Intrinsic Scintillation Research Articles

Structure analysis and luminescence properties of red-emitting iodide scintillators with potassium-hexachloroplatinate(IV) type structure

Halide compounds with chemical formula A2MX6 (A: alkali metal or Tl, M: tetravalent metal, X: halogen) have previously been reported as intrinsic scintillators. However, some application fields e.g., utilizing waveguides, require novel scintillators with emission wavelengths longer than that of Cs2HfI6 as bright and red/infrared scintillators. Thus, in this study, cation-substituted Cs2HfI6 crystals (Rb2HfI6, K2HfI6, Cs2ZrI6, Rb2ZrI6, and K2ZrI6) were grown using the vertical Bridgman method, and their crystal structures were analyzed via single-crystal X-ray diffraction. The crystal structures of Cs2HfI6 and Cs2ZrI6 were determined to be cubic (space group: Fm3̅m), Rb2HfI6 and Rb2ZrI6 were tetragonal (space group: P4/mnc), and K2HfI6 and K2ZrI6 were monoclinic (space group P21/c). In addition, the emission spectra of Rb2HfI6, K2HfI6, Rb2ZrI6, and K2ZrI6 showed that their peaks exhibited a 25 nm red-shift in comparison with that of Cs2HfI6, whereas those of Cs2ZrI6 did not. For A2MI6 compounds, the emission was effectively determined by A-site cation substitution. Moreover, the pulse-height spectra were successfully measured for A2HfI6. The scintillation light outputs of Cs2HfI6, Rb2HfI6, and K2HfI6 were estimated to be approximately 60,000, 40,000, and 33,000 photons/MeV, respectively.

Bulk crystal growth and characterization of intrinsic scintillator CaNb2O6

Single crystal CaNb2O6 was grown by the optical floating-zone method and characterized for a new intrinsic scintillator. Under Ar and O2 growth atmospheres, the as-grown crystal rods (Ф 4–5 mm) exhibit a greyish and a colorless appearance, respectively, which are related with the oxygen vacancy. They both show a broad emission band with the maximum at 480 nm under UV excitation, while the one grown in O2 is brighter with a photoluminescence quantum efficiency of 44%. Consequently, a 25% higher scintillation light yield was obtained in the crystals grown in O2 compared to those in Ar. The other scintillation properties including radioluminescence spectrum, pulsed X-ray decay and afterglow were also characterized and compared.

Simulation of Electron-Hole Pairs Created by γ-Rays Interaction in Scintillation Crystals Using Geant4 Toolkit

Understanding the electron-hole pair (e-h pair) production process in scintillators is essential for further studies of the scintillation mechanism. The number of e-h pairs can be derived from theoretical and empirical approaches. However, simulation approaches have been chosen to study all the related physical processes and spatial information in scintillation crystals. Due to the complexity of particle interaction and physical properties of solid-state, the simulation approach could be better than the analytical approach. This work aims to develop a Geant4-based simulation toolkit for studying scintillation mechanisms in single-crystalline scintillators since Geant4 can accurately calculate particle energy loss. Due to the lack of a solid-state physic model for insulators, the plasmon creation and phonon loss process for electrons are implemented in the Geant4 toolkit. The electrons, which are created by γ-rays interaction in crystals, can make further ionization to generate secondary electrons or create plasmons, or lose energy to phonons. Then the created plasmons decay into e-h pairs. These electrons and holes are tracked with information on energy and position. Therefore, the number of created e-h pairs and their spatial distribution can be extracted. In this work, we will present the result of the creation process of e-h pairs per MeV in scintillation crystals induced by γ-rays interaction. The number of e-h pairs created by γ ray in some popular inorganic crystals has been calculated. Assuming maximum conversion efficiency from the e-h pair to the luminescence light, we can calculate the maximum light yield of the intrinsic scintillator.

Tungsten gallium-phosphate glasses as promising intrinsic scintillators

Tungsten gallium-phosphate glasses with composition NaPO3-20Ga2O3-xNa2WO4 (x = 0, 1, 3, 5 and 10% mol) were synthesized by the melt-quenching technique with high chemical stability and excellent optical properties, and evaluated as potential intrinsic glass scintillators. Fourier-transform infrared measurements showed a decrease in the amount of OH- groups for increasing Na2WO4 contents, while X-ray photoelectron spectroscopy revealed the presence of sole W6+ species. Optical transmission measurements showed a high level of transmittance (∼90%) over a broad spectral range (400 to 2500 nm). Luminescence was found to correspond to a broad emission characteristic of the WO42− complex that can be excited by ultraviolet and X-rays with average lifetimes ranging from 32 to 20 μs. At cryogenic temperatures, the NaPGaW glasses showed a significant increase in the luminescence emission.

Tungsten Gallium-Phosphate Glasses as Promising Intrinsic Scintillators

Tungsten Gallium-Phosphate Glasses as Promising Intrinsic Scintillators

Cs2HfCl6 doped with Zr: Influence of tetravalent substitution on scintillation properties

Homeland security is an emerging application field for inorganic scintillators. Detection and identification of radioactive materials in both individual and cargo transport requires scintillators with excellent energy resolution and high light output. On the other hand, the time characteristics of the scintillation are less crucial. Furthermore, price is a key parameter due to the large volume of the detectors. State-of-the-art halide scintillators e.g. SrI2:Eu or LaBr3:Ce can satisfy the requirements for light yield and energy resolution, but the production costs are very high due to complicated crystal growth and the high prices of rare earth metals. Cesium hafnium chloride (Cs2HfCl6) is a promising intrinsic scintillator with high light yield and excellent resolution. Moreover, its low hygroscopicity and cubic crystal structure could allow lower production costs compared to SrI2 or LaBr3. Intrinsic luminescence of Cs2HfCl6 is commonly attributed to self-trapped excitons. However, the luminescence of Cs2HfCl6 shows complex structure and temperature dependence. This might be explained by the presence of zirconium which is a common impurity in hafnium compounds. We present a study of the influence of zirconium doping on the scintillation properties of Cs2HfCl6. Single crystals of Cs2HfCl6 with zirconium doping from 0 to 1 mol% were grown by the vertical Bridgman method. Their phase purity, radioluminescence, scintillation decay, light yield, and afterglow were evaluated. Results show that zirconium doping can affect the scintillation properties of Cs2HfCl6 even at concentrations comparable to impurity levels of zirconium in hafnium compounds.

Sustainable preparation of ixora flower-like shaped luminescent powder by recycling crab shell biowaste

We report on cost-effective recycling of crab shell biowaste and synthesis of luminescent powders. XRD, thermogravimetric and, FTIR analysis revealed that calcination temperature must be beyond ∼800 °C to recycle biowastes crab (Ucides cordatus) shell into crystalline powder composed of CaO, and a small amount of CaCO3, NaCa(PO4), MgO, and Ca₁₀(PO₄)₆(OH)₂. SEM analysis revealed that calcination of crab shell biowaste at 800 °C/2 h produced the homogeneous cauliflower- and ixora flower-like shaped intrinsic luminescent powders. XEOL and thermoluminescence analyses suggest that F+-centers of the CaO structure are responsible for the efficient intrinsic scintillation of the crab shell powder. Therefore, our study shows a biomaterial promise derived from seafood wastes as natural dosimeter materials for industrial applications.

Air-Stable Bulk Halide Single-Crystal Scintillator Cs3Cu2I5 by Melt Growth: Intrinsic and Tl Doped with High Light Yield.

Ternary metal halides with large exciton binding energy have recently gained considerable attention in the optoelectronic field due to their high photoluminescence quantum yield and large Stokes shift. Here, efficient scintillators are designed based on these advantageous properties. For the first time, bulk Cs3Cu2I5 is grown using a melt method other than the intensively reported solution growth, and behaved as an intrinsic scintillator, emitting bright blue (∼450 nm) light under X-ray and γ-ray irradiation. Successful Tl doping at Cs sites tune the emission band over the entire visible range (400-700 nm) due to the synergetic effects of self-trapped excitons (STEs) and Tl centers. Notably, after doping with 1% Tl+, the scintillation light yield of Cs3Cu2I5 increases by nearly three times to 51 000 ± 2000 ph/MeV (Cs-137, 662 keV). Cs3Cu2I5:Tl shows a higher energy resolution of 4.5% at 662 keV than that of NaI:Tl and an excellent nonproportionality (<3%) in the γ-ray energy range of 60-1275 keV. A model of energy relaxation in Cs3Cu2I5:Tl scintillators is proposed and discussed. In particular, it is the first Cu-based halide scintillator that has air stability, good stopping power, and the ability to grow large bulk single crystals for practical application. This work provides a strategy for tuning and broadening the spectral range of STE emitters, and bridges the lead-free halide derivatives with scintillators.

Tl₂ZrCl₆ and Tl₂HfCl₆ Intrinsic Scintillators for Gamma Rays and Fast Neutron Detection

Thallium zirconium chloride (Tl2ZrCl6) and its hafnium analog (Tl2HfCl6) are intrinsic scintillators that have been recently reported, primarily for gamma-ray detection and spectroscopy. These crystals are especially useful for dual-mode detection, gammas exploiting the highly effective $Z$ of the materials, and fast neutrons using the (n, p) and (n, $\alpha$ ) reactions with chlorine. $Z_{\mathrm {eff}}$ (70.7) and high photoelectric conversion efficiency of the zirconium variant without any activator makes it particularly attractive in this regard. We report here on the growth of undoped Tl2ZrCl6 and Tl2HfCl6 crystals using vertical Bridgman technique. The X-ray emission spectrum shows a broadband in the range of 400–600 nm with a peak around 470 nm for both crystals. The light yields (LYs) of these crystals were measured under irradiation by a 137Cs gamma source (662 keV) and compared with that from the commercial Tl-doped NaI crystal. Normalized for the quantum efficiency of the photomultiplier (PMT) at the relevant emission wavelengths, the respective LYs of Tl2ZrCl6 and Tl2HfCl6 are estimated to be 40 000 and 18 000 photons/MeV with corresponding energy resolution of 5% and 9%, respectively. We have demonstrated for the first time the neutron detection capability of these materials showing excellent pulse shape discrimination (PSD) with figure of merit on the order of 2.4. PSD measurement of Tl2ZrCl6 using a 241AmBe source demonstrates well-separated gamma, recoiled proton, and alpha particle bands.

Advanced high-performance large diameter Cs2HfCl6 (CHC) and mixed halides scintillator

This paper reports on successful growth and performance evaluation of two large diameter Cs2HfCl6 (CHC) and Cs2HfCl4Br2 (CHCB), both recently developed scintillator crystals. The discovery of Cs2HfCl6 (CHC) as a scintillator has lately generated much interest in this material and its family, which belongs to the K2PtCl6 cubic crystal structure. CHC is an intrinsic scintillator that is non-hygroscopic, has no self-radioactivity, provides excellent energy resolution, and has excellent non-proportionality. CHC has a moderate density of 3.9 g/cm3 and an effective atomic number of 58. Reported in this paper are transparent crack-free single crystal CHC and CHCB boules of one inch in diameter, both grown using the vertical Bridgman method. Samples retrieved from the boules, sized 23mm dia. x 30mm and 23mm dia. x 26mm, respectively, are characterized for their optical and scintillation properties. Energy resolutions of 3.5% and 3.7% (FWHM) at 662 keV, respectively, are reported. Results comparable to previously reported results for smaller crystals have been obtained. Studies on light yield, decay time, non-proportionality, as well as detector characterization are also reported.

Neutron detection performance of gallium nitride based semiconductors

Neutron detection is crucial for particle physics experiments, nuclear power, space and international security. Solid state neutron detectors are of great interest due to their superior mechanical robustness, smaller size and lower voltage operation compared to gas detectors. Gallium nitride (GaN), a mature wide bandgap optoelectronic and electronic semiconductor, is attracting research interest for neutron detection due to its radiation hardness and thermal stability. This work investigated thermal neutron scintillation detectors composed of GaN thin films with and without conversion layers or rare-earth doping. Intrinsic GaN-based neutron scintillators are demonstrated via the intrinsic 14N(n, p) reaction, which has a small thermal neutron cross-section at low neutron energies, but is comparable to other reactions at high neutron energies (>1 MeV). Gamma discrimination is shown to be possible with pulse-height in intrinsic GaN-based scintillation detectors. Additionally, GaN-based scintillation detector with a 6LiF neutron conversion layer and Gd-doped GaN detector are compared with intrinsic GaN detectors. These results indicate GaN scintillator is a suitable candidate neutron detector in high-flux applications.

Investigation of CeBr3−xIx scintillators

Abstract CeBr3 is an intrinsic scintillator that has promising performance capabilities such as 60,000 ph/MeV and ~4% energy resolution at 662 keV. It has been shown that halide mixing can improve the light yield of single halide scintillators. In this work we attempted to improve on CeBr3 by creating a mixed halide composition CeBr3−xIx. Analysis of melt-freeze samples revealed that the light yield of CeI3 rapidly increases as bromine is substituted. Thirteen mm diameter crystals were grown using the vertical Bridgman technique, with x values between 0.03 and 1. The CeBr2.5I0.5 crystal segregated into two sections, colorless and green, while the CeBr2I crystal was entirely green. X-ray diffraction analysis was used to identify the two phases. Decay time increased as iodine concentration increased. All colorless phase crystals reached 65,000 ph/MeV, with the CeBr2.94I0.06 crystal achieving 3.9% energy resolution at 662 keV. The green phase crystals both reached light yields of 78,000 ph/MeV, the highest ever recorded for a CeI3 phase crystal.

Tl2HfCl6 and Tl2ZrCl6: Intrinsic Tl-, Hf-, and Zr-based scintillators

The re-discovery of Cs2HfCl6 (CHC) as a scintillator has lately generated much interest in this material and its family, which belongs to the K2PtCl6 cubic crystal structure [1]. CHC is an intrinsic scintillator that is non-hygroscopic, has no self-radioactivity, provides good energy resolution, and has good non-proportionality. In this paper we are reporting growth and scintillation properties of two new and Tl-, Hf- and Zr-based compounds of Tl2HfCl6 and Tl2ZrCl6, with physical densities of 5.1 g/cm3 and 4.5 g/cm3, respectively and effective atomic numbers (Zeff) of 71 and 69, respectively. Samples from successfully grown 16 mm diameter boules were cut, processed, and characterized for their scintillation and radiometric properties. Energy resolutions of 3.7% (FWHM) for THC and 3.4% for TZC (FWHM) at 662 keV are measured. Primary luminescence decay times of 1.1 μs and 2.3 μs, for Tl2HfCl6 and Tl2ZrCl6, respectively, as well as good proportionality for both materials have been observed.

15 lutetium compounds screened for Ce[formula omitted] activated scintillation

We report the scintillation luminosities, decay times, and emission wavelengths of eight new Ce3+ activated lutetium scintillators, two of which are also new compounds. Candidate compounds were (1) selected by empirical estimates of densities, bandgaps, and the potentials for Ce3+ activation; (2) synthesized as crystalline granules in arrays of automated furnaces; (3) verified by X-ray diffractometry; and (4) measured by X-ray luminescence spectroscopy and pulsed X-rays. By calibrating with Lu2SiO5:Ce crystals of similar grain size (average 0.5 mm) we estimate that the luminosities of the eight new scintillators range from 1000 to 27,000 photons/MeV and all show the characteristic Ce3+ doublet emission. The same undoped compounds have intrinsic scintillation with lower luminosities and different emission spectra. While most of these scintillators do not have properties that make them candidates for widespread use, their data will contribute to a deeper understanding of the factors that limit scintillation performance. In addition, we list seven lutetium compounds whose intrinsic scintillation was quenched by the incorporation of Ce3+.

New Intrinsic Scintillator with Large Effective Atomic Number: Tl2HfCl6 and Tl2ZrCl6 Crystals for X-ray and Gamma-ray Detections

New Intrinsic Scintillator with Large Effective Atomic Number: Tl2HfCl6 and Tl2ZrCl6 Crystals for X-ray and Gamma-ray Detections

Tl2ZrCl6 crystal: Efficient scintillator for X- and γ-ray spectroscopies

Crystal growth and luminescence properties of the intrinsic Tl2ZrCl6 scintillation crystal are presented. The proposed crystal is a promising candidate for γ-ray detection owing to its high light yield, good energy resolution, and high effective atomic number (Z = 69). It has a cubic structure, which is desirable for a large-size-crystal growth. In this study, the crystal is grown from its melt using the Bridgman technique. The emission peak under X-ray excitation is found at 460 nm, with a primary decay time of 2.7 μs. The light yield and energy resolution are estimated to be 47,000 photons/MeV and 4.3% (full-width-at-half-maximum), respectively, obtained from the pulse-height spectra under a 137Cs 662-keV γ-ray excitation.

Scintillation yield of hot intraband luminescence

Cathodoluminescence yield of hot intraband luminescence (IBL) under ∼100 keV pulsed electron excitation was evaluated in several halide and oxide compounds. Its values lie in the range of 5–35 ph/MeV, which is in accordance with the general assumptions on the mechanism of IBL. The expected inverse correlation between the IBL yield and highest phonon energies was found for binary compounds, therefore the IBL yield dependence on charge carrier thermalisation rate was experimentally confirmed. Consequently, CsI was found to have the highest IBL yield (33 ph/MeV) compared to other studied crystals due to its lowest phonon energy among them. The studied sample of pure CsI shows both cathodoluminescence and scintillation yields significantly higher than the value frequently cited earlier (2000 ph/MeV), pointing out that its intrinsic scintillation properties might be currently underestimated. CsI shows good coincidence time resolution (CTR) up to 110 ps FWHM in a TOF-PET-like geometry, which surpasses all the materials studied so far except the much more expensive scintillators from the lutetium-yttrium silicates family (CTR values up to 73 ps). Therefore, CsI can be considered as a promising material for low-cost TOF-PET machines. For complex (ternary) compounds the phonon energies and IBL yield are not noticeably correlated, and the highest spectral yield (at 2.5 eV) among the studied compounds is shown by Na2Mo2O7.

Optimization, evaluation and calibration of a cross-strip DOI detector

This study depicts the evaluation of a SiPM detector with depth of interaction (DOI) capability via a dual-sided readout that is suitable for high-resolution positron emission tomography and magnetic resonance (PET/MR) imaging. Two different 12 × 12 pixelated LSO scintillator arrays with a crystal pitch of 1.60 mm are examined. One array is 20 mm-long with a crystal separation by the specular reflector Vikuiti enhanced specular reflector (ESR), and the other one is 18 mm-long and separated by the diffuse reflector Lumirror E60 (E60). An improvement in energy resolution from 22.6% to 15.5% for the scintillator array with the E60 reflector is achieved by taking a nonlinear light collection correction into account. The results are FWHM energy resolutions of 14.0% and 15.5%, average FWHM DOI resolutions of 2.96 mm and 1.83 mm, and FWHM coincidence resolving times of 1.09 ns and 1.48 ns for the scintillator array with the ESR and that with the E60 reflector, respectively. The measured DOI signal ratios need to be assigned to an interaction depth inside the scintillator crystal. A linear and a nonlinear method, using the intrinsic scintillator radiation from lutetium, are implemented for an easy to apply calibration and are compared to the conventional method, which exploits a setup with an externally collimated radiation beam. The deviation between the DOI functions of the linear or nonlinear method and the conventional method is determined. The resulting average of differences in DOI positions is 0.67 mm and 0.45 mm for the nonlinear calibration method for the scintillator array with the ESR and with the E60 reflector, respectively; Whereas the linear calibration method results in 0.51 mm and 0.32 mm for the scintillator array with the ESR and the E60 reflector, respectively; and is, due to its simplicity, also applicable in assembled detector systems.

Light yield determination in large sodium iodide detectors applied in the search for dark matter

Application of NaI(Tl) detectors in the search for galactic dark matter particles through their elastic scattering off the target nuclei is well motivated because of the long standing DAMA/LIBRA highly significant positive result on annual modulation, still requiring confirmation. For such a goal, it is mandatory to reach very low threshold in energy (at or below the keV level), very low radioactive background (at a few counts/keV/kg/day), and high detection mass (at or above the 100 kg scale). One of the most relevant technical issues is the optimization of the crystal intrinsic scintillation light yield and the efficiency of the light collecting system for large mass crystals. In the frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project large NaI(Tl) crystals from different providers coupled to two photomultiplier tubes (PMTs) have been tested at the Canfranc Underground Laboratory. In this paper we present the estimates of the NaI(Tl) scintillation light collected using full-absorption peaks at very low energy from external and internal sources emitting gammas/electrons, and single-photoelectron events populations selected by using very low energy pulses tails. Outstanding scintillation light collection at the level of 15 photoelectrons/keV can be reported for the final design and provider chosen for ANAIS detectors. Taking into account the quantum efficiency of the PMT units used, the intrinsic scintillation light yield in these NaI(Tl) crystals is above 40 photons/keV for energy depositions in the range from 3 up to 25 keV. This very high light output of ANAIS crystals allows triggering below 1 keV, which is very important in order to increase the sensitivity in the direct detection of dark matter.

Intrinsic scintillators: TlMgCl3 and TlCaI3

Two intrinsic scintillators TlMgCl3 and TlCaI3 with excellent energy resolution and fast scintillation decay are presented. Crack-free transparent crystals were successfully grown by the Bridgman method. The scintillation light yields of TlMgCl3 and TlCaI3 are approximately 30,600ph/MeV and 42,200ph/MeV, respectively. Their energy resolutions at 662keV are 3.7% for TlMgCl3 and 6.2% for TlCaI3. The scintillation decay times of TlMgCl3 are 46ns, 166ns, and 449ns. The scintillation decay times of TlCaI3 are 62ns, 200ns, and 1.44μs. Under X-ray irradiation, the emission of TlMgCl3 is between 300nm and 540nm with the peak at 409nm. TlCaI3 has a broad emission band between 300nm and 750nm. Due to their excellent scintillation properties, these two scintillators can be developed for gamma-ray detection. Additionally, TlMgCl3 is non-hygroscopic.