (3H6,5d1)S=1/2 → 2F7/2 transition has only been identified in Tm2+-doped iodides and some bromides but never before in a Tm2+-doped chloride. Its presence in orthorhombic BaCl2:Tm2+ is explained by the absence of a (3H6,5d1)S=1/2 → (3H6,5d1)S=3/2 energy transfer process. As the temperature increases, both 4f125d1 → 4f13 emissions undergo rapid quenching and are no longer observed at 120 K, resulting in an intensity increase of the 4f13 → 4f13 emission. However, above 100 K, the intensity of the 4f13 → 4f13 emission also decreases, most likely due to quenching via (3H6,5d1)S=3/2 → 2F7/2 interband crossing, as enabled by the exceptionally large 4f125d1 Stokes shift.