Advances in perovskite-based scintillation materials for gamma-ray spectroscopy: issues and opportunities

Abstract

The accurate detection of radiation is crucial for assessing health risks, ensuring safety, and optimizing its use in various fields including medicine, industry, security, space exploration, and scientific research. As part of the exploration of novel detectors, this review considers the recent progress in halide perovskite scintillators derived from alkali and alkaline earth metal halides, covering high-to low-dimensional configurations. This review begins with the key figures of merit for scintillation and physics of radiation detection, followed by crystal growth techniques. Moreover, it provides a comprehensive overview of various energy ranges in gamma-ray spectroscopy research, emphasizing strategies including dimensional tuning, compositional engineering and use of activators to enhance the scintillator performance. This study reveals that low-dimensional perovskite scintillators generally outperform their higher-dimensional counterparts. Notably, 2D CsBa₂I₅:4% Eu demonstrated exceptional results with a light yield (LY) of 198 000 photons per MeV and an energy resolution (ER) of 1.8% at 1332 keV, which are much better than the values obtained for conventional scintillators. In addition, fine-tuning of the composition in the host structure and incorporating suitable dopants significantly contribute to improving the performance of scintillators. Finally, this investigation identifies certain limitations such as the challenges in crystal growth, limited structural and dopant variety, underreported stability and radiation hardness, and a predominant focus on specific gamma energies, particularly 662 keV. In addition, despite the long decay times of some scintillators, research has yet to fully utilize this advantageous property in imaging systems. Moreover, standardized testing protocols are essential to ensure the authentication of their performance all over the world. Future research should focus on addressing existing challenges to ease the transition of perovskite-based scintillators from laboratory to commercial use.

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1. Introduction

In November 1895, a significant discovery was made by Wilhelm Conrad Roentgen when he noticed the glow of a barium platino-cyanide screen located next to his operating discharge tube.¹ This led to the discovery of a new form of invisible and penetrating radiation, which came to be known as X-ray or ‘Roentgen radiation’, with barium platino-cyanide regarded as the first radiation detector by the scientific community.^1,2 Following his discovery of radioactivity, Becquerel looked for methods to quantify and monitor the radiation emitted by radioactive materials.³ A photographic plate was one of the first tools used to record any type of information from radioactivity. However, the high energy radiation (X-rays and γ-rays) registration process revealed that a basic photographic film was quite ineffective, prompting an immediate exploration for materials capable of converting high energy radiations into visible light to enhance their compatibility with sensitive photographic film-based detectors.⁴ Pupin introduced a CaWO₄ powder, which served this purpose for over 75 years.⁵ Alongside the ZnS-based powders introduced a few years later, the CaWO₄ powder represents the most surviving material utilized for high energy radiation detection.⁴ In 1897, Braun invented the cathode ray tube, a device that converts the energy of an accelerated electron beam into visible light through a phosphor material, which was also a fascinating development in technology.^6,7 The process of this transformation of particle energy into visible light is notably akin to that observed in the scenario of X-rays or γ-rays conversion. At the end of the nineteenth century, two dynamic types of radiation including photons and particles have emerged, and their vast potential for application significantly propelled the advancement of phosphor and scintillator materials for their utilization. Along with the development of scintillator materials, the scientific community has placed emphasis on counting the converted visible photons. During World War II, Samuel Curran and W. Baker developed the first electronic scintillation counter as part of the Manhattan Project.⁸ They utilized an RCA Type 1P21 photomultiplier tube to capture the scintillations induced by alpha particles striking on a silver-activated ZnS screen. The credit for the first electronic alpha scintillation detector is sometimes given to Marietta Blau and B. Dreyfus (1945) or John Coltman and Hugh Marshall (1947) due to the delay in publication of the Manhattan Project.²

In addition, Blau and Dreyfus measured the intensity of alpha radiation using PMT current, whereas Coltman and Marshall recorded pulses.^14,15 Ruby Sherr made the first use of a PMT in an alpha survey meter in 1947.¹⁶ It is impossible to say enough about how important the photomultiplier tube was. It was necessary for many of the scientific methods we use today, such as gamma scintillation spectroscopy, liquid scintillation counting, and thermo-luminescent dosimetry. The advent of the photomultiplier tube catalyzed the emergence of an extensive array of scintillating materials including naphthalene, anthracene, stilbene phenanthrene, NaI, CaF, KI, fluoranthene, fluorine, CaWO₄, and CdWO₄. The chronological history of the research and development of various scintillators is listed in Fig. 1. These scintillators are classified as organic scintillators, inorganic scintillators, and composite scintillators. The latter is produced by combining organic and inorganic materials to harness the advantages of both and produce synergistic effects. Although organic scintillators offer advantages such as chemical versatility, ease of modification and cost-effectiveness, it has a lot of issues including non-linearity of light output, dependence of LY on gas pressure and magnetic field, decrease in LY with time, and vulnerability to foreign elements and radiation damage.^17–20 To overcome the limitations of organic scintillators, the scientific community has adopted inorganic scintillators owing to their high stopping power, hardness to high energy, relatively high LYs and fast response times.²¹ The most used inorganic scintillator is thallium-doped sodium iodide (NaI:Tl) due to its high LY and efficiency in detecting gamma rays and X-rays.^22,23 In addition, cesium iodide (CsI:Tl or Na) is often used in medical imaging and high-energy physics experiments due to its improved scintillation properties.^24,25 Moreover, bismuth germanate (BGO) is also known for its high density and effective atomic number and is used in positron emission tomography.^26–28 Furthermore, zinc sulfide (ZnS) is commonly used in alpha particle detection and in some neutron detectors.^29,30 Finally, gadolinium oxysulfide (Gd₂O₂S) is employed in X-ray imaging and computed tomography scanners.^31,32 Despite being widely used, classic inorganic scintillators have a number of drawbacks that limit their practical applications. For instance, some have low LY (such as CdWO₄), others have significant afterglow (such as CsI:Tl), and many are hygroscopic (such as NaI:Tl). Additionally, although semiconductor detectors have a higher energy resolution (ER) than scintillation-based detectors, they are quite expensive.³³ This emphasizes how important it is to develop or explore new materials with better scintillation qualities to replace conventional materials for ionizing radiation detection.

Fig. 1 Timeline of the discovery of various scintillators.^4,9–13

The application of perovskite materials for ionizing radiation spectroscopy with the scintillation technique has been investigated because of their advantageous properties including high average atomic number, tunable bandgaps, high density, significant LY, and fast response time.^10,34 However, the best scintillator detector with a perovskite material can distinguish between two gamma-ray peaks that are separated by 15.226 keV at 662 keV,³⁵ while HPGe exhibits 1.8 keV at the same energy of 662 keV.³⁶ Thus, it is imperative to focus on tailoring the chemical composition, doping elements, and crystal structure of perovskites to enhance their performance for ionizing radiation sensing.

According to the research and development of scintillation, alkali halides and alkaline earth metal halides have demonstrated excellent performances in gamma-ray spectroscopy as commercial scintillators namely CsI, NaI, and SrI₂. When combined in stoichiometric ratios, these halides form perovskite structures that can integrate the advantageous properties of both compounds. The objective of this review is to present the recent progress in halide perovskite scintillators, which encompasses a variety of structural configurations ranging from higher-to lower-dimensional frameworks. Additionally, this study explores key figures of merits for scintillation performance, fundamental principles of radiation detection, and techniques for synthesizing high-quality single crystals. Moreover, a comprehensive overview of gamma-ray spectroscopy research is provided, highlighting the strategies and approaches employed to enhance the performance of scintillators. Furthermore, the comparative performance of perovskites with varying dimensionalities, as well as with conventional commercial scintillators, is discussed. Finally, this review addresses the current drawbacks in the development of perovskite-based detectors for gamma-ray spectroscopy and outlines potential directions for future research. The essence of our investigation is depicted in Fig. 2.

Fig. 2 Brief essence of this review.

2. Material and structural layout

In 1839, German mineralogist Gustav Rose identified CaTiO₃, which he named perovskite in honor of Russian mineralogist Count Lev Alekseyevich von Perovski.³⁷ Over time, any compound following the stoichiometric ratio of ABX₃ came to be known as a perovskite. In this formula, the symbol A recommends a monovalent inorganic/organic cation/mixed of both cations, B represents a divalent cation, and X generally implies a halide anion.³⁸ Additionally, multiple cations and anions with different valences can be incorporated into the ABX₃ formula. For example, chalcogenides, nitrogen, oxygen, or carbon can be placed at the X-site to develop ABX₃ perovskites, where the valences of the A and B cations are adjusted accordingly to attain charge neutrality. Structurally, cubic perovskites are composed of corner-sharing BX₆⁴⁻ octahedra that form a 3D network, with the A-site cations occupying 12-fold coordinated cuboctahedral voids to preserve charge balance.^39,40 Additionally, perovskite materials can be viewed as a cubic close-packed AX₃ sublattice, with divalent B-site cations located within six-fold coordinated octahedral sites. Besides the cubic phase, perovskites can also exist in the form of tetragonal and orthorhombic phases.

Moreover, by carefully picking and coordinating the components in organometal halide perovskites, materials with different dimensionalities including three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D), can be attained. These classifications are based on the connectivity and spatial arrangement of the metal halide octahedra within the crystal structure.⁴¹

2D organometal halide perovskites can be observed as layers or sheets derived from 3D perovskite structures by slicing along specific crystallographic directions.⁴² When one or more layers of corner-sharing metal halide octahedra are separated by bulky cations, the resulting structures are known as Ruddlesden–Popper-type perovskites.⁴¹ In addition, in 1D perovskites, the metal halide octahedra attach through corner-sharing, edge-sharing, or face-sharing configurations to form continuous 1D chains or nanowires. These inorganic chains are typically isolated by surrounding bulky cations.⁴¹ Moreover, in 0D perovskites, individual metal halide octahedra are surrounded and isolated by bulky cations.⁴³

The Goldschmidt tolerance factor (GTF) is sometimes used to predict the potential of perovskites for photovoltaic applications, though it has seen limited application in the development of scintillators. The GTF is a dimensionless quantity that is employed to predict the geometrical distortion and phase stability of perovskite compounds based on the ionic sizes of the atoms in ABX₃. It is expressed by t = (R_A + R_X)/2(R_X + R_B), where R_A, R_B and R_X correspond to the ionic radius of the A, B, and X atoms in ABX₃ or mixed perovskite, respectively. For a stable 3D perovskite, this value should be between 0.80 and 1.0.⁴⁴ The GTF of a given ABX₃ structure not only acts as an indicator for evaluating its likelihood to be a 3D formation but also assists as a projecting tool for defining whether it will display an ideal cubic phase (t ≈ 1) or transition to tetragonal phases (0.9 < t < 1.0) or to an orthorhombic phase (0.8 < t < 0.9).⁴⁵ In addition, GTF values below 0.8 or above 1.0 tend to lead to non-photovoltaic perovskites.⁴⁶

Another important parameter, known as the octahedral factor (μ), evaluates the ability to form BX₆⁴⁻ octahedra and the stability of perovskite compounds. This factor is determined by the ratio of R_B and R_X. For the formation of stable BX₆⁴⁻ octahedra, μ should lie within the range of 0.414 < μ < 0.732.⁴⁷ Maintaining a 3D structure is crucial to facilitating charge transport throughout the system, which in turn ensures the efficient accumulation of photo-generated charges.⁴⁸ In devices with semiconductors such as perovskite solar cells, the cubic phase of the material and a lower band gap are highly desirable for efficient charge carrier extraction and transport. A lower band gap enables better absorption of light, while the cubic phase contributes to the isotropic charge mobility, which enhances the overall performance. However, in the case of scintillators, materials with relatively higher band gaps are often preferred to facilitate efficient energy transfer between the host material and the activators (dopants). This higher band gap ensures that the energy levels of the activators align properly within the host, allowing efficient recombination processes that generate scintillation light. Additionally, the cubic phase of the material in scintillators is crucial because it supports fast and isotropic charge generation and recombination, contributing to a higher LY and faster response and decay times, which are key for high-performance radiation detection applications. Therefore, although both solar cells and scintillators benefit from the cubic phase structure, the optimal band gap for each application varies based on the functional requirements. Schematic diagrams of the various dimensional configurations are depicted in Fig. 3.

Fig. 3 Schematic diagram of the perovskite structures of various dimensions.

3. Required properties of materials to be used as scintillators

This section delves into the detailed material characteristics and their influence on performance. The material properties explored include high stopping power, LY, afterglow, non-proportionality, emission spectrum, refractive index, ER, and radiation hardness. These intrinsic properties of the materials directly shape the behavior and effectiveness of the devices. This analysis focuses solely on understanding how these material properties impact the overall device functionality and performance.

3.1. Stopping power

The stopping power of a material represents the energy loss of an incident particle per unit distance as it travels through the material. The attenuation formula is used to predict the radiation stopping ability, as follows in eqn (1).^49–51

I = I_oe^[−μx] (1)

where μ and x represent the linear attenuation coefficient, and thickness, respectively.

The attenuation of incident radiation occurs due to the interaction of incident radiation with materials through mainly the photoelectric effect (PE), Compton effect (CE), pair production (PP) processes, etc. The cross-section of interaction for these three phenomena largely depends on the atomic number of the constituent atoms of the substance. In addition, the cross-section, σ_PE ∝ Z^3to5, σ_CE ∝ Z, and σ_PP ∝ Z³ are estimated for PE, CE and PP, respectively.^52,53

The linear attenuation coefficient in inverse length units can be expressed as follows in eqn (2).⁵⁴

where Z indicates the atomic number of the material, A specifies the atomic mass, and E represents the energy of the incident radiation. However, depending on interaction methods such as photo-electric effect, Compton scattering and pair production, the value of n will change. As the energy increases, the dependence of the attenuation coefficient on atomic number becomes less pronounced, reflecting the variations in cross-sections for these interactions. Therefore, although the attenuation coefficient strongly correlates with atomic number for low-energy photons, the dependence weakens with an increase in energy due to the changing dominance of different interactions. Thus, the radiation blocking ability or stopping power of a material is directly proportional to ρZ_effⁿ, where the power of the effective atomic number will change based on the interaction mechanism.^10,34 Different research estimated a high atomic number and moderate density for perovskite, making it a promising candidate for radiation detection.^50,55

3.2. Light yield

The light yield is a key performance metric for scintillators. It is defined as the number of photons emitted per unit of energy through the process of scintillation.⁵⁶where E is the energy of the incident X-ray photons, E_gap represents the band gap of the scintillator, S stands for the transmission efficiency of electron–hole pair energy to the luminescent center, and Q refers to the quantum yield. Parameter and E_eh = energy required to generate one electron–hole pair. The luminescence efficiency Q plays a dominant role in scintillation; however, the LY of scintillators does not always directly correlate with Q, as it reflects only the luminescence efficiency. The efficiency of electron–hole pair transport to the optical emission centers is also critical in determining LY.⁵⁷

In practical applications, the actual LY is often diminished due to factors such as self-absorption, non-radiative recombination caused by defects, and suboptimal light extraction efficiency. Some effective measures to enhance the light extraction, and thus improve the performance of scintillators include applying reflective coatings to minimize light loss, using single crystals with low defect concentrations, and incorporating optical coupling agents between the scintillator and photodiode to optimize light transfer.^58,59 These approaches help to accumulate more light, leading to improved performances. Given that the band gap of perovskite materials is lower than that of commercially available scintillators (BGO: 5 eV, NaI: 5.9 eV and CsI: 6.4 eV), the theoretical LY of perovskite-based scintillators can reach up to 129 000–250000 photons per MeV; however, no perovskite has yet to achieve this level of LY.^60,61 In addition, a few strategies including modulation of the exciton confinement effect, minimization of self-absorption in perovskite, constructing energy-transfer channels, and defect modification can enhance the LY of perovskite-based scintillators.

3.2.1 Modulation of the exciton confinement effect. In semiconductor materials, an exciton, sometimes called a quasiparticle, represents a bound state of an electron and a hole, held together by electrostatic (Coulombic) attraction. An exciton has a finite lifetime; hence, when the electron and hole eventually recombine (the electron falls back into the hole), the energy is typically released in the form of a photon (UV/visible photons) or as heat (non-radiative recombination). The emission of light is primarily dependent on excitonic recombination. Therefore, the LY of scintillators is substantially determined by the parameters of excitons, such as their binding energy (energy required to break apart excitons into free electrons and holes). Materials with low exciton binding energies can indeed dissociate excitons more easily into free charge carriers, which may enhance the LY if the material relies primarily on free charge recombination. However, in the case of perovskite-based scintillators, the situation is slightly different because excitonic recombination is the primary mechanism of light emission. In perovskite-based scintillators, a low exciton binding energy can lead to significant thermal dissociation of excitons at room temperature, resulting in a lower LY because excitons are less likely to survive long enough to recombine radiatively and produce photons. Therefore, in the case of perovskite-based scintillators specifically, increasing the exciton binding energy can indeed improve LY, given that this helps to stabilize excitons at room temperature and enables them to recombine radiatively rather than thermally dissociate. Bulk 3D halide perovskite-based scintillators exhibited very low LY of 1000 photons per MeV at room temperature because of their low exciton binding near to a few tens of meV, indicating loosely bound exciton states and strong thermal dissociation of excitons.^61,62 Techniques such as reducing the dimensionality materials or altering their structural distortions can help increase the exciton binding energy, thereby increasing the likelihood of radiative recombination and enhancing the photon output.⁶³
3.2.1.1. Enhancement of light yield by structural modification. Reducing the dimensionality of perovskite materials has enormous potential for altering their electrical and optical characteristics by altering the quantum confinement and dielectric effects.⁴² Quantum dots, nanowires, and 2D perovskite layers have size- and thickness-dependent light absorption and emission spectra.⁶⁴ In perovskite structures, when the A-site cation is larger than the ideal size, it disrupts the typical 3D framework, leading to a 2D layered structure.⁶⁵ In this modified structure, the oversized A-site cation functions as a spacer, separating the crystal along a particular direction. This results in alternating layers, where the inorganic BX₆⁴⁻ octahedral units form one set of layers, while larger organic/inorganic cations form the other. Electrostatic forces bind these layers, maintaining a stable 2D perovskite structure.⁶⁶ The inclusion of large organic/inorganic cations enhances the stability of 2D layered perovskites. Although these bulky cation layers may restrict carrier transport within the 2D layers, they play a crucial role in adjusting the electrostatic forces influencing electron–hole pairs. In this layered structure, the organic/inorganic cationic layers serve as barriers due to their larger energy gap, while the inorganic octahedral layers function as well because of their smaller energy gap.⁶⁷ Consequently, excitons are confined within the inorganic octahedral layers. The quantum confinement effect on these trapped excitons within the quantum well structure results in a higher binding energy and oscillator strength, providing the excitons in 2D perovskites with improved thermal stability and leading to a high LY.⁹ Due to the increase in exciton binding energy and band gap of lower-dimensional structures compared with 3D perovskites, the electrical conductivity and thermal quenching of lower-dimensional materials remain limited. In this situation, excitonic radiative recombination-dominated light emission could be found more. For example, the LY of 3D europium-doped RbSrI₃ was 78 700 photons per MeV. However, it was enhanced to 90 400 photons per MeV for the 2D perovskite of europium-doped RbSr₂I₅.^68,69 In addition, the 3D perovskite CsCaI₃:Eu had LY of 40 000 photons per MeV, whereas 0D perovskites such as Cs₄CaI₆:Eu showed LY of 69 000 photons per MeV, respectively.^70,71 In another study, the highest LYs of 3D perovskite of RbCaBr₃:Eu and 0D perovskite of Rb₄CaBr₆:Eu reached 45 000 photons per MeV and 71 000 photons per MeV, respectively.⁷² Moreover, perovskites also exhibit thermal quenching due to their low exciton binding energy. For instance, the research team led by V. B. Mykhaylyk reported that the LY of MAPbBr₃ was estimated to be 116 000 photons per MeV at 8 K; however, it was reduced to 90 000 photons per MeV due to increasing the temperature to 77 K.⁷³ To tackle the issue of thermal quenching, researchers have explored reducing the size of single crystals to lower-dimensional forms. This approach effectively confines excitons in a restricted space, enhancing the overlap of exciton wavefunctions in nanocrystals. By limiting excitons within a reduced Bohr radius, this method boosts the exciton binding energy.⁷⁴ For example, the research group led by prominent researchers V.B. Mykhaylyk studied the temperature-dependent LY of a 3D CsPbBr₃ single crystal and found that the intensity of light decreases with an increase in temperature.⁷⁵ In addition, they claimed that the highest LY of the crystal was of ∼50 000 photons per MeV at a temperature of 7 K. Later another research group, Q. Xu et al., reported that embedding 3D CsPbBr₃ single crystals in 0D Cs₄PbBr₆ nanocrystals is a promising strategy to enhance their LY at room temperature by inhibiting thermal quenching.⁷⁶ Additionally, they found that the LY is ∼64 000 photons per MeV at room temperature for the scintillator of CsPbBr₃@Cs₄PbBr₆ nanocrystals.

In addition to the quantum confinement effects in lower-dimension perovskite structures that confine excitons within the inorganic octahedral layers, thereby enhancing their binding energy, the research group led by Asai et al. discovered that structural distortions in the adjoining PbBr₆²⁻ octahedra, specifically the Pb–Br–Pb bond angles, as well as distortions of the internal PbBr₆²⁻ octahedra (Br–Pb–Br bond angles), can further contribute to exciton confinement. These distortions are thought to reduce the exciton Bohr radius, thereby enhancing the confinement effects.⁷⁷ The distortion of these octahedra can be tuned by partially replacing Pb²⁺ with various divalent cations. The research group led by Yanagida incorporated various divalent cations including Ni²⁺, Sr²⁺, Zn²⁺, Sn²⁺ and Ca²⁺ in 2D perovskites to partially substitute Pb²⁺, specifically C₆H₅C₂H₄NH₃)₂PbBr₄.^78–82 As a result, this modification enhances the structural distortions in the inorganic octahedral layers due to the differences in cationic radii between the introduced cations and host cation Pb²⁺. Due to this distortion, the exciton confinement effect is increased, hence enhancing LY. For instance, the baseline LY for pure (C₆H₅C₂H₄NH₃)₂PbBr₄ was 14 000 photons per MeV, which increased to 19 700 photons per MeV and 18 500 photons per MeV at doping levels of x = 0.1 and x = 0.25, respectively, due to the incorporation of Sr²⁺ into this 2D structure [(C₆H₅C₂H₄NH₃)₂Pb_1−xSr_xBr₄].⁷⁸ Similarly, the Cd²⁺-doped structure [(C₆H₅C₂H₄NH₃)₂Pb_1−xCd_xBr₄] demonstrated LYs of 16 000 photons per MeV, 18 000 photons per MeV, 20 000 photons per MeV, and 19 000 photons per MeV at the doping level of x = 0.05, x = 0.1, x = 0.25, and x = 0.5, respectively.⁸¹

In recent years, self-trapped exciton (STE) emission has gained popularity as a viable approach for increasing the LY of perovskite-based scintillators.^83–85 The strong interaction between excitons and phonons is the result of localized lattice deformation, where excitons are trapped, limiting their mobility inside the crystal. In addition, the soft lattice structure of halide perovskites makes them more susceptible to distortions, facilitating the formation of STEs.⁸⁶ The research group led by Smith reported that the ratio of STE emission to free exciton emission increases with the level of distortion in the PbX₆⁴⁻ octahedra.⁸⁷ Additionally, another research team, Ma et al., discovered that lower-dimensional halide perovskite structures promote the formation of STEs by enabling the more effective confinement of excitons and enhanced interaction with phonons.⁸⁸ The prominent research group led by Tang recently investigated the scintillation characteristics of copper-based lower-dimensional perovskites named Rb₂CuBr₃, K₂CuBr₃ and Rb₂CuCl₃, which exhibited strong STE emissions driven by Jahn–Teller distortion.^83,89,90 The STE state had a lower energy than the excited state, resulting in a significant Stokes shift. These materials also showed impressive LYs of approximately 91 056 photons per MeV, 23 806 photons per MeV, and 16 600 photons per MeV, respectively, due to their minimal self-absorption.

3.2.2 Enhancement of light yield through activators. For a material to act as a scintillator, it is essential that it has a luminescent center. These centers originated from lattice defects or the transition of electrons from the core to valence band in an intrinsic scintillator, or from the purposeful addition of impurities in intrinsic scintillators.⁹¹ The impurities intentionally added to scintillators act as activators. Activators raise the LY of scintillators by launching additional energy states within the band gap of the host material, which may lead to more efficient light emission.⁹² When radiation excites the scintillator, energy is transferred from the excited states in the host lattice to these activator centers. As the activators return to their ground state, they emit light by converting its absorbed radiation during excitation into visible photons. This process lessens energy loss as heat rather than light through non-radiative recombination, thereby improving the scintillation efficiency. Furthermore, activators can control the emission wavelength and frequently extend it into the visible region, which can be detected more effectively. In addition, matching the emission to the sensitivity range of photodetectors improves the light collecting efficiency. In some circumstances, several activators or co-dopants can be utilized to raise LY even further by promoting energy transfer pathways that increase the number of photons released. Activators also alter the decay time, with some emitting quicker, resulting in a larger light output inside the detection window of fast detectors. Thus, activators play an important role in increasing the brightness and efficiency of scintillator materials.

Rare-earth elements are commonly used as effective activators in scintillators due to their unique electronic properties. These elements have well-defined energy levels within the band gap of the host material, allowing efficient energy transfer and photons emission when exposed to radiation. Trivalent rare-earth ions including Eu³⁺, Yb³⁺, and Tb³⁺ are commonly known to demonstrate intrinsic 4f-4f transitions although these transitions are parity-forbidden but occur due to the mixing of states caused by the crystal field and spin–orbit coupling.⁹ Conversely, the divalent elements including Eu²⁺ and Tb²⁺ display 4fⁿ–4fⁿ⁻¹5d transitions, which are parity-allowed, and thus about 10⁶ times stronger and emit much more intense light than 4f–4f transitions in trivalent rare-earth ions.⁷² By introducing these types of ions into halide perovskite structures, their distinctive emissions can effectively amplify the Stokes shift of halide perovskite scintillators, and subsequently enhance the scintillation efficiency. For example, Yang and colleagues reported the in situ growth of Eu³⁺-doped CsPbBr₃ quantum dots and observed a significant Stokes shift between their luminescence and absorption spectra under exposure to X-rays.⁹³ This shift is attributed to the intrinsic ⁵D₀ → ⁷F_J (J = 1, 2, 3, and 4) transition of Eu³⁺, leading to reduced self-absorption and light production of 10 100 photons per MeV. In another study conducted by Katelyn A. Dagnall et al., they reported that Yb³⁺-doped CsPbCl₃ displayed a significant amount of light production of 102 000 photons per MeV under X-ray illumination at room temperature, which is brighter than the cutting-edge commercial scintillators.⁹⁴ Alternatively, the research group led by L. Stand observed high light production of 90 400 photons per MeV from RbSr₂I₅ doped with Eu²⁺, while RbSr₂Br₅ doped with Eu²⁺ exhibited light production of 64 700 photons per MeV.^68,69

Alongside rare-earth elements, a variety of other elements are now being used as activators in perovskite structures to improve the performance of scintillators. These alternative activators are being explored to optimize the LY, decay times, and overall efficiency of the scintillation process, broadening the range of materials available for enhanced detection applications. Dongsheng Yuan developed Cs₃Cu₂I₅ doped with a Tl activator and obtained the scintillation LY of 51 000 photons per MeV.⁹⁵ In addition, the prominent research group led by L. Stand developed two single crystals of undoped and Tl-doped Cs₃Cu₂I₅ through Bridgman crystal growth.⁹⁶ The undoped Cs₃Cu₂I₅ had an LY of 41 500 photons per MeV. Alternatively, Tl doping at 0.5 mol% resulted in an improved performance with LY of 98 200 photons per MeV at under the same level of energy.

3.3. Afterglow

The afterglow in scintillator materials, particularly those based on perovskites, is the persistent emission of light after the excitation source has been withdrawn. This effect can have a major influence on the performance of perovskite-based scintillators, particularly in applications that require accurate timing and rapid response, such as fast radiation detection. The afterglow in perovskite scintillators is frequently attributed to traps within the structure of the material, which can momentarily hold excited carriers and slowly release them over time. These traps may arise from defects, impurities, or inherent characteristics of the perovskite crystal lattice.^97,98 Although perovskite scintillators are promising due to their high LY and tunable emission wavelengths, minimization of afterglow is crucial for achieving accurate and high-speed detection. In addition, to achieve lag-free radiation detection and high-contrast imaging, particularly in the case of CT where the imaging demands are as high as 0.1% at 3 ms, minimizing afterglow is essential.⁹⁹ Cs₂Ag_0.6Na_0.4In_0.85Bi_0.15Cl₆ perovskite scintillators exhibit an afterglow of only 0.1% at 16 μs, which is significantly shorter than that of the commonly used CsI scintillator (1.5% at 3 ms).⁵⁷ This reduction in afterglow paves the way for real-time, dynamic X-ray imaging and medical CT applications. Thus, research efforts are directed at improving the composition and crystallinity of perovskites to decrease their trap states and increase their temporal performance in scintillator applications.

3.4. Non-proportionality in performance

Non-proportionality, where the conversion factor between deposited energy and produced light is not constant, have been studied in scintillators for over 70 years. However, at first, it was thought that the cause of nonproportionality is the thermalization of energy carriers.¹⁰⁰ In the 1950s, researchers thought that particles of the same energy but different types (e.g., gamma rays, electrons, and alpha particles) produced different amounts of scintillation light.¹⁰¹ Later, it was established that type and energy of particles were strongly correlated with the ionization density and due to the fact that the interaction of these particles with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the LY depends on the ionization density.¹⁰² This was attributed to the differences in ionization density, with heavier particles such as alpha particles having higher ionization density, and thus lower light output due to saturation of luminescence centers.^13,103 Particularly, by the 1960s, it was understood that luminescent centers could only process one electron–hole pair at a time, and excess pairs in high-ionization-density regions led to reduced light output.¹⁰⁰ In 1991, the discovery of the LSO scintillator, which had a much higher light output (4–5 times) than BGO but almost similar ER, highlighted that ER was not dominated by photon counting statistics but by non-proportionality and complex energy deposition processes as well. Non-proportionality, combined with Compton scattering and photoelectric interactions, introduces event-to-event variations in light production.^101,104 More clearly, Compton scattering creates multiple discrete energy deposits, while photoelectric interactions trigger cascade processes involving fluorescent X-rays, Auger electrons, and delta rays further dividing the energy into smaller fractions.¹⁰¹ These processes are involved in non-proportionality and degraded ER. The “electron response” of scintillators is considered a more fundamental probe of scintillation mechanisms because the electrons produced by Compton scattering involve fewer confounding interactions than photoelectric interactions. However, even electron-excited interactions show ER degradation due to delta rays (knock-on electrons) and Landau fluctuations, where energetic electrons deposit energy non-uniformly, creating multiple smaller deposits with varying ionization densities.^105,106 As electrons slow down, their ionization density increases, leading to further variations in light output. Thus, non-proportionality arises from a combination of ionization density effects, saturation of luminescence centers, cascade processes, and non-uniform energy deposition, all of which contribute to event-to-event variations in light production and degraded ER. Despite the extensive progress, gaps still remain in understanding the fundamental causes of non-proportionality and their quantitative relationship with ER.¹⁰¹ Very recently, the research group led by J. Kaewkhao conducted a study on the non-proportionality of Cs₂LiYCl₆:Ce (CLYC) adopting the Compton coincidence technique. They reported that the electron response of the CLYC crystal decreased by approximately 7% for energies below 179 keV, which indicates greater LY non-proportionality at lower energies. However, in the case of electron energies above 200 keV, the crystal exhibited good proportionality. They also proposed that the non-proportional response primarily influences the intrinsic ER, which is linked to the crystal structure.^107,108

3.5. Matching emission spectrum with the response spectrum of photodetectors

After the generation of light through the scintillation process, the detection of these photons is crucial. To minimize post-scintillation losses and maximize the detection efficiency, it is imperative to ensure a strong overlap between the radioluminescence spectrum of the scintillator and the peak response window of the photodetector. This spectral alignment reduces energy losses during the detection process, leading to an overall improvement in the performance of the detector in applications including radiation detection, imaging, and high-energy spectroscopy.^109,110 Many effective scintillating materials predominantly generate ultraviolet photons with very short attenuation lengths that are rapidly absorbed within the material. This absorption causes severe loss of information. Thus, to alleviate this problem, a popular option is to include additional scintillators in the bulk material. This secondary scintillator captures ultraviolet photons and emits them as visible light.¹¹⁰ Visible photons have substantially longer attenuation lengths; thus, they stay in the material for longer periods of time, increasing the total efficiency of the detector. These secondary scintillators are also known as wavelength shifters.

3.6. Refractive index

The refractive index of a scintillator material must be compatible with the refractive index of the coupling material or the refractive index of the PMT tube glass to improve the light transmission and collection efficiency in scintillation-based detection systems.¹¹¹ When scintillation photons are generated, they are emitted isotopically from the scintillator, covering a full 4π solid angle, and their migration to the photodetector is affected by the refractive index mismatch among the scintillator materials, coupling material, and window glass of the photodetector. In addition, according to Snell's law, photons can only exit the material and reach a photodetector if their angle of incidence is less than the critical angle at interfaces. Photons with incidence angle ≥ critical angle are subject to total internal reflection. For example, in LYSO (refractive index 1.82), only about 8% of the light escapes at the crystal–air interface.¹¹¹ Thus, a significant mismatch can initiate total internal reflection, trapping photons within the scintillator and reducing the light available for detection. Moreover, the reflection losses at interfaces and increased scattering and absorption due to mismatched refractive indices further reduce the number of photons reaching the photodetector. Minimizing the refractive index mismatch can enhance the light collection efficiency and improve the system performance. To minimize the refractive index mismatch, applying an optical grease or fluid with an intermediate refractive index between the scintillator and the photodetector creates a refractive index gradient that enhances photon transmission. This method can increase LY to 50–60%;¹¹² for example, Singh and research group demonstrated that coupling LYSO (n = 1.82) with a PMT using optical grease (n = 1.52) raised the light output to 50%.¹¹³

3.7. Energy resolution

The energy resolution of scintillators is calculated by dividing the full width at half maximum peak intensity (FWHM) by the peak position in gamma ray pulse height spectra measured by PMT. In addition, it can be formulated from three contributions, namely, inhomogeneity contribution, contribution due to nonproportional response and statistical contribution. Mathematically, it can be expressed as a squared sum of these contributions according to eqn (4).^10,114where R_inh represents the contribution of inhomogeneity. This may result from the inhomogeneity in crystal quality or existence of defects or nonuniform distribution of activators, causing variable LY based on the interaction point within the crystal. In addition, the nonuniform properties of the scintillator wrapping or the irregularities across the photocathode area may also affect R_inh. R_npr stands for the contribution resulting from the nonproportional response of the scintillator. R_stat corresponds to the statistical contribution arising from the variance in the number of detected photons N_dp, in addition to the variance (υar(M)) resulting from electron multiplication in the PMT. This contribution is accurately characterized by Poisson statistics according to eqn (5).¹¹⁵where νar(M) is the variance in the gain when photoelectrons are multiplied in the PMT and N_dp is the number of photoelectrons created when scintillation photons interact with the PMT photocathode. The influence of R_stat on the overall ER of the detector diminishes as the LY in the scintillator increases and the dispersion caused by the PMT also decreases. The maximum LY is predominantly influenced by the emission wavelength and the band gap width of the scintillator.¹¹⁶ The fundamental limit in N_dp at 662 keV can be expressed as eqn (6).

Momentum conservation in the electron–electron interactions accounts for the value of 2.5.¹¹⁶

A high energy resolution (ER) corresponds to a lower R value. According to eqn (4), for a perfectly uniform scintillator crystal with ideal energy transfer, the contribution R_inh can be considered zero. In this case, the overall ER primarily depends on the statistical component and the nonproportionality component. To get a high ER, R_npr should be the lowest possible value. This is because R_stat is fixed and shows the theoretical limit set by the number of photons detected (N_dp) in the PMT. Unfortunately, R_npr is not negligible. For instance, the statistical limit of ER for NaI:Tl and LSO:Ce³⁺ scintillators at 511 keV is 3.8% and 4.2%, respectively.¹¹⁷ However, the actual measured ERs are 7% and 9%, respectively. The nonproportionality contributions are of and , respectively. Similarly, the relatively poor ER of 6–8% demonstrated by conventional scintillators including NaI:Tl, Lu₂SiO₅:Ce³⁺, CsI:Tl, BGO, is caused by poor proportionality.^107,115 This shows that eliminating R_npr could nearly halve the ER values. Thus, understanding and reducing the LY nonproportionality in scintillators is critical for improving their ER.

3.8. Radiation hardness

Radiation hardness denotes the capacity of a material to preserve its scintillation efficacy without deterioration when subjected to extended or intense high-energy radiation, including X-rays or gamma rays. This property is essential in applications involving high-radiation environments, such as high-energy physics experiments, space missions, and nuclear reactors, where materials are consistently exposed to extreme radiation flux.¹¹⁸ Conversely, in traditional medical imaging applications such as PET, CT, and SPECT scans, the radiation doses are relatively moderate and administered intermittently.¹¹⁹ Consequently, although radiation hardness is a significant criterion for evaluating long-term material stability, its influence on routine medical imaging performance is typically minimal and receives less emphasis during the selection of materials for these clinical technologies.

4. Interaction of radiation with matter and production mechanism of scintillation light

High-energy electromagnetic radiation possesses energy typically in the range of 100 eV to 100 keV (X-rays) and 100 keV to MeV (γ-rays). The spectroscopy of this high energy radiation has an important role in the diversified field including medical imaging, nuclear nonproliferation, industrial monitoring, nuclear site inspection, astrophysical study, homeland security and basic scientific research. Due to the broad applications and great importance of this high-energy radiation detection, it is also essential to understand the interaction of radiation with materials during the detection procedures. Mainly three phenomena including PE, CE and PP are observed during the interaction of X-rays and γ-rays with the detector materials. However, there is a possibility of no interaction, Rayleigh scattering, and photodisintegration during interaction also. Fig. 4(a) depicts the transmission, Rayleigh scattering, PE, CE, and PP.

Fig. 4 (a) Graphical representation of light-matter interaction. (b) Probability of the occurrence of PE, CE and PP, depending on the incident radiation's energy and proton number of the absorber¹²¹. (c) Depiction of energy band scheme of the scintillation process.⁹¹ (d) Flowchart of the scintillation in an inorganic scintillator.⁹¹ (e) Schematic diagram of a high energy radiation detection procedure through scintillation.

The PE is predominantly observed in the energy range of 10 keV to 500 keV.¹²⁰ When the incident energy interacts with the detecting materials, more specifically with the orbital electrons of the atoms, the orbital electrons are ejected from atoms by getting sufficient energy from the incident radiation. The passage of the ejected electron initiates via the nearby matter and it starts to rapidly lose its momentum and moves only a comparatively shorter path from its origin. Thus, the energy handover is a two-step route. The first one is the PE in which the photons relocate their energy to the electron and second is the depositing of the energy in the neighboring matter by the electron. As the interaction produces a vacancy in the electronic shells, usually K or L, an electron travels to fill the vacancy by releasing energy, which is often called the characteristic X-ray photon energy. The strength of the characteristic energy relies on the binding energy of the involved electrons. Sometimes, this process transforms X-ray photons into light photons. Conversely, the CE occurs dominantly in the energy range of 50 keV to 3 MeV.¹²⁰ In CE, the incident radiation partially transfers its momentum to the orbital electrons of the atoms, and subsequently the orbital electrons are ejected from the atoms. The rest of the incoming energy will be deviated with respect to its incident direction.

Contrariwise, PP is usually observed for high energy of at least 1.022 MeV.¹²⁰ Due to the interaction of the high energy radiation with the electromagnetic field in the vicinity of a heavy nucleus, a pair of electron-positron is produced. Later the produced positron can disappear through interaction with free electrons by releasing two identical gamma energies with energy of 511 keV in opposite direction. The cross-section of interaction for these three phenomena largely depends on the proton number of the constituent atoms of the substance. The cross-section, σ_PE ∝ Z^3to5, σ_CE ∝ Z, and σ_PP ∝ Z³ are estimated for PE, CE and PP, respectively. A graphical representation of the dependence of PE, CE and PP on the incident radiation energy and proton number of the absorber is provided in Fig. 4(b).

A scintillator basically operates as an energy converter by transforming high-energy photons into low-energy photons in the visible range, which can be sensed by PMT or SiPM. For a material to be selected as a scintillator, it is essential for it to have a luminescent center. This center originates from lattice defects or the transition of electrons from the core to valence band in an intrinsic scintillator, or from the intentional addition of impurities in intrinsic scintillators. In both intrinsic and extrinsic scintillators, a radiative transition occurs from an excited quantum state to a low-energy quantum state. In addition, during the scintillation process, typically energy losses are observed owing to non-radiative defects, deep-level electron traps, and the de-excitation of e–h pairs through interaction with phonons during thermalization. The conversion of high-energy photons to visible photons can be divided into four steps, as follows: Step (1): energy conversion: absorption of high energy radiation produces e-h pairs. During the interaction of the high-energy photons with the scintillator material, they (photons) transfer their energy to electrons in the material, causing them to reach higher energy levels and form e–h pairs. Step (2): relaxation and thermalization: the produced e–h pairs undergo relaxation and thermalization by losing their energy until they have an energy equivalent to the band gap of the material. This process includes the interaction of the e-h pairs with the lattice, which commonly causes phonon emission. Step (3): transfer to luminescence centers: the e–h pairs are then transferred to the luminescent centers, which are specific spots in the material that allow radiative recombination. These centers might be inherent defects or impurity sites (dopants) that contain energy levels within the band gap. Step (4): light emission: when the e–h pairs recombine at these luminescence centers, the energy is released as photons in the visible range. Thus, the luminescence centers emit light, which is detected as scintillation light.

Steps (3) and (4) of the light emitting process are depicted in Fig. 4(c) by illustrating single crystalline inorganic scintillators in an energy-band scheme. Generally, rare earth elements Eu²⁺, Ce³⁺, etc. are employed to create luminescent centers into scintillator materials.¹² These rare earth elements display parity allowed electronic transitions among the electronic configurations of 4f^n-15d and 4fⁿ, resulting in rapid luminescence decay, which is essential for the scintillation technique.¹²² The fundamentals of the scintillation process are depicted in Fig. 4 for an inorganic scintillator owing to the absorption of incident high-energy radiation.

4.1. Luminescence without activators (intrinsic luminescence)

Scintillators can be classified into two categories based on their luminescence mechanisms, i.e., intrinsic and extrinsic luminescence. Their emission mechanisms are crucial in determining essential properties, including the emission wavelength and decay time. Consequently, these characteristics substantially affect the efficacy of scintillation detectors. Intrinsic luminescence arises from the inherent properties of the scintillator material including free-exciton luminescence, self-trapped exciton luminescence, Auger-free luminescence, and self-activation luminescence. Free-exciton luminescence results from the recombination of free electron–hole pairs, which is witnessed in wide bandgap scintillating semiconducting materials with direct transitions and characterized by fast scintillation decay time having a sharp emission peak in the spectrum. The examples of these scintillating materials exhibiting free-exciton luminescence include ZnO, GaN, and Ga₂O₃.^123–125

Self-trapped exciton (STE) luminescence occurs in wide bandgap insulating materials when excitons become localized due to self-induced lattice distortion. The common scintillators based on alkali and alkaline earth metal halides include BaF₂, SrF₂, CaF₂, LiSrF₃, and LiCaF₃.^126–130 The characteristics of STE luminescence are a broad emission peak in the spectrum, a significant Stokes shift, relatively high scintillation LY, and a decay time ranging from several hundred nanoseconds to a few microseconds. Additionally, the scintillation LY of STE exhibits strong temperature dependence.

Auger-free luminescence refers to emission occurring without Auger processes, typically in wide-bandgap materials including CsF, BaF₂, and Cs₂ZnCl₄, which is characterized by a band gap energy exceeding the energy difference between the core and valence bands.^131,132,133 Auger free luminescence exhibits a rapid decay with a short emission wavelength. However, a controversial study was performed on the temperature dependent performance of scintillators.^134,135

Self-activation luminescence occurs when the host material naturally contains luminescent activators or consists of luminescent ions as a primary structural component. The common scintillators in this category include BGO, CeBr₃, TlMgCl₃ and TlCaI₃.^136–138 In BGO, TlMgCl₃ and TlCaI₃, scintillation arises from the ns² transitions of Bi³⁺ ions, and Tl⁺, while in CeBr₃, it is due to the 5d–4f transitions of Ce³⁺ ions. The emission properties are determined by the luminescent ions within the host material, and notably this type of scintillator does not experience concentration quenching.

4.2. Luminescence with activators (extrinsic luminescence)

Extrinsic luminescence occurs when external activators or dopants are introduced into the host material. Among the various electronic transitions, activator-related transitions influence the luminescence characteristics of scintillators, playing a crucial role in defining their emission wavelength, decay time, and overall scintillation efficiency. The 1s ↔ 2p transition occurs in color centers (F-centers), where an electron is trapped in an anion vacancy within a crystal lattice. These centers can serve as luminescence centers by emitting light when they are subjected to irradiation. Common materials exhibiting F-centers are MgO and Al₂O₃.^139,140 In addition to F-centers, there are other related defect centers, such as F_D-centers and F⁺-centers. An F_D-center involves an F-center located adjacent to a cation vacancy, while an F⁺-center is a defect where an anion vacancy captures two instead of one electron. These defect centers play a significant role in the luminescence properties of scintillator materials. Conversely, the ns² ↔ nsnp transition is characteristic of materials with heavy elements exhibiting an ns² electronic configuration including Ga⁺, In⁺, Tl⁺, Ge²⁺, Sn²⁺, Pb²⁺, Sb³⁺, and Bi³⁺.⁹² The prevalent scintillators are Tl-doped NaI, Tl-doped CsI, and BGO.^141–144 The spectral characteristic of the ns² ↔ nsnp transition is defined by a broad emission band, with emission wavelengths ranging from near ultraviolet to visible light. The decay time ranges from several hundred nanoseconds to several microseconds, making it suitable for pulse-height-based radiation detectors.

Conversely, the 3d ↔ 3d transitions observed in transition metals including Mn²⁺, Cr³⁺, and Fe³⁺ are typically parity-forbidden transitions.¹⁴⁵ However, these transition are allowed due to the state mixing induced by the crystal field and spin–orbit coupling. These transitions are characterized by long decay times on the millisecond scale and the emission spectra can sometimes be affected by self-absorption. Thus, these transition metals are frequently used in dosimetry applications. Similar transitions occurred in trivalent rare-earth ions (Eu³⁺, Yb³⁺, Nd³⁺, and Tb³⁺), which demonstrate intrinsic f–f transitions with a long decay time although these transitions are parity-forbidden but occur due to the mixing of states caused by the crystal field and spin–orbit coupling.¹⁴⁶ Recently, Eu³⁺-doped (Lu,Gd)₂O₃ has gained interest for X-ray CT, offering higher emission intensity in the red region.¹⁴⁷ However, less investigations have been on the actinide series (5f–5f transition).

Instead, divalent and trivalent elements including Eu²⁺, Tb²⁺, Ce³⁺ display 4fⁿ–4f^n-15d transitions, which are parity-allowed, and thus about 10⁶ times stronger and emit much more intense light than f–f transitions in the trivalent rare earth ions.⁷² Emissions due to the transitions (5d–4f transitions) of trivalent and divalent rare-earth ions are very important for scintillation detectors because they show intense and fast emissions by spin- and parity-allowed transitions. In the research conducted by Dagnall et al., they reported that Yb³⁺-doped CsPbCl₃ displayed a significant amount of LY under X-ray illumination at room temperature, which is brighter than the cutting-edge commercial scintillators.⁹⁴ Conversely, the research group led by Stand observed high LY and better ER from RbSr₂I₅ doped with Eu²⁺ under gamma energy.⁶⁸

Lastly, charge transfer luminescence can occur in certain atomic groups, including Yb³⁺-doped materials and compounds containing VO₄³⁻ and WO₄²⁻. Examples are Lu₂O₃:Yb, YVO₄ and CaWO₄.^148–150 Among the materials with charge-transfer luminescence, CdWO₄ has been used with Si-PD for X-ray detectors for security applications.¹⁵¹

5. Single crystal development method

The quality of crystals is essential to efficiently absorb and convert ionizing radiation into detectable light in the scintillation process. High-quality crystals ensure the optimal performance in terms of light output, ER, and reliability. The development of high-quality single crystals (ingots) involves specific crystal growth techniques that produce highly ordered structures with a continuous lattice and no grain boundaries. These methods are primarily used in the semiconductor, photovoltaic, and optical industries. Here, the most common methods for the development of single crystals are discussed.

5.1. Czochralski method

The Czochralski method, developed in 1916 by Polish scientist Jan Czochralski, is a widely used technique for growing high-quality and large single crystals of semiconductors, metals, salts, and synthetic gemstones for different applications in opto-electronics.^152,153 Additionally, this crystal growth technique allows precise control of the crystal orientation by using a seed crystal with a specific orientation and enables the production of single crystals with minimum defects at a relatively faster growth rate. The process begins with a crucible containing the raw material, which is usually melted at high temperatures in an inert or vacuum atmosphere to prevent contamination. Then, a seed crystal of the chosen material is carefully dipped into the molten. This seed is gradually lifted up along with rotation, enabling the molten material to solidify on its preferential surface and create a single-crystal structure. In addition, the pulling rate and the temperature gradient are precisely controlled to ensure uniform crystal growth. Moreover, factors such as the rotation speed and the ambient conditions also directly influence the quality of the crystal. Similarly, using this method, R. Zheng and group effectively developed a single-crystal perovskite of Cs₃Cu₂I₅ doped with thallium.¹⁵⁴ Their findings discovered an LY of 81 000 photons per MeV and ER of 5.2% under the exposure of 662 keV. A schematic diagram of the Czochralski method is presented in Fig. 5(a).

Fig. 5 Schematic of the (a) Czochralski method; (b) Bridgman–Stockbarger method; (c) floating zone method; (d) inverse-temperature crystallization; (e) temperature-lowering crystallization; (f) slow solvent evaporation method and (g) antisolvent vapor-assisted crystallization for SC development.

5.2. Bridgman–Stockbarger method

The Bridgman–Stockbarger (BS) scheme is one of the various types of crystallization processes from a melt, in which the crystallization happens from a melt when the melt is cooled, lowering its icing point and leading to the fusion, and then solidification of the pure substance. During the synthetization of SCs, there are no probable sources of pollutants apart from the contamination from the crucible substance as well as the surroundings. Additionally, the crystallinity and growth rate are generally higher than that of other existing approaches. According to this approach, the composition of perovskite will be melted during the passing of the silica ampule filled by perovskite precursor through the hot region, and then the perovskite material will be nucleated and grown into a SC due to the movement of the ampule toward the cold region. The eminent research group led by V. B. Mykhaylyk developed a single crystal of CsPbBr₃ to explore its scintillating properties by means of the Bridgman-Stockbarger method.¹⁵⁵ The CsPbBr₃ crystal produces 50 000 photons per MeV when excited by X-rays at 12 keV and 109 000 photons per MeV when excited by α-particles from ²⁴¹Am at 7 K. A schematic diagram showing the different components of the Bridgman–Stockbarger method for SC development is provided in Fig. 5(b).

5.3. The edge-defined film-fed growth

The edge-defined film-fed growth method is another method to grow crystals, which builds on the Czochralski process by introducing a floating graphite die on the melt surface.¹⁵⁶ At the top of the die, there is a pool, which is filled by molten material through narrow tiny channel due to capillary action principles. Then, a seed crystal is lowered to contact the melt in the pool, and crystallization occurs under the optimal thermal conditions. As the seed is pulled upward at a controlled rate (1–10 mm h⁻¹), the molten substance solidifies, and the crystal grows with a cross-section defined by the shape of the die.¹⁵⁷ By controlling the environmental condition and pulling speed, the crystal formation can be smooth and uniform.

This technique is great for forming crystals with consistent shapes including sheets or ribbons because the die directly controls the size and shape of the crystal. The prominent research group of G. Calvert and colleagues developed a strontium iodide (SrI₂) scintillator crystal for the first time by the edge-defined film-fed growth method.¹⁵⁸ They claimed that the LYs were in the range of 90 000 to 107 000 photons per MeV and ER in the range of 5.96% to 3.69% for different crystals of SrI₂ doped with europium. Similarly, using this method, R. Zheng and team successfully developed a single crystal of Cs₃Cu₂I₅ doped with thallium.¹⁵⁴ Their findings revealed an LY of 79 000 photons per MeV and ER of 5.4%.

5.4. Floating zone method

The floating zone technique is another effective method for developing high-quality single crystals.^159–161 In this procedure, a feed rod of the material is vertically positioned, and a tiny molten zone is formed at the center using targeted heating, often with a laser, halogen lamp, or radio frequency induction. The molten zone is held together by surface tension between the feed rod and the developing crystal. As the molten zone travels up the rod, the material solidifies with a controlled crystal orientation, resulting in a defect-free single crystal. The floating zone approach is particularly useful since it eliminates contamination from crucibles and allows exact control over crystal formation factors, such as temperature gradients and growth rates. This makes it ideal for developing high-quality crystals for advanced optical, electronic, and scintillation applications. R. Takahashi and colleagues developed a single crystal of Ca₃TaGa₃Si₂O₁₄ doped with thulium by the floating-zone method.¹⁶² The scintillator with 0.1%, 0.5%, and 1% Tm-doping displays the LYs of 1300 photons per MeV, 1800 photons per MeV, and 1500 photons per MeV under γ-ray irradiation, respectively. In addition, the well-known research group of Yusuke Endo and colleagues developed a perovskite Ca_1-xMg_xHfO₃ scintillator crystal doped with terbium by the floating zone method and they found the highest LY of 17 400 photons per MeV.¹⁶³ In another experiment, Y. Endo and team developed CaHfO₃ single crystals doped with Dy using the floating zone technique, and they confirmed from the pulse height spectra of ¹³⁷Cs that 3.0% Dy:CaHfO₃ exhibited an LY of 20 000 photons per MeV.¹⁶⁴ A schematic diagram of the floating zone method is presented in Fig. 5(c).

5.5. Solution-based single crystal growth techniques

The crystal growth methods based on melting suffer from high energy costs and challenges to form uniform crystals due to the differing solidification points (temperatures) of constituent compounds. Often they lead to phase segregation, compositional inhomogeneity, and structural defects, which affect the performance of scintillators. Therefore, future research should place greater emphasis on alternative cost effective and solution-based crystal growth methods. Perovskite material-based single crystals can be synthesized using simple solution processes at temperatures below 150 °C. Some prominent solution methods for growing single crystals including inverse-temperature crystallization (ITC), slow solvent evaporation method, temperature-lowering crystallization, and antisolvent vapor-assisted crystallization techniques are utilized to create crystals.^165–168 Schematic diagrams of a few solution-based SC growth methods are presented in Fig. 5(d–g).

6. Performance analysis of perovskite scintillators derived from alkali metal halides and alkaline earth metal halides

Scintillators, as a primary category of radiation detectors, hold significant importance in the realms of radiation detection and imaging. The evolution of scintillators has emerged with alkali and alkaline metal halides as first-generation scintillators, developed between the 1940s and 1970s, consisting of simple chemical compounds such as NaI and CsI, along with natural phosphor materials such as CaF₂ and CdWO₄, which were widely used due to their basic scintillation properties. Then, second-generation scintillators emerged in the 1970s, with BGO (bismuth germanate) being a notable advancement, offering a higher density and improved gamma-ray detection efficiency, making it valuable for medical imaging and high-energy physics applications. Since the 1990s, third-generation scintillators have been developed, primarily based on host materials doped with an emission center, such as Ce-doped LSO and LYSO. These modern scintillators follow three main trends of rare-earth silicates, rare-earth garnets, and halide materials.¹³ Among them, halide materials from alkali and alkaline groups of periodic tables and their combination are a niche area of research because of their enhanced LY, faster decay times, and improved ER, making them highly effective for advanced imaging and radiation detection technologies. Among the initial inorganic scintillators, alkali metal halides exhibit promising scintillation properties and better performance in high energy radiation detection. More specifically, thallium (Tl)-doped NaI and CsI are extensively utilized in medical imaging, high energy/space physics and homeland security.^169,170 In addition, CsI:Tl stands as a prominent scintillator, having found utility across a multitude of applications over several decades. With an impressive LY of 54 000 photons per MeV and minimal sensitivity to moisture, making it nearly non-hygroscopic, CsI:Tl is distinguished as a reliable and efficient gamma-ray detector, featuring an emission spectrum that peaks at around 540 nm.¹⁷⁰ Moreover, the ER typically reaches approximately 4.42% to 6% at 662 keV for CsI:Tl when paired with photodetectors.^171–173 Thallium-doped sodium iodide (NaI:Tl) scintillators are commonly employed in radiation detection, and in general demonstrate an ER of 6.5 to 7% at 662 keV.^170,174 A recent investigation has paid attention to improving their performance through various strategies. Co-doping with alkaline metals including Sr²⁺ and Ca²⁺ has shown promising results by improving the ER to 5.3% and reducing the decay time to 170 ns.¹⁷⁴ In addition, lithium halides, potassium halides, and rubidium halides doped with different activators show promising scintillation properties and performances for detecting high-energy electromagnetic radiation and particles.^175–177 Thus, undoped alkali metal halides and doped with various activators exhibit promising scintillation properties and better performances in the detection of high energy radiation.

Conversely, alkaline earth metal halides more specially, beryllium halides, magnesium halides, calcium halides, strontium halides, barium halides, radium halides (less studied due to the radioactivity of radium) doped with various activators have been studied for deciphering their scintillation properties and applied to detect high-energy electromagnetic radiation and particles. In addition, alkaline halide materials exhibit a high effective atomic number, which enhances their photoelectric cross-section.¹⁷⁸ Among the alkaline earth metal halides, europium-doped strontium iodide (SrI₂:Eu) has surfaced as a noteworthy scintillator material for gamma-ray spectroscopy and radio-isotope identification. Although it was first developed by Hofstadter in 1968, its remarkable characteristics have come to light day by day due to advancements in crystal growth methods and characterizations techniques.¹⁷⁹ Moreover, SrI₂:Eu demonstrates an impressive LY surpassing 120 000 photons per MeV, along with outstanding ER of 3% at 662 keV.^179,180 In addition, BaBrI doped with Eu also shows a high light output of 71 000 photons per MeV and ER of 4.3% at 662 keV.¹⁸¹ Moreover, CaI₂ scintillator was also confirmed to have a high light output of 107 000 photons per MeV and ER of 3.2% at 662 keV, which was 2.7 times higher than that of NaI(Tl).¹⁸² Thus, undoped alkaline earth metal halides and doped with various activators demonstrate promising scintillation properties along with better performance in the detection of high-energy radiation.¹⁸³

In recent years, the outstanding scintillation performance of alkali metal halides (e.g., CsI) and alkaline earth metal halides (e.g. SrI₂) have motivated the scientific community to investigate the combination of both halides materials, integrating the benefits of both classes of halides. The combination of these two halides in various ratios results in the formation of different dimensional perovskite materials featuring a variety of structural configurations, including AX:BX₂ = ABX₃ (3D), 2AX:BX₂ = A₂BX₄(2D), AX:2BX₂ = AB₂X₅ (2D) and 4AX:BX₂ = A₄BX₆ (0D) frameworks.^69,184–187

In addition, elements from these two categories can be combined with other groups of elements to form various types of perovskites, such as A₃B³⁺₂X₉ (2D) and A₂B⁺B³⁺X₆ (3D) structures. The electronic and optical properties of these perovskite materials can be adjusted for better scintillation performances including high LY for outstanding ER, which can position them as strong candidates for future scintillation technologies. In addition, reducing the dimensionality of perovskite materials holds significant promise for modifying their bandgap properties, ultimately resulting in changes to their optoelectronic features.²⁰¹

When perovskite structures are restricted to two, one, or even zero dimensions, the effects of quantum confinement and dielectric confinement become increasingly significant, leading to adjustable bandgaps and unique optoelectronic properties and enabling precise manipulation of their light absorption and emission spectra.^202,203 The ability to alter the bandgap of perovskites through dimensionality reduction presents exciting opportunities in various applications, where meticulous control of optoelectronic properties is essential for optimal device performance. By utilizing the combined characteristics of alkali and alkaline earth metal halides in these dimensional structures, experts seek to tackle issues related to the detection of high-energy electromagnetic radiation and particles, with potential uses in medical imaging, nuclear spectroscopy, and particle physics. This new category of materials presents an exciting opportunity for developing high-performance scintillators. Photographs of successfully grown single crystals with varying compositions, activator concentrations and dimension are displayed in Fig. 6(a–r).

Fig. 6 (a–r) Photographs of successfully grown single crystals with varying compositions, activator concentrations and dimensions. The images reflect high optical transparency, uniformity, and luminescent properties under ambient and UV illumination. (a) Reprinted from ref. 69 with permission from Elsevier, Copyright 2024; (b) reprinted from ref. 188 with permission from Elsevier, Copyright 2022; (c) reprinted from ref. 189 with permission from Elsevier, Copyright 2015; (d) reprinted from ref. 190 with permission from Elsevier, Copyright 2017; (e and f) reprinted from ref. 191 with permission from Elsevier, Copyright 2025; (g) reprinted from ref. 68 with permission from Elsevier, Copyright 2017; (h) reprinted from ref. 192 with permission from IEEE, Copyright 2016; (i) reprinted from ref. 193 with permission from Elsevier, Copyright 2015; (j) reprinted from ref. 194 with permission from Elsevier, Copyright 2018; (k) reprinted from ref. 195 with permission from Wiley, Copyright 2023; (l) reprinted from ref. 196 with permission from Elsevier, Copyright 2020; (m) reprinted from ref. 70 with permission from Elsevier, Copyright 2019; (n) reprinted from ref. 197 with permission from RSC, Copyright 2018; (o) reprinted from ref. 198 with permission from Elsevier, Copyright 2016; (p) reprinted from ref. 199 with permission from Elsevier, Copyright 2022; (q) reprinted from ref. 200 with permission from Elsevier, Copyright 2022; and (r) reprinted from ref. 138 with permission from Elsevier, Copyright 2017.

6.1. ABX₃ perovskite scintillators

The most common and widely recognized structure of perovskite is ABX₃, which can be produced using an equal mixture of alkali metal halides (AX) and alkaline earth metal halides (BX₂). The scientific community seeks to harness and integrate the beneficial features of both groups of materials, given that each exhibits remarkable efficacy in the detection of high-energy particles. In the pursuit of advancing intrinsic ABX₃-based perovskite scintillators, various research teams have encountered challenges, as these scintillators have not demonstrated the desired performance metrics including LY, ER, and energy linearity. For example, the prominent research group led by K. S. Pestovich successfully synthesized an intrinsic scintillator of RbSrI₃ through the precise mixing of RbI and SrI₂ in an optimal ratio with help of the widely used Bridgman-Stockbarger single-crystal growth technique.⁶⁹ They reported essential performance metrics including an LY of 8000 photons per MeV, ER of 7.9%, and fast decay time of 375 ns under exposure from a radioactive source of 137Cs (662 keV). Generally, undoped scintillators often exhibit a relatively lower performance compared to doped scintillators, including reduced light production because of the lack of activators, which facilitate radiative relaxation. Dopants introduce additional efficient recombination pathways, which slow the decay times along with high LY compared to undoped materials. Moreover, undoped scintillators emit intrinsic wavelengths that may not effectively align with the detector response curve, which may be another issue that reduces the overall performance. These variables cumulatively lead to a diminished scintillation performance. The same research group assessed the effects of introducing various elements as an activator into the same framework. Upon doping RbSrI₃ with 5% Eu²⁺, outstanding performance metrics were documented including a high LY of 78 700 photons per MeV, decay time of 1.03 μs, and ER of 2.8% under the same radiation source.⁶⁹ In addition, upon the introduction of 0.5% Yb²⁺ as a dopant, the LY of the scintillator reduced to 24 000 photons per MeV, accompanied by a scintillation decay time of 795 ns and ER of 4.9% under the 137 Cs gamma source. However, the observed reduced LYs in the structure of Yb-doped scintillators can be attributed to an inadequate concentration of the dopant, which might consequently lead to a reduced number of radiative relaxation pathways. Another prominent research group, headed by M. Zhuravleva, developed two RbCaBr₃ scintillators doped with 5% and 7% Eu²⁺, respectively.⁷² The reported LYs were 37 000 photons per MeV and 43 000 photons per MeV with the corresponding ERs of 7.6% and 4% of each device doped with 5% and 7% Eu²⁺, respectively. Furthermore, a scintillator based on CsSrI₃ with different concentrations of Eu²⁺ doping was developed by the group of K. Yang et al..²⁰⁴ According to them, their structure worked best with 8% Eu²⁺ doping. However, when Eu²⁺ completely replaced Sr²⁺, they observed a drop in performance. In particular, the LYs of CsSrI₃:1% Eu, CsSrI₃:8% Eu and CsEuI₃ (totally replaced Sr²⁺ with Eu²⁺) were 32 000 photons per MeV, 65 000 photons per MeV and 24 000 photons per MeV, with the corresponding ER of 9%, 5.9% and 11%, respectively. More importantly, the fully Eu-substituted structure (CsEuI₃) showed similar features to that of the undoped ABX₃ material and may not have extra pathways for additional light emission that would improve the scintillation performance of the device. Alternatively, the performance can be optimized through compositional engineering of the ABX₃ framework by maintaining a fixed activator concentration. For instance, the Japanese research team headed by K. Mizoi investigated the scintillation characteristics of ABX₃ via composition engineering, while keeping the dopant concentration constant.²⁰⁵ By maintaining the Yb²⁺ dopant concentration at 1% through whole study, they completely replaced Sr²⁺ in compounds of CsSrCl₃ and CsSrBr₃ with Ca²⁺. As a result, they observed an enhanced LY from 16 000 photons per MeV (CsSrCl₃) to 48 000 photons per MeV (CsCaCl₃) and 23 000 (CsSrBr₃) to 120 000 photons per MeV (CsCaBr₃), respectively. In addition, another research group adopted a similar strategy to improve the scintillator performance. Specifically, two devices were developed with CsCaBrI₂:Eu 7% and CsSrBrI₂:Eu 7% (by replacing Ca by Sr) and they demonstrated an enhanced performance in both LY (from 51 800 photons per MeV to 65 300 photons per MeV) and ER (from 3.8% to 3.4%), respectively.²⁰⁶

The non-proportionality of LY in scintillators represents a multifaceted phenomenon that arises from the intricate interactions between microscopic and macroscopic processes occurring within the scintillator material. More specifically, it emerges from the intricate interaction of energy deposition, thermalization, transport, and recombination mechanisms, all of which are affected by the types of incident particles and energies, ionization density and nature of the energy carriers generated during the scintillation process.¹⁰⁰ Various research teams investigated the non-proportionality issue in ABX₃ perovskite structures, although these studies are still limited in these compounds. Notwithstanding these endeavors, the above-mentioned issue remains prevalent in ABX₃ perovskite systems. For example, the research team of A. C. Lindsey explored the non-linearity in the scintillator performance of KCaI₃:Eu 3% with different sizes.¹⁸⁹ The findings revealed that the device with ∅ 2 mm × 5 mm crystal achieved an ER of 3% when it was subjected to 662 keV energy emitted from a¹³⁷Cs source. Alternatively, relatively lower ER of 6.1% was recorded when the same device was subjected to 122 keV energy emitted from a⁵⁷Co source. Moreover, the device with larger dimensions of ∅ 17 mm × 17 mm crystal under excitation from ¹³⁷Cs and ⁵⁷Co source demonstrated the ER of 4.4% and 7.3%, respectively. The observation of outcomes underscores a divergence in performance, suggesting a lack of proportionality in the response of the scintillator. The research group investigated the factors contributing to the fluctuation in ER and determined a non-proportionality degree of roughly 0.015, which is similar to the high-performance scintillator LaBr₃:Ce, documented to exhibit a non-proportionality of 0.013 in existing literature.²¹⁹ Furthermore, the statistical impact on the decrease in performance was around 2.3% for the small crystal and 2.8% for the big crystal at 662 keV. They determined that the principal factor affecting the decline in ER was the inhomogeneity of faults, impurities, or activators inside the crystal. Nonetheless, they observed that statistical factors probably exert a significant influence on the total performance. Another research team, headed by Y. Wu, also observed the non-proportionate ER of a perovskite-based scintillator with dimensions of ∅ 50 mm × 45 mm, developed from KCa_0.8Sr_0.2I₃:Eu 0.5%.¹⁹⁰ Particularly, when the device was exposed to 662 keV radiation from a¹³⁷Cs source, it demonstrated an ER of 4.1%, whereas the device exhibited a substantially lower ER of 7.9% under exposure from 122 keV radiation from a⁵⁷Co source. Similar observations by the research team of Y. Wu et al. for a scintillator made from KCa_0.835Sr_0.165I₃:Eu 3% found different ER of 2.5% and 5.9% under exposure from ¹³⁷Cs (662 keV) and ⁵⁷Co (122 keV), respectively. However, a remarkable LY of 74 000 photons per MeV was observed under exposure from ¹³⁷Cs (662 keV). Finally, the degree of non-proportionality of the structure was 0.015.

A contemporary research trend emphasizes the integration of alkali or alkaline earth metal halides with other halide materials to investigate their feasibility and performance over diverse energy ranges. As part of their efforts, A. Khan and team developed an intrinsic scintillator by combining TlCl and CaCl₂ in a proportionate ratio using the Bridgman–Stockbarger method.²¹⁸ Similar to other intrinsic scintillators, the performance of TlCaCl₃ remained limited, which may be due to the absence of additional pathways for radiative recombination, which are typically introduced through activators in scintillating materials. Specifically, the device exhibited an LY of 30 600 photons per MeV, scintillation decay time of 727 ns, and ER of 5% when exposed to a ¹³⁷Cs (662 keV) radiation source. In addition, the research group also investigated the non-proportionality of the device performance under different ranges of energy from diverse radio isotopes. At higher energy levels, the response exhibited a linear proportionality; however, at a lower energy of incident gamma rays (less than 350 keV), this proportionality diminishes. A more important observation was that the scintillation response of the TlCaCl₃ crystal demonstrates a notable linearity compared to traditional alkali halide crystals such as NaI:Tl and CsI:Tl.²¹⁸ Another research team developed two intrinsic scintillators named TlCaI₃ and TlMgCl₃.¹³⁸ However, their performance metric remains inadequate compared to activated scintillators. In particular, TlCaI₃ showed an LY of 42 200 photons per MeV alongside an ER of 6.2%, whereas TlMgCl₃ revealed an LY of 30 600 photons per MeV with an ER of 3.7%. A notable discovery was their rapid intrinsic band-to-band decay with a decay time of 200 ns and 449 ns for TlCaI₃ and TlMgCl₃, respectively, which is essential for obtaining precise and fast high-energy spectroscopy. The investigation further analyzed the non-proportionality of the two devices and observed that TlCaI₃ demonstrated a nearly linear correlation between incident energy and LY, while TlMgCl₃ displayed non-linearity at lower radiation energies. Although TlCaI₃ demonstrates praiseworthy proportionality, it exhibits a moderate ER of 6.2% at 662 keV, which can be attributed to its comparatively low crystal quality. Furthermore, the research group of M. Arai and colleagues developed various scintillator crystals using the self-seeding solidification method to investigate the photoluminescence properties of (Tl_1-xA_x)MgCl₃, where A represents an alkali metal dopant.²²⁰ They observed varying LYs when 0.5% of different alkali metals was substituted at the A site in the perovskite structure. Particularly, the LYs for (Tl_0.995Li_0.005)MgCl₃, (Tl_0.995Na_0.005)MgCl₃, (Tl_0.995K_0.005)MgCl₃, (Tl_0.995Rb_0.005)MgCl₃, (Tl_0.995Cs_0.005)MgCl₃, and TlMgCl₃ were of 28 000 photons per MeV, 27 000 photons per MeV, 55 000 photons per MeV, 35 000 photons per MeV, 49 000 photons per MeV, and 42 000 photons per MeV, respectively. Additionally, the study confirmed a fast decay rate similar to previous findings, with decay times of 319 ns, 197 ns, 269 ns, 295 ns, 259 ns, and 350 ns for the respective scintillators. Various studies have established that a proportionate ratio of alkali or alkaline earth metal halides with other substances to for the ABX₃ framework yields intrinsic scintillators, which reliably exhibit rapid decay due to band-to-band decay scheme, moderate ER, and a linear performance response across diverse energy levels. Several promising perovskite scintillators along with their performance metrics are presented in Table 1 and SI Table S1.

Table 1 Performance metrics of ABX₃ perovskite scintillators under various ranges of gamma energy

Material		Crystal growth method	Volume (mm^3)	Emission wavelength (nm)	Source	LY photons per MeV	Decay time (μs)	ER (%)	Ref.
ABX3	CsSr0.93Eu0.07I3	Vertical gradient freeze(VGF)	—	455	^137Cs (662 keV)	73 000	3.2	3.9	207
	Cs1.06Sr0.8742Eu0.0658I2.94	BS	5702.7	469	^137Cs (662 keV)	44 000	3.7	3.4	208
					^57Co (122 keV)	—	—	6.8
	RbSrI3:5% Eu	BS	77	462	^137Cs (662 keV)	78 700	1.03	2.8	69
	RbSrI3:0.5% Yb			457		24 000	0.795	4.9
	CsBaI3:2% Eu	BS	—	429	^137Cs (662 keV)	55 000	0.760	3.8	209
	CsBaBr3:2% Eu		—	425	^137Cs (662 keV)	22 000	1.193	5.5
	CsSrI3:2% Eu		—	450	^137Cs (662 keV)	20 000	0.700	5.5
	CsSrBr3:2% Eu		—	431	^137Cs (662 keV)	17 000	0.973
	KCaI3:3% Eu	BS	4	—	^137Cs (662 keV)	70 000	1.0	3.8	210
	CsCaI3:5% Eu		8	—	^137Cs (662 keV)	39 000	1.0 s	3.8
	CsSrBr3:5% Eu		—	—	^137Cs (662 keV)	40 000	3.0	3.8
	CsSrI3:7% Eu		20	—	^137Cs (662 keV)	73 000	3.0	3.7
	CsSr0.92Eu0.08I3	VGF	—	457	^137Cs (662 keV)	65 000	3.3	5.9	204
	KCaI3:0.5% Eu	BS	43 096	—	^137Cs (662 keV)	59 100	—	∼4	211
	KCaI3:3% Eu	BS	16	466	^137Cs (662 keV)	72 000	1.06	3	189
					^57Co (122 keV)	—	—	6.1
	RbCaBr3:7% Eu	BS	115	436	^137Cs (662 keV)	43 000	2.8	4.0	72
	CsSrBr3:5% Eu	BS	125	430	^137Cs (662 keV)	55 600	2.16	5.2	212
	CsSrBr3:5% Eu	BS	125	437	^137Cs (662 keV)	55 000	—	5.6	213
	KCa0.8Sr0.2I3:0.5% Eu	BS	88 357	472	^137Cs (662 keV)	51 000	2.37	4.1	190
					^57Co (122 keV)	—	—	7.9
	CsSrBrI2:7% Eu	BS	50	449	^137Cs (662 keV)	65 300	1.8	3.4	206
	CsCaBrI2:7% Eu		50	452	^137Cs (662 keV)	51 800	2.1	3.8
	CsSrClBr2:10% Eu		468	441	^137Cs (662 keV)	35 100	3.5	3.9
	CsSrCl3: 0.5% Ce	BS	9	365	^137Cs (662 keV)	8600	0.626	7.2	214
	KCa0.835Sr0.165I3:3% Eu	BS	64	470	^137Cs (662 keV)	74 000	1.06	2.5	215
					^57Co (122 keV)	—	—	5.9
					^241Am (59.5 keV)	—	—	8.0
ABX3 (Other)	MAPbBr0.05Cl2.95	ITC	—	—	^137Cs (662 keV)	18 000	0.00014	10.5	216
	TlMgCl3	BS	16	405	^137Cs (662 keV)	46 000	0.350	5	217
	TlCaI3	BS	28	533	^137Cs (662 keV)	42 200	0.200	6.2	138
	TlMgCl3	—	396	409		30 600	0.449	3.7
	TlCaCl3	BS	—	425	^137Cs (662 keV)	30 600	0.727	5	218

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6.2. AB₂X₅ perovskite scintillators

This section reviews the scintillation performance of various undoped and doped AB₂X₅ framework materials composed of a proportionate ratio of alkali (AX) and alkaline earth metal halides (2BX₂). Lower-dimensional materials can sometimes offer advantages in scintillation performance due to their unique electronic and optical properties, such as enhanced quantum confinement effects and reduced defect densities over bulk scintillators. Many research groups investigated the performances of intrinsic AB₂X₅ scintillators. Under K. S. Pestovich, one of these groups developed an intrinsic scintillator with RbSr₂Br₅ using the Bridgman–Stockbarger technique.¹⁹¹ However, the first performance of this device fell short of expectations with an LY of 2400 photons per MeV, scintillation decay time of 2.2 μs, and ER of 12.9%. However, they used composition engineering of the AB₂X₅ framework to improve its performance and obtained notable results when they replaced bromine with iodine. Upon exposure to 662 keV radiation (¹³⁷Cs), the revised scintillator showed an LY of 14 000 photons per MeV, a lower decay time of 1.9 μs, and enhanced ER of 7.9%. The general performance of the scintillator was improved by this compositional change. Notwithstanding these advances, other intrinsic scintillator designs have shown comparably poor performances; this could be explained by the lack of extra radiative recombination channels.³⁵ Different research groups have added various activators, such europium (Eu) and samarium (Sm), into scintillator materials to solve this constraint and increase the number of radiative pathways. These activators are included to increase the efficiency of radiative recombination, thus enhancing the general scintillator performance. For example, prominent researcher K. S. Pestovich and colleagues incorporated different concentrations of divalent europium into RbSr₂Br₅ and RbSr₂I₅ scintillators.¹⁹¹ Significant improvements in performance were noted including an increase in LY from 2400 photons per MeV to 73 000 photons per MeV for RbSr₂Br₅ and 14 000 photons per MeV to 90 000 photons per MeV for RbSr₂I₅ upon activation with 5% Eu. Furthermore, the ER was enhanced from 12.9% to 3.9% for RbSr₂Br₅ and 7.9% to 2.9% for RbSr₂I₅. These findings indicate that the incorporation of Eu activators successfully established multiple radiative recombination pathways, which lead to a notable improvement in the scintillation efficiency and overall performance of the materials. In addition, using a similar approach, the research team led by P. Dorenbos improved the performance of the intrinsic scintillator CsBa₂I₅.³⁵ Key performance indicators were significantly improved by including 5% divalent europium as an activator. Upon exposure to 662 keV radiation from ¹³⁷Cs, the LY climbed from 22 000 photons per MeV to 80 000 photons per MeV and the ER improved from 9.6% to 2.3%. These results show how well the inclusion of Eu creates extra radiative recombination channels, thus improving the general performance of the scintillator. The co-doping strategy is another effective way to improve the performance of scintillators. W. Wolszczak and associates adopted a co-doping approach and developed a CsBa₂I₅ scintillator with 2% europium and 1% samarium.²²¹ The scintillator showed an LY of 45 000 photons per MeV and ER of 3.2% upon 662 keV gamma radiation. By means of emission source analysis, they discovered that the 5d-4f transition of Sm²⁺ at 755 nm accounted for 95% of the light emission; a minor contribution came from Eu²⁺, emitting weakly around 430 nm. This work emphasizes the main influence of Sm²⁺ in the scintillation process and the complementary effect of Eu²⁺ in the general performance of the material.

Another strategy for optimizing the performance of scintillators involves composition engineering within the AB₂X₅ framework by maintaining a constant doping concentration. The prominent research group L. Stand and colleagues employed this approach to enhance the radiation detection performance of RbSr₂Br₅ doped by 2.5% europium.⁶⁸ By replacing bromine with iodine and by keeping the doping concentration constant, they developed a modified scintillator named RbSr₂I₅:Eu 2.5%. This modification led to significant performance improvements, including an increase in LY from 64 700 photons per MeV to 90 400 photons per MeV and an improvement in ER from 4% to 3% when exposed to 662 keV radiation from a¹³⁷Cs source. This demonstrates the effectiveness of composition engineering in enhancing the properties of scintillators without altering the dopant concentration. In addition, the better performance might be attributed to the fact that the stopping power is increased by inserting iodine also. A similar approach has been used to enhance the performance of the KBa₂I₅:Eu 4% scintillator. By substituting barium with strontium and maintaining the same dopant concentration, the research group improved the LY of the modified scintillator (KSr₂I₅:Eu 4%) from 90 000 photons per MeV to 94 000 photons per MeV when it was subjected to 662 keV radiation (¹³⁷Cs) with 2.4% ER.^198,222 Similarly, by simply replacing potassium in the KBa₂I₅:Eu 4% scintillator, a research group achieved an enhancement in LY from 90 000 photons per MeV to 102 000 photons per MeV.²²⁶ These studies underscore that by strategically applying compositional engineering at various sites within the scintillator structure AB₂X₅, researchers can achieve the optimal performance from the material.

The non-proportionality in the performance of AB₂X₅ compounds has been investigated by various researchers although studies are still limited on these compounds. Notwithstanding these endeavors, issues remain prevalent in AB₂X₅ perovskite systems. For example, the research team of L. Stand explored the non-linearity in the scintillator performance of KSr₂Br₅:Eu 5% under various levels of energy.¹⁹³ The findings revealed that the device achieved an ER of 3.5% when it was subjected to 662 keV energy emitted from a¹³⁷Cs source. Alternatively, a comparatively lower ER of 6.7% was recorded when the same device was subjected to 122 keV energy emitted from a⁵⁷Co source. Additionally, when the device was exposed to ²⁴¹Am (59.5 keV), an ER of 7.9% was noted. The results in ER show performance differences, which indicate a non-proportional response from the scintillator. Despite this, the research group asserted that within the measured gamma-ray energy range of 59 keV to 662 keV, there was a minimal deviation of about ±4% in the LY per unit energy. A separate study found that substituting bromine with iodine and fine-tuning the dopant concentration improved both the LY and ER.²²² The device with KSr₂I₅:Eu 4% demonstrated the highest LY of 94 000 photons per MeV and impressive ER of 2.4% when it was exposed to ¹³⁷Cs (662 keV). However, ERs of 5.7% and 7.2% were observed for ⁵⁷Co (122 keV) and ²⁴¹Am (59.5 keV), respectively. In the linearity test, the devices were excited with energies ranging from 14 keV to 1275 keV and minimal variation in LY within 3% across the entire energy range was observed. Another prominent research team focused on the scintillation performance of CsBa₂I₅:Eu 4% in the presence of frequently used radio isotopes.²²⁶ The performance metrics indicate that the LY and ER of the devices increased in the range of 2600 photons per MeV to 198 000 photons per MeV and 21.3% to 1.8%, respectively, due to the change in the excitation energy from 18 keV to 1332 keV. In addition, the device exhibited a better performance for higher energy spectroscopy. Particularly, under the sources of ⁶⁰C o(1173 keV), ²²Na (1274 keV) and ⁶⁰Co (1332 keV), the LY of 174 600 photons per MeV, 190 000 photons per MeV and 198 000 photons per MeV and ER of 2.1%, 1.9% and 1.8% were observed, respectively. In addition, excited by the popular source ¹³⁷Cs (662 keV), the device shows a high LY of 102 000 photons per MeV with an ER of 2.55%. This trend implies a strong scintillation response over a broad energy range. A few promising perovskite scintillators along with their performance metrics are summarized in Table 2 and SI Table S2.

Table 2 Performance metrics from AB₂X₅ perovskite scintillators under various concentrations of dopants and varying gamma-ray energy

Material		Crystal growth method	Volume (mm^3)	Emission wavelength (nm)	Source	LY photons per MeV	Decay time (μs)	ER (%)	Ref.
AB2X5	RbSr2Br5:1% Eu	BS	154	423	^137Cs (662 keV)	58 000	0.8	3.9	191
	RbSr2Br5:3% Eu			431		73 000	1	4.1
	RbSr2Br5:5% Eu			433		73 000	1.18	3.9
	RbSr2Br5:7% Eu			438		71 000	1.22	4.0
	RbSr2I5:1% Eu			438		74 000	0.7	3.5
	RbSr2I5:5% Eu			444		90 000	1	2.9
	RbSr2I5:7% Eu			445		89 000	1.1	2.8
	RbSr2Br5:2.5% Eu	BS	37	429	^137Cs (662 keV)	64 700	0.78	4.0	68
	RbSr2I5:2.5% Eu			445		90 400	0.89	3.0
	CsBa2I5:0.5% Eu	BS	30	430	^137Cs (662 keV	62 000		5.1	35
	CsBa2I5:5% Eu		50	435	^137Cs (662 keV)	80 000	∼1	2.3
					^241Am(59.5 keV)	—	—	7.5
	KSr2Br5:5% Eu	BS	24	427	^137Cs (662 keV)	75 000	1.07	3.5	193
					^57Co (122 keV)	—	—	6.7
					^241Am (59.5 keV)	—	—	7.9
	KSr2I5:4% Eu	BS	2	445	^137Cs (662 keV)	94 000	0.99	2.4	222
			8		^57Co (122 keV)	—	—	5.7
			8		^241Am (59.5 keV)	—	—	7.2
	RbBa2I5:3% Eu	BS	226	436	^137Cs (662 keV)	58 200	0.800	8.18	223
	LiSr2I5:3% Eu	BS	3619	497	^137Cs (662 keV)	60 000	1.331	3.5	192
	LiCa2I5:Eu			472		90 000	1.416	5.6
	LiSr2Br5:Eu			407, 476		32 000	1418	6.1
	KSr2I5:0.5% Eu	BS	—	448	^137Cs (662 keV)	—	—	3.5	224
					^57Co (122 keV)	—	—	6.5
	KSr2I5:2% Eu	BS	12	—	^137Cs (662 keV)	87 900	0.85	2.9	194
					^57Co (122 keV)	—	—	5.5
					^241Am (59.5 keV)	—	—	7.8
	KBa2I5:4% Eu	BS	4	444	^137Cs (662 keV)	90 000	0.91	2.4	198
					^241Am (59.5 keV)	—	—	7.6
	KSr2I5: 4% Eu	BS	2	445	^137Cs (662 keV)	94 000	0.99	2.4	222
			8		^57Co (122 keV)	—	—	5.7
			8		^241Am (59.5 keV)	—	—	7.2
	KSr2I5:4% Eu	BS	—	—	^137Cs (662 keV)	94 000	0.99	2.4	210
	KBa2I5:5% Eu		—	—		84 000	0.91	2.6
	CsBa2Br5:2% Eu	—	—	—	—	91 800	1.26	—	225
	CsBa2I5:4% Eu	BS	10	435, 466	^137Cs (662 keV)	102 000	1.20	2.55	226
					^241Am(59.5 keV)	9000	—	8.1
					^133Ba(81 keV)	12 300	—	7.2
					^133Ba (303 keV)	48 600	—	3.7
					^133Ba(356 keV)	53 250	—	3.3
					^133Ba (384 keV)	57 400	—	3.1
					^22Na (511 keV)	76 350	—	2.9
					^60Co(1173 keV)	174 600	—	2.1
					^22Na(1274 keV)	190 000	—	1.9
					^60Co(1332 keV)	198 000	—	1.8
	CsBa2I5:2% Eu; 1%Sm	BS	—	755, 430	^137Cs (662 keV)	45 000	2.1	3.2	221

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6.3. A₄BX₆ perovskite scintillators

A₄BX₆ is another zero-dimensional framework of perovskites, which is composed of a 4 : 1 ratio of alkali (AX) and alkaline earth metal halides (BX₂). In an effort of advancing zero-dimensional perovskite materials, the prominent research group led by L. Stand developed Cs₄CaI₆ with 4% Eu doping by means of reliable Bridgman–Stockbarger method and evaluated its performance metrics.²²⁷ They reported an LY of 51 800 photons per MeV and ER of 3.6% under exposure to ¹³⁷Cs (662 keV). To further enhance the performance of scintillators, they employed compositional engineering by substituting calcium with the iso-valent element strontium. This modification resulted in significant improvements in the performance of the new Cs₄SrI₆:Eu 4% device including a higher LY of 62 300 photons per MeV, better ER of 3.3%, and faster scintillation decay time of 1.9 μs compared to its predecessor. These findings underscore the potential of these materials for high-resolution gamma-ray spectroscopy, and Cs₄SrI₆:Eu 4% emerging as the most efficient and promising scintillator. Later, Rutstrom et al. explored the effects of europium doping concentration on the scintillation performance under ¹³⁷Cs (662 keV) excitation of Cs₄SrI₆ and Cs₄CaI₆ devices developed via the Bridgman-Stockbarger approach.⁷⁰ Due to the enhanced doping concentration of Eu in Cs₄SrI₆, the LY increased from 23 000 photons per MeV at 0.5% Eu dopant to 71 000 photons per MeV at 7% Eu along with improved ER from 6.6% to 3.2%. Similarly, Cs₄CaI₆:Eu showed an increased LY from 22 000 photons per MeV at 0.5% to 69 000 photons per MeV at 7% and improved ER from 4.8% to 3.6%. The decay times remained relatively stable, ranging from 1.63 μs to 1.90 μs, indicating the minimal impact from the dopant concentration. These findings indicate that the LY is enhanced due to additional activators, which necessarily augment the radiation recombination centers. However, the concentration of the activator should be optimized; otherwise, it would act as a self-absorber. They also examined the proportionality with respect to the excitation energy and observed a satisfactory performance, with the exception of the 0.5% Eu-doped Cs₄SrI₆ device at a lower energy. A subsequent study conducted by the same group introduced a new activator named ytterbium at different concentrations and evaluated the performance of both scintillators under ¹³⁷Cs (662 keV).¹⁹⁶ Their study revealed that the scintillator with Cs₄CaI₆ doped by 1% Yb attained an impressive LY of 43 000 photons per MeV and exhibited the optimal ER of 3.5%. Nonetheless, the decay time diminished with an increase in the Yb concentration. In the case of the Cs₄SrI₆ scintillator, the highest LY of 40 000 photons per MeV was noted with 3% Yb doping, whereas the optimal ER of 5.1% was attained with 1% Yb doping. In addition, the decay time exhibited a remarkable consistency across all concentrations for the Cs₄SrI₆ structure. Very recently, the prominent researcher Pestovich and colleagues investigated the scintillation properties of Rb₄CaBr₆:Eu with varying europium doping concentrations of 3%, 7%, and 9%.⁷² Their study revealed that increasing the Eu doping concentration significantly enhanced the LY, from 21 000 photons per MeV at 3% to 71 000 photons per MeV at 9%, when the devices were subjected to ¹³⁷Cs (662 keV) excitation. However, this improvement came at the cost of increased decay times, which increased from 2.07 μs at 3% to 2.83 μs at 9%. The ER also improved slightly from 7.7% at 3% to 6.9% at 9%. Although this study did not pay attention to the response against higher energy and proportionality performance, it reflects that Eu works efficiently as an activator to enhance the radiative recombination centers. Several promising perovskite scintillators and their performance metrics are shown in Table 3.

Table 3 Performance metrics of A₄BX₆, A₂BX₄ and A₂BB′X₆ perovskite scintillators under various dopant concentrations and varying gamma-ray energies

Material		Crystal growth method	Volume (mm^3)	Emission wavelength (nm)	Source	LY photons per MeV	Decay time (μs)	ER (%)	Ref.
A4BX6	Rb4CaBr6:3% Eu	BS	116	455	^137Cs (662 keV)	21 000	2.07	7.7	72
	Rb4CaBr6:7% Eu		—	456		63 000	2.77	7.5
	Rb4CaBr6:9% Eu		—	457		71 000	2.83	6.9
	Cs4SrI6:0.5% Eu	BS	27	460	^137Cs (662 keV)	23 000	1.73	6.6	70
	Cs4SrI6:3% Eu			460		57 000	1.74	3.9
	Cs4SrI6:7% Eu			460		71 000	1.88	3.2
	Cs4CaI6:0.5% Eu			462		22 000	1.63	4.8
	Cs4CaI6:3% Eu			462		52 000	1.65	4.3
	Cs4CaI6:7% Eu			462		69 000	1.90	3.6
	Cs4SrI6:4% Eu	BS	8	467	^137Cs (662 keV)	62 300	1.9	3.3	227
	Cs4CaI6:4% Eu			467		51 800	2.1	3.6
	Cs4CaI6:0.5% Yb	BS	125	446	^137Cs (662 keV)	30 000	3.50	4.6	196
	Cs4CaI6:1% Yb			446		43 000	3.08	3.5
	Cs4CaI6:3% Yb			446		42 000	2.15	4.8
	Cs4SrI6:0.5% Yb			446		27 000	2.04	9.2
	Cs4SrI6:1% Yb			446		35 000	2.28	5.1
	Cs4SrI6:3% Yb			—		40 000	2.11	5.5
A2BX4	K2BaI4:7% Eu	BS	36	—	^137Cs (662 keV)	57 000	—	2.9	228
	K2BaI4:7% Eu	BS	4	448	^137Cs (662 keV)	63 000	0.720	2.9	198
A2BB′X6	Cs2NaLaI6:2% Ce	BS	—	—	^137Cs (662 keV)	39 000	—	—	229
	Cs2NaLaI6:5% Ce			—	458	46 000	—	5
	Cs2NaLaI6:10% Ce			—	—	34 000	—	—
	Cs2NaYBr3I3:2% Ce	BS	9	—	^137Cs (662 keV)	41 000	0.056	3.5	210
	Cs2NaLaBr3I3:5% Ce		98	—		58 000	0.078	3.3
	Tl2LiGdCl6:10% Ce	BS	101	387	^137Cs (662 keV)	58 000	0.034	4.6	230
	Tl2LiGdCl6:1% Ce		101	387		45 700	—	—
	Tl2LiGdBr6:10% Ce	BS	75	422	^137Cs (662 keV)	17 400	0.029	17	231
	Tl2LiYCl6:3% Ce	BS	864	435	^137Cs (662 keV)	26 000	0.431	4.2	232
	(Cs,Tl)2LiLaBr6:3% Ce	BS	—	—	^137Cs (662 keV)	—	0.529	3.4	200
	Tl2NaScCl6	BS	—	430	^137Cs (662 keV)	20 000	0.181	5.3	233
	Tl2NaYCl6	BS	25	390	^137Cs (662 keV)	23 000	0.350	6.3	234
	Tl2LiScCl6	BS	—	385	^137Cs (662 keV)	26 000	1.147	8.3	235
	Tl2LiGdBr6	BS	20	435	^137Cs (662 keV)	27 000	0.105	7.2	236
	Tl2LiYCl6:1% Ce	BS	—	—	^137Cs (662 keV)	30 500	0.459	4.8	237
	Tl2LiYCl6:10% Ce		—	—		11 000	0.560	18

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6.4. A₂BX₄ perovskite scintillators

A₂BX₄ represents another distinct framework of perovskites characterized by its two-dimensional structure, consisting of a 2 : 1 ratio of alkali (AX) and alkaline earth metal halides (BX₂). In an effort to develop the A₂BX₄ perovskite framework, the prominent research group led by L. Stand developed an Eu-activated K₂BaI₄ scintillator for the first time via the Bridgman–Stockbarger method.²¹⁰ Throughout the synthesis process, the concentration of europium (Eu) was methodically adjusted between 5% and 10% to ascertain the ideal doping level for improving the scintillation efficacy. The optimization process revealed that a 7% concentration of Eu produced the most favorable outcomes. At the optimal doping concentration of 7% Eu, the K₂BaI₄:Eu crystals exhibited remarkable scintillation characteristics including the light output of roughly 57 000 photons per MeV and ER of 2.9% at 662 keV. The research examined the correlation between light output and the energy of the incoming radiation and reported that the light output increased proportionally with the radiation energy. In the case of γ-ray energies exceeding 100 keV, the light output was determined to be directly proportional to the deposited energy, exhibiting a minimal deviation of less than approximately 2%. Conversely, at energies below 50 keV, the crystals retained approximately 96% of their scintillation LY, demonstrating their efficacy in detecting low-energy radiation. This property is especially beneficial for applications necessitating sensitivity over a wide energy range. Numerous promising perovskite scintillators along with their performance metrics are displayed in Table 3.

6.5. Halide double perovskite scintillators

Initiating compositional engineering using various sized cations or anions in their respective sites in ABX₃ can result in the formation of a huge number of derivatives of PVSK materials. However, among them, A₂B⁺M³⁺X₆, called halide double perovskite (HDP) (given that its unit cell is twice that of lead halide perovskites), is more efficient and stable.^238,239 Using common X atoms at the octahedral vertices, a strong rigid link connects the B⁺X₆ and M³⁺X₆ octahedra.²⁴⁰ Because of their possible uses in radiation detection and medical imaging, the study of scintillator materials, especially those based on A₂B⁺M³⁺X₆ structures, has been a major focus of attention. Usually, when using the Bridgman–Stockbarger approach, a proven method for producing premium single crystals, the performance of these materials is critically explained based on their key performance parameters including LY, ER, and decay time. As part of developing intrinsic HDP-based scintillators, various research groups have adopted alkali metal halides or monovalent metal halides in combination with trivalent metal halides. With decay times in the range of 181 ns to 350 ns, and ERs between 5.3% and 6.3%, intrinsic materials such Tl₂NaScCl₆ and Tl₂NaYCl₆ show moderate LYs ranging from 20 000 photons per MeV to 23 000 photons per MeV.^233–236 These materials offer a basis for comprehending the basic features of the A₂B⁺M³⁺X₆ family. Doping is another strategy to enhance LY by creating additional recombination centers. However, doping these materials with rare earth elements including cerium, has shown to greatly improve their scintillation characteristics. The prominent researcher J. Glodo and research team developed high-performance scintillators based on Ce doping in Cs₂NaLaI₆ at concentrations of 2%, 5%, and 10%.²²⁹ The LYs were measured to be 39 000 photons per MeV, 46 000 photons per MeV, and 34 000 photons per MeV, respectively, and the light emission attributed to the d-f transitions of Ce³⁺ ions. Among them, the 5% Ce-doped Cs₂NaLaI₆ was identified as the optimized structure along with an ER of 5%. The non-proportionality of light output versus excitation energy for Cs₂NaLaI₆:5% Ce was investigated under exposure to gamma-ray sources (¹³⁷Cs, ²²Na, ⁵⁷Co, and ²⁴¹Am) in the energy range of 14.4 keV to 1274 keV. The photopeak positions were normalized to the incident energy and plotted relative to the 662 keV reference. The results demonstrated excellent linearity with deviations of less than ±5%. A slight deviation was observed only at lower energies (14 keV). This near-ideal linearity suggests that ER is primarily governed by photons statistics, with a calculated ER of 3.1% at 662 keV, highlighting the superior performance of the material. In a similar study, Cs₂NaLaBr₃I₃ doped with 5% Ce demonstrated an exceptional performance, with an LY of 58 000 photons per MeV, decay time of 78 ns, and excellent ER of 3.3%.²¹⁰ These findings highlight that Ce doping not only enhances the LY but also contributes to shorter decay times and improved ER, underscoring its critical role in optimizing scintillator materials. A higher Ce doping content does not always result in an improved performance, according to systematic studies on doping concentrations. For instance, Tl₂LiYCl₆:Ce 10% shows a much smaller LY of 11 000 photons per MeV and a longer decay time of 560 ns than the 1% doped variant, with an LY of 30 500 photons per MeV and decay time of 459 ns.²³⁷ This emphasizes the need of the optimizing doping levels to attain the intended scintillation characteristics. The research group of R. Hawrami et al. grew (Cs,Tl)₂LiLaBr₆:3% Ce crystals with different Cs-to-Tl ratios to address the non-congruence issue in Tl₂LiLaBr₆:Ce.²⁰⁰ The 50% Cs–50% Tl crystal grew as a single and crack-free crystal but exhibited a poor scintillation performance along with an ER of 6.3% at 662 keV, which is lower than that expected for Cs₂LiLaBr₆:Ce. Additionally, a dark last-to-freeze section was observed, likely due to thallium segregation during growth. To improve the performance, the Tl content was reduced, resulting in a 75% Cs–25% Tl crystal with better crystallinity, which was also single and crack-free. This composition demonstrated a significantly improved ER of 3.4% at 662 keV, which is comparable to that of Cs₂LiLaBr₆:Ce. Both samples exhibited radioluminescence peaks at 390 nm and 422 nm, and the decay time profiles of 81 ns and 529 ns, making it a more promising scintillator. All things considered, the methodical study of intrinsic and doped A₂B⁺M³⁺X₆ materials reveal that doping, especially with Ce, can greatly improve the scintillation characteristics. To reach the intended balance among LY, decay time, and ER, the dopant concentration and material composition must be carefully tuned. These findings also provide a lack of insightful extensive research on various HDP combinations with different activators, aiming to develop next-generation scintillator materials tailored for specific applications. Several promising perovskite scintillators along with their performance metrics are presented in Table 3 and SI Table S3.

7. Comparative study of various dimensional perovskite scintillators

Our research has explored high-dimensional to low-dimensional metal halide perovskite scintillators, especially comprised of alkali and alkaline earth metal halides as promising alternatives to traditional inorganic scintillators.^76,241 The unique quantum confinement effects and dielectric confinement in lower-dimensional perovskites enable precise tuning of their light absorption and emission properties.^42,63 This arises from the spatial isolation of metal halide octahedra, which are spaced apart by bulky cations. In these structures, the separated octahedral units behave similarly to discrete nanocrystals, leading to strong exciton localization.^{43,88,197,242} As a result, these confined excitons predominantly undergo radiative recombination, contributing to efficient light emission. These advancements in low-dimensional scintillators offer potential applications in various fields, including medical imaging, homeland security, and high-energy physics.²⁴³ Our investigation into perovskite scintillators reveals that their scintillation performance is strongly influenced by their structural dimensionality. Specifically, our analysis of 3D perovskites (ABX₃) shows that these detectors produce a high LY, which is greater than that of conventional scintillators such as CsI:Tl and NaI:Tl but to some extent, it is lower than their lower-dimensional perovskite counterparts.^69,111,222 Additionally, these 3D scintillators exhibit promising ER; however, their relatively slow decay times may limit their suitability for fast-timing applications. Notably, the RbSrI₃:Eu 5 mol% scintillator achieved the highest recorded LY of 78 700 photons per MeV and an ER of 2.8% when exposed to a¹³⁷Cs (662 keV) gamma source, though it exhibited a decay time of 1.03 μs.⁶⁹ In contrast, a better performance was also observed in the KCa_0.835Sr_0.165I₃:Eu 3% scintillator, which showed a high LY of 74 000 photons per MeV, highest ER of 2.5%, and decay time of 1.06 μs under the same radiation source.²¹⁵

In contrast to the 3D ABX₃ structures, the most extensively studied 2D perovskites with the AB₂X₅ configuration have demonstrated superior performances overall. These materials exhibited the highest LYs, best ERs, and shortest decay times among all dimensional variants investigated. Notably, the CsBa₂I₅:Eu 4% scintillator achieved an impressive LY of 198 000 photons per MeV and an ER of 1.8% under exposure to a⁶⁰Co source (1332 keV). This study further highlighted this consistent response of this scintillator across a wide energy range from 303 keV to 1332 keV. Additionally, high-performing candidates include KSr₂I₅:Eu (4%), with an LY of 94 000 photons per MeV and ER of 2.4% under ¹³⁷Cs (662 keV); KBa₂I₅:Eu (4%) with 90 000 photons per MeV and 2.4% ER; and CsBa₂I₅:Eu (5%), delivering 80 000 photons per MeV and an ER of 2.3% under the same source.^35,198,222

In contrast, the zero-dimensional perovskite structure (A₄BX₆) has been relatively less explored. Among this class, the most notable performer is Cs₄SrI₆:Eu (7%), which achieved an LY of 71 000 photons per MeV, ER of 3.2%, and decay time of 1.88 μs when tested under a¹³⁷Cs (662 keV) source.⁷⁰ Another strong candidate is Cs₄CaI₆:Eu (7%), which delivered an LY of 69 000 photons per MeV, ER of 3.6%, and decay time of 1.90 μs under the same irradiation source.⁷⁰ Additionally, Cs₄SrI₆:Eu (4%) also showed a promising performance, with an LY of 62 300 photons per MeV, ER of 3.3%, and decay time of 1.90 μs, again under ¹³⁷Cs exposure.²²⁷ Alternatively, the A₂BX₄ structure remains the least explored among the perovskite scintillator types. The best-performing material in this category is K₂BaI₄:Eu (7%), which demonstrated an LY of 63 000 photons per MeV, ER of 2.9%, and relatively fast decay time of 720 ns.²²⁸ These comparative findings highlight the importance of tailoring the dimensionality and composition of perovskites to specific application requirements, whether for high-sensitivity spectroscopy or ultrafast particle detection. Further investigation into low-dimensional systems, particularly A₄BX₆ and A₂BX₄ phases, and mixed-dimensional structures could unlock enhanced scintillation properties and novel functional applications. However, according to the review of a few studies, it can be concluded that the thermal stability decreases with reduced dimensionality, though a systematic study has not been reported to date.^206,227,247 Fig. 7, 8 and 9 present a comparative analysis of the LY, ER, and non-proportionality across scintillators of varying dimensionalities, respectively.

Fig. 7 Performance metric (light yield) of scintillation crystals as a function of (a–c) dopant concentration, (d–f) dimensional variation, and (g–i) thickness (volume). (a and i) Reprinted from ref. 194 with permission from Elsevier, Copyright 2018; (b) reprinted from ref. 244 with permission from Elsevier, Copyright 2019; (c and g) reprinted from ref. 196 with permission from Elsevier, Copyright 2020; (d) ref. 209 and 225; (e) ref. 72 and 245; (f) reprinted from ref. 197 with permission from RSC, Copyright 2018; and (h) reprinted from ref. 189 with permission from Elsevier, Copyright 2015.

Fig. 8 (a–k) Energy resolution comparison of various scintillation crystals with different compositions, dopants, and dimensions. (a) Reprinted from Ref. 69 with permission from Elsevier, Copyright 2024; (b) reprinted from ref. 189 with permission from Elsevier, Copyright 2015; (c) reprinted from ref. 246 with permission from Wiley, Copyright 2016; (d) reprinted from ref. 222 with permission from Elsevier, Copyright 2015; (e) reprinted from ref. 35 with permission from Elsevier, Copyright 2014; (f and g) reprinted from ref. 198 with permission from Elsevier, Copyright 2016; (h) reprinted from ref. 227 with permission from Elsevier, Copyright 2018; (i and j) reprinted from ref. 244 with permission from Elsevier, Copyright 2019; and (k) Reprinted from ref. 196 with permission from Elsevier, Copyright 2020.

Fig. 9 (a–i) Comparison of the non-proportional (relative light yield) behavior under various range of energies between conventional scintillators and perovskite-based scintillators with varying dimensions. The data are standardized to the light yield measured at 662 keV from a¹³⁷ Cs source. (a) Reprinted from ref. 249 with permission from IEEE, Copyright 2015; (b and c) reprinted from ref. 170 with permission from MDPI, Copyright 2022; (d) reprinted from ref. 250 with permission from Elsevier, Copyright 2012; (e) reprinted from ref. 209 with permission from IEEE, Copyright 2011; (f) reprinted from ref. 194 with permission from Elsevier, Copyright 2018; (g) reprinted from ref. 222 with permission from Elsevier, Copyright 2015; (h) reprinted from ref. 244 with permission from Elsevier, Copyright 2019; and (i) reprinted from ref. 198 with permission from Elsevier, Copyright 2016.

8. Comparative study with conventional scintillators

Commercially available scintillators including NaI(Tl), CsI:Tl, LYSO:Ce, and BGO stand out for their historical success and application in medical imaging, security screening, and nuclear science. However, each scintillator comes with trade-offs in terms of performance parameters including LY, decay time, and ER. The emergence of halide perovskite scintillators has advanced a new class of materials that is not only comparable but also in some cases surpasses these commercial standards. NaI:Tl scintillators are the most established radiation detectors having a moderate LY of approximately 42 000 photons per MeV and ER around of 5.4%.¹⁷⁰ However, although its decay time is very fast at only 234 ns, it is limited by the fact that it is hygroscopic and has a low ER. In addition, another widely used scintillator is CsI:Tl and it performs better in LY, reaching up to 87 000 photons per MeV, but with a relatively longer decay time of about 500 ns and better ER near 4.9%.¹⁷⁰ Furthermore, LYSO:Ce is recognized for its ultrafast response near to ∼45 ns, but suffers from poor ER ranging between 8% to 20%, depending on its crystal quality and configuration.²⁴⁸ Moreover, BGO is dense and robust but has one of the lowest LYs ranging from 8000 photons per MeV to 10 000 photons per MeV and poor ER, which is generally worse than 10%.²⁴⁸

In contrast, recent advancements in halide perovskite scintillators comprised of alkali and alkaline earth metal halides demonstrate significantly improved performances across several key metrics. For example, europium-doped AB₂X₅ perovskites including KSr₂I₅ and CsBa₂I₅ have shown LYs of 94 000 photons per MeV and 102 000 photons per MeV under ¹³⁷Cs (662 keV), respectively, which not only surpass NaI:Tl and CsI:Tl but also exceed even the best-performing commercial scintillators.^222,226 These materials maintain excellent ER values between 2.4% and 2.55% respectively, a major improvement over their traditional counterparts.

In addition, CsBa₂I₅:Eu demonstrated the highest performance under high energy ⁶⁰Co(1332 keV) with LY of 198 000 photons per MeV and ER of 1.8%.²²⁶ Additionally, their structural dimensionality plays a significant role in optimizing the scintillation properties. Similarly, 3D perovskites such as KCa_0.835Sr_0.165I₃:Eu and RbSrI₃:Eu exhibit high LYs of 74 000 photons per MeV to 78 700 photons per MeV and ERs as good as 2.5% to 2.8%.^69,215 However, their decay times are longer than that of commercial materials ranging from ns to μs (shown in Table 4). Meanwhile, 0D perovskites such as Cs₄SrI₆:Eu demonstrate slightly lower performances, with LYs of around 71 000 photons per MeV and ER near 3.2%.⁷⁰ However, they show better performances than NaI:Tl and CsI:Tl. Another promising composition is K₂BaI₄:Eu, which exhibits a balanced performance with an LY of around 70 000 photons per MeV, ER of 2.9%, and decay time near 720 ns.²²⁸ These values make it highly competitive with commercial scintillators and indicate its strong potential for practical deployment.

Table 4 Comparative performance metrics of perovskite scintillators to those of traditional scintillators

Material	Source	LY (photons per MeV)	Decay time (μs)	ER (%)	Ref.
NaI(Tl)	^137Cs (662 keV)	42 000	0.234	5.4	170
CsI:Tl	^137Cs (662 keV)	87 000	0.500	4.9
Lu1.95Y0·05SiO5:Ce	^137Cs (662 keV)	39 900		8.2	250
Lu2SiO5:Ce	^137Cs (662 keV)	35 900		10.6
BGO		8000–10 000	0.300	9.7–16	248
LYSO(Ce)		30 000–33 000	0.045	8–20%
CsI(Na)		38 000–44 000	0.630	5.8
CsF	^137Cs (662 keV)	1900	0.002–0.004	19	252
BaF2		1430	0.0008	10	130
CaWO4	^137Cs (662 keV	4800	6.800	6.6	253
CdWO4		19 700	0.104	6.5	21 and 254
RbSrI3:5% Eu	^137Cs (662 keV)	78 700	1.03	2.8	69
KCa0.835Sr0.165I3:3% Eu	^137Cs (662 keV)	74 000	1.060	2.5	215
CsBa2I5:5% Eu	^137Cs (662 keV)	80 000	∼1	2.3	35
KSr2I5:4% Eu	^137Cs (662 keV)	94 000	0.990	2.4	222
KBa2I5:4% Eu	^137Cs (662 keV)	90 000	0.910	2.4	198
CsBa2I5:4% Eu	^137Cs (662 keV)	102 000	1.200	2.55	226
	^60Co (1332 keV)	198 000		1.8
Cs4SrI6:7% Eu	^137Cs (662 keV)	71 000	1.88	3.2	70
Cs4SrI6:4% Eu	^137Cs (662 keV)	62 300	1.9	3.3	227
K2BaI4:7% Eu	^137Cs (662 keV)	63 000	0.720	2.9	198

---

The non-proportional performances between commercial and perovskite scintillators over a broad gamma energy range are illustrated in Fig. 9. Traditional scintillators including CsI:Na and NaI:Tl display significant non-proportionality, where their LY deviates notably from linearity.^170,249 This behavior underlines the constraints of these materials for precise spectroscopic applications, where a proportional response is essential. Conversely, LSO:Ce and LYSO:Ce both display a steadily increasing LY with gamma energy. LYSO:Ce performs better, with approximately 35% non-proportionality compared to 42% from LSO:Ce.²⁵⁰ This improved proportionality is one of the reasons behind the superior ER of LYSO:Ce. In contrast, halide perovskite scintillators containing Cs₄CaI₆:Eu and Cs₄SrI₆:Eu show that the concentration of Eu significantly impacts their LY response.⁷⁰ Lower concentrations (0.5%) result in more proportional responses for the Cs₄CaI₆:Eu scintillator, whereas higher concentrations (5%) are more effective for the Cs₄SrI₆:Eu scintillator. The most promising performance is observed in europium-doped materials including KSr₂I₅:Eu, KBa₂I₅:Eu, and KBa₂I₅:Eu.^194,198,251 These scintillators demonstrate nearly ideal LY proportionality across the full gamma energy spectrum, with minimal variation from a constant response. This behavior signifies their capability for gamma-ray detection with high-resolution over a broad energy range, as they overcome one of the fundamental limitations named non-proportionality. In summary, this comparative study underscores the remarkable progress of halide perovskite scintillators comprised of alkali and alkaline earth metal halides, particularly in achieving superior LY and ER and non-proportionality over traditional commercial materials.

Alternatively, limited studies have focused extensively on the moisture stability of newly developed scintillator materials derived from alkali halides and alkaline metal halides. However, it is apparent from existing data that many of them demonstrate significantly higher vulnerability to moisture compared to traditional scintillators such as NaI and SrI₂. For example, as shown in Fig. 10, NaI(Tl) consistently demonstrates minimal weight change under humid conditions, which indicates its superior moisture resistance.

Fig. 10 (a–f) Comparison of the hygroscopicity of perovskite and conventional scintillators. (a) Reprinted from ref. 69 with permission from Elsevier, Copyright 2024. (b) Reprinted from ref. 247 with permission from Elsevier, Copyright 2013. (c) Reprinted from ref. 207 with permission from IEEE, Copyright 2010. (d) Reprinted from ref. 255 with permission from Elsevier, Copyright 2012. (e) Reprinted from ref. 245 with permission from IEEE, Copyright 2023. (f) Reprinted from ref. 227 with permission from Elsevier, Copyright 2018.

In contrast, perovskite materials including RbCaBr₃, Rb₄CaBr₆, and CsSr_xBa_1-xI₃:Eu display much higher weight changes, which reflect how much they are susceptible to moisture. However, some structures including CsCaCl₃:Eu exhibit better moisture resistance. Moreover, zero-dimensional perovskites achieve better moisture resistance compared to their 3D counterparts, even close to some conventional scintillators (shown in Fig. 10f). Therefore, when aiming for commercial viability and large-scale application, it is necessary to prioritize enhancing the environmental stability of these materials. Without improvements, their prospective as replacements for traditional scintillators remains limited due to their compromised performances under humid conditions.

9. Economic viability of perovskite-based scintillators compared to conventional commercial scintillators

High-bandgap perovskite materials, i.e., materials derived from alkali and alkaline earth metal halides including CsSrI₃, KCaI₃, Cs₄CaBr₆, and CsCa₂Br₅, have recently emerged as promising candidates for scintillator and associated applications. In addition to their scintillation properties, these materials offer economic advantages over conventional scintillators. The raw materials used in their synthesis, including cesium bromide, strontium iodide, calcium bromide, and potassium iodide, are generally more abundant and less expensive than the rare-earth based compounds required for traditional commercial scintillators including LaBr₃:Ce, LYSO:Ce, and Gd₂O₃:Eu (https://www.sigmaaldrich.com/). For example, materials like LaBr₃, Lu₂O₃, and Eu₂O₃ are costly due to the scarcity and processing difficulty of rare-earth elements (https://www.sigmaaldrich.com/). Another key benefit of halide perovskite-related compounds lies in their relatively low melting points, especially compared to oxide-based scintillation materials. Most alkali halides and alkaline earth metal halides exhibit melting points below 1000 °C, with the exception of fluorine-containing compounds (https://www.webelements.com/caesium/compounds.html). In contrast, the compounds in traditional oxide scintillators containing Lu₂O₃, Y₂O₃, SiO₂, CeO₂, etc. possess significantly higher melting points, often up to 2700 °C, making them expensive to grow via melt-based crystal growth techniques (https://www.webelements.com/caesium/compounds.html). Furthermore, single crystals from low-bandgap halide perovskites have already been successfully synthesized using low-temperature solution-based growth methods such as inverse-temperature crystallization, slow solvent evaporation, temperature-lowering crystallization, and antisolvent vapor-assisted crystallization for radiation detection.⁵⁵ These methods can be adapted for the growth of high-bandgap perovskite single crystals, which offers substantial reductions in manufacturing costs by avoiding high-temperature processing. Overall, perovskite-based scintillators have a very bright economic future because of their potential to drastically reduce manufacturing costs through solution processability, easily accessible precursors, and less energy-intensive manufacture. Their outstanding performance-to-cost ratio and superior intrinsic scintillation qualities further improve their value proposition.

10. Perspective and prospective

Our studied halide perovskite-based scintillators have demonstrated impressive scintillation performances that surpasses many conventional commercial scintillators; however, their performances are still inferior compared to that of direct detectors including CsPbBr₃ with an ER of 1.4% or HPGe detectors.^256,257 In addition, current research shows the limited diversity of crystal growth techniques and most studies rely predominantly on the Bridgman–Stockbarger method. However, although the Bridgman-Stockbarger technique is an effective method for growing large-scale perovskite single crystals, with their size reaching inch-scale dimensions; however, it presents several limitations. The resulting crystals often suffer from non-uniform compositional distribution and localized structural defects, which can lead to inconsistencies in the performance of scintillators. Furthermore, this method is relatively expensive due to the high energy requirement during melting of compounds. A major challenge arises from the differing melting points of the constituent compounds. These disparities can cause one compound to solidify, while another remains in the liquid phase, which reduces the likelihood of complete chemical reactions, resulting in phase segregation or compositional inhomogeneity. This makes it particularly difficult to achieve uniformity across the entire crystal. Therefore, future research should place greater emphasis on alternative growth methods, specifically solution-based techniques at room temperature, which can offer better control over phase purity and compositional uniformity throughout the crystal.

Current studies largely focus on structures including ABX₃ and AB₂X₅, whereas there remains substantial unexploited potential in less-explored configurations such as A₄BX₆ and A₂BX₄. Thus, future research should place emphasis on exploring a broader range of structural dimensionalities. In addition, by strategically manipulating the cation and anion compositions within these structures, it is possible to fine-tune their emission spectra for aligning with the peak sensitivity of PMTs, which enhances their overall performance. Furthermore, emission tunability, LY enhancement and tunable decay time can be achieved by incorporating suitable activator ions in the host crystal. The choice of activator plays a pivotal role in modulating the decay time, which facilitates either rapid or slow scintillation responses based on the application, such as swift timing for high-rate counting systems or sharp imaging system. However, a significant limitation in the current studies is the narrow focus on a limited set of activators, predominantly europium, with only few explorations of samarium and ytterbium. Other promising dopants, including Nd, Tb, Ce, Pr, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, Mn, Cr, and Fe, have been largely overlooked and should be the focus of further investigation.

Most studies on the performance of scintillators have been reported as the best performer or carried out performance tests under an energy of 662 keV from ¹³⁷Cs sources. This limited report on a broad energy range constrains our understanding of scintillator behavior across a broader spectrum. Although a few studies have investigated non-proportionality over a wide energy range and showed performances that surpasses conventional scintillators, comprehensive and systematic evaluations remain limited. Thus, to develop reliable and high-performance devices over a broad energy range, it is crucial to conduct detailed non-proportionality analyses including the sources of non-proportionality across a wide energy range given that it has a direct impact on ER. Although certain scintillators display fast decay times, others demonstrate slower decay times. While slower decay is normally less desirable for fast counting applications, it can be advantageous in specific imaging systems. However, none of the studies that were reviewed explored or capitalized on this prospective advantage. Moreover, long-term stability under environmental conditions and radiation hardness under various ranges of energy are essential for practical deployment. However, these aspects have not been thoroughly investigated, and it raises concerns about the real-world applicability of these scintillators. Finally, the lack of a unified and standardized evaluation framework for perovskite scintillators poses a significant challenge in benchmarking their performance consistently across different studies. As a result, this constraint hampers meaningful comparisons and limits the reproducibility of findings and slows down technological optimization. Ultimately, this creates a barrier in the progress of these scintillators toward commercial viability and integration into practical radiation detection systems.

Finally, in addition to the quantum confinement effect, nanophotonic strategies, particularly the Purcell effect, which can be activated by impinging quantum dot or nanoparticles into the host matrix,^258,259 have gained interest to improve the scintillator efficiency. However, they are typically restricted to the nanoscale or very thin films due to the localized nature of plasmonic field enhancements. Recently, for the first time, the research group led by M. Makowski overcame this limitation by dispersing 100 nm plasmonic nanoparticles (Ag-spheroidal and cuboidal in shape) into a 5-mm-thick bulk composite containing CsPbBr₃ perovskite nanocrystals.²⁵⁹ The result is a significant boost in performance including an increase in the light emission intensity and decay rate by up to 3.2 and 4.2 times, respectively. This enhancement also translated to a measurable doubling of the scintillation yield under gamma-ray radiation from a²⁴¹Am source. This type of approach can be implemented in the case of perovskite scintillators derived from alkali and alkaline metal halide perovskites to enhance their scintillation performances.

11. Conclusion

This review presents the recent progress in halide perovskite scintillators, particularly those derived from alkali and alkaline earth metal halides, covering a range of structures from higher-dimensional frameworks to low-dimensional configurations. In addition, this review begins with key figures of merit of scintillation, fundamental principles of radiation detection and methods for fabricating high-quality single crystals. This study also provides a comprehensive overview of gamma-ray spectroscopy-related research, detailing various strategies and approaches adopted to enhance the performance of scintillators. These perovskite-based scintillators are proving to be strong contenders to traditional inorganic scintillators, offering improved performances for gamma-ray detection. Among the various dimensional structures explored, ABX₃ and AB₂X₅ types have been more extensively studied, whereas A₄BX₆ and A₂BX₄ structures remain relatively underexplored. However, our investigation demonstrated that low-dimensional perovskite scintillators tend to outperform their higher-dimensional counterparts. Notably, CsBa₂I₅:Eu 4% demonstrated a superior performance across energies ranging from 303 keV to 1332 keV, achieving its best results under ⁶⁰Co (1332 keV) with an LY of 198 000 photons per MeV and ER of 1.8%. Although it showed a slightly lower ER of 2.55% at 662 keV with an LY of 102 000 photons per MeV, interestingly the same structure with different concentrations of Eu (CsBa₂I₅:Eu 5%) delivered the best ER of 2.3% at 662 keV under ¹³⁷Cs. In the ABX₃ category, the best performance was observed in KCa_0.835Sr_0.165I₃:3% Eu with an LY of 74 000 photons per MeV and ER of 2.5% at 662 keV. In the A₄BX₆ class, Cs₄SrI₆:Eu 7% achieved the best results with LY of 71 000 photons per MeV and 3.2% ER at 662 keV, while for AB₂X₄, K₂BaI₄:Eu 7% stood out with an LY of 63 000 photons per MeV and 2.9% ER under ¹³⁷Cs irradiation. Finally, this review also covers the perovskite configuration including ABX₃ and A₂B⁺M³⁺X₆, which integrate alkali or alkaline earth metal halides with other halide compounds, and highlights their potential with enhanced scintillation performance.

However, although halide perovskite-based scintillators have demonstrated impressive scintillation performances over many conventional commercial scintillators, they are still inferior compared to the performance of direct detectors including CsPbBr₃ with an ER of 1.4% under ¹³⁷Cs irradiation. Current crystal growth methods (crystallization from melting) suffer from high energy costs and challenges due to differing solidification points (temperatures) of the constituent compounds. Often they lead to phase segregation, compositional inhomogeneity, and structural defects, which affect their scintillator performance. Therefore, future research should place greater emphasis on alternative cost effective and room temperature crystal growth methods. Current studies give more attention to certain dimensional structures, leaving others comparatively understudied. Future exploration should focus on a broader range of structural dimensionalities and compositional tuning to optimize the emission spectra for better alignment with PMT sensitivity. In addition, emission tunability, LY enhancement, and tailored decay times can be attained by integrating an appropriate activator for specific scintillation applications. However, current investigations are limited by their narrow focus on a few activators such europium, while many promising dopants such as Nd, Tb, Ce, Pr, Tl, and In remain underexplored. Most studies focus on the performance of scintillators at 662 keV (¹³⁷Cs), which limits insight into their behavior across a broader energy spectrum. In addition, comprehensive non-proportionality analyses and assessments of their long-term stability and radiation hardness remain less explored. Thus, addressing these gaps is necessary for obtaining high-performance and reliable scintillators for real-world applications.

In summary, although perovskite scintillators show significant potential, their effective transition from the laboratory scale to industrial phase requires the development of alternative cost-effective and room-temperature crystal growth methods, expansion of structural and compositional engineering, exploration of a wider range of activators, deciphering the performance behavior under broad energy range, enhancing the stability and radiation hardness, and implementing standardized testing protocols.

Consent for publication

All authors of this work have agreed and are ready to sign the transfer of copyright which empowers the publisher to protect the work against unauthorized use and to maintain the integrity of the work from a bibliographical and archival standpoint.

Author contributions

Md. Helal Miah: literature review, designed and executed the study, data analysis and writing of the original draft; Mohammad Aminul Islam, Phannee Saengkaew, Arshid Numan, and Mohammad Aminul Islam: data curation, discussion, software and validation; Mayeen Uddin Khandaker: conceptualization, supervision, and reviewing and editing of the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All data are available in the manuscript.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ra04980f.

Acknowledgements

The Sunway University Scholarship support received by Md. Helal Miah is acknowledged. During the preparation of this work, the authors used some online services (such as Grammarly and Gemini) in order to improve language and readability. After using these tools/services, the authors reviewed and edited the content as needed, and they take full responsibility for the content of the publication.

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