CdWO4 Single Crystals Research Articles

Dislocation motion, interaction and twinning during microindentation of CdWO4 single crystals

In this paper dislocations and twins formed under Vickers indentation of different planes of CdWO4 crystals are studied. It has been found that on (010) and (100) faces fresh dislocations run along [001] from an imprint of Vickers pyramid. On (001) twins with (010) twinning plane induced by indenter propagate along [100]. Twins with {011} twin planes are induced by indentation on (100). Possible dislocation dissociation reactions and role of the resulting partial dislocations in twinning during microindentaion in cadmium tungstate are discussed.

Dislocations in monoclinic cadmium tungstate CdWO4

Several types of dislocations were found in CdWO4 single crystals. In-grown dislocations slip on (100) plane parallel to [001]. Dislocation pits were revealed in mechanically polished samples on (100) and (010) planes. Based on the results of chemical etching, it was inferred that the lattice dislocations gliding along < 001 > and < 100 > can split into partial dislocations. In the samples annealed at T = 600ºC for 2 h dislocation density decreased noticeably. After annealing shape and sizes of helical etch pits on (100) and (001) planes transform to those typical of lattice dislocations.

Evaluation of Cerium-Doped Lanthanum Bromide (LaBr3:Ce) Single-Crystal Scintillator’s Luminescence Properties under X-ray Radiographic Conditions

In the present study, the response of the crystalline scintillator LaBr3:Ce when excited with X-rays at tube voltages from 50 kVp to 150 kVp was investigated, for possible use in hybrid medical-imaging systems. A single crystal (10 × 10 × 10 mm3) was irradiated by X-rays within the aforementioned tube-voltage range, and the absolute efficiency (AE), as well as the detective quantum efficiency for zero spatial-frequency (DQE(0)), were measured. The energy-absorption efficiency (EAE), the quantum-detection efficiency (QDE) and the spectral compatibility with various optical photodetectors were also calculated. The results were compared with the published data for the LaCl3:Ce, Bi4Ge3O12 (BGO), Lu2SiO5:Ce (LSO), and CdWO4 single crystals of equal dimensions. The AE values of the examined crystal were found to be higher than those of the compared crystals across the whole X-ray tube-voltage range. Regarding the EAE, LaBr3:Ce demonstrated a comparatively better performance than the LaCl3:Ce crystal. The emitted-light spectrum of LaBr3:Ce was found to be compatible with various types of photocathodes and silicon photomultipliers. Moreover, the LaBr3:Ce crystal exhibited excellent performance concerning its DQE(0). Considering these properties, the LaBr3:Ce crystal could be considered as a radiation-detector option for hybrid medical-imaging modalities, such as PET/CT and SPECT/CT.

Complex nature of the interaction of AFM tip with the freshly splitted (010) surface of CdWO4 and ZnWO4 single crystals

The discovery of new low-dimensional materials formed by splitting known bulk crystals into separate elementary layers has enabled future realization of still undiscovered potential of long-studied materials. High-resistivity two-dimensional films obtained by splitting ionic layered crystals are of great interest for creating new vertical van der Waals heterostructures. For unveiling the potential of using two-dimensional monocrystalline films of CdWO4 and ZnWO4, it is necessary to gain a more penetrating insight into the processes that occur on the freshly splitted surface of the crystals. In the present study, for the first time we investigated the effect of AFM probe on the (010) surfaces of freshly splitted CdWO4 and ZnWO4 single crystals. It is shown that the observed “protruding areas” on the splitted surfaces of these crystals are of a complex nature, and they are mainly related with the transfer of charges, as well as with the formation of complete bonds on the surface of the broken crystal lattice of examined crystals. The charge spots observed on the splitted surface of the crystals arise as a result of local charging by AFM probe. As a result of charge diffusion, as well as under the influence of ambient medium, the complete dissipation of charges occurs in less than 2 h.

Slip and twinning during deformation of CdWO4 single crystals

Cadmium tungstate, CdWO4, is widely used as a material for scintillating detectors in medicine, prevailingly, in computer tomography, as well as in high energy physics. Many studies consider luminescent properties of these crystals. However, there are few publications concerning structural defects, which can influence practically important properties of cadmium tungstate. In this work strain induced twinning and dislocation slip of CdWO4 single crystals are investigated.

Efficiency Properties of Cerium-Doped Lanthanum Chloride (LaCl3:Ce) Single Crystal Scintillator under Radiographic X-ray Excitation

The aim of this study is to evaluate the suitability of crystalline scintillator LaCl3:Ce for possible use in hybrid medical imaging systems, such as PET/CT and SPECT/CT scanners. For this purpose, a single crystal (10 × 10 × 10 mm3) was irradiated by X-rays within the tube voltage range from 50 to 150 kVp, and the absolute efficiency (AE) was measured experimentally. The energy absorption efficiency (EAE), quantum detection efficiency (QDE), and the spectral compatibility with various optical detectors were also calculated with the use of mathematical formulas. The results were compared with published data for Bi4Ge3O12 (BGO), Lu2SiO5:Ce (LSO), and CdWO4 single crystals of equal dimensions, commonly used in medical imaging applications. The luminescence efficiency values of the examined crystal were found to be higher than those of LSO, BGO, and CdWO4 crystals, within the whole X-ray tube voltage range. In the matter of EAE, LaCl3:Ce demonstrated reduced performance with respect to LSO and CdWO4 crystals. The emission spectrum of LaCl3:Ce was found to be compatible with various types of photocathodes and silicon photomultipliers (SiPMs). Considering these properties, LaCl3:Ce crystal could be considered suitable for use in hybrid medical imaging systems.

Luminescence Efficiency of Cadmium Tungstate (CdWO4) Single Crystal for Medical Imaging Applications

Background: In this study, the light output of a cadmium tungstate (CdWO4) single crystal was measured under various X-ray radiographic energies. Methods: A CdWO4 single crystal (10 × 10 × 10 mm3) was exposed to X-rays in the 50–130 kVp range. Measurements were evaluated against published data for single crystals of equal dimensions (CaF2:Eu and Lu3Al5O12:Ce). Since the crystal was examined for application in medical imaging detectors, the emitted optical spectrum was classified with respect to the spectral compatibility of numerous commercial optical sensors. Results: The luminescence efficiency (LE) was found to constantly increase with X-ray energy and was higher than that of CaF2:Eu for energies above 90 kVp. However, the efficiency of the previously published Lu3Al5O12:Ce was found to be constantly higher than that of CdWO4. The light emitted from CdWO4 can be optimally detected by certain charge-coupled devices (CCDs), amorphous silicon photodiodes, and photocathodes. Conclusions: The high density (7.9 g/cm3) of CdWO4 and the luminescence signal of this material make it suitable for medical imaging (such as dual energy), high-energy physics or for applications of scintillators in harsh environments.

Fluorescence properties of divalent and trivalent europium ions in CdWO4 single crystals grown by the Bridgman method

Optical absorption, excitation, and fluorescence were investigated in Eu ion-doped CdWO4 single crystal grown by a modified Bridgman method. The results indicate that Eu2+ and Eu3+ ions coexist in CdWO4 crystal and an energy transfer occurs between these Eu2+ and Eu3+ ions. When the crystal is excited by 266-nm light, the energy corresponding to the 4f65d to 8S7/2 transition of Eu2+ ions results in the excitation of the Eu3+ ions to the 5DJ level. The effect on fluorescence of annealing in oxygen at various temperatures was investigated. The excitation intensity of Eu2+ ions at 266 nm decreases as annealing temperature increases from 300 K to 1073 K, but it remains at a certain equilibrium level when the annealing temperature is further increased.

First-principles calculations of the structural, electronic and elastic properties of ZnWO4 and CdWO4 single crystals at the ambient and elevated pressure

Two technologically important tungstate crystals – CdWO4 and ZnWO4 – are studied theoretically using the plane wave based first-principles calculations. The optimized crystal structures were used to calculate the structural, electronic and elastic properties of both materials under varying hydrostatic pressure. It was shown that the band gaps, which are direct at ambient pressure, turn into indirect ones in both compounds in the pressure range from 5 to 10 GPa, whereas the values of the band gaps themselves depend only slightly on the applied pressure. Differences in compressibility along the crystallographic axes and the W–O, Cd–O, Zn–O chemical bonds were revealed and quantified by calculating the pressure coefficients for all these characteristic distances. The first estimations of the complete elastic tensor constants for both materials are reported.

Ab-initio studies of the electronic and optical properties of ZnWO4 and CdWO4 single crystals

Two technologically important materials – cadmium tungstate CdWO4 and zinc tungstate ZnWO4 – are studied in details using the plane wave based first-principles calculations. The optimized crystal structures were used to calculate the electronic and optical properties of both materials. Comparison of the calculated results with the available in the literature experimental X-ray photoelectron spectra (XPS) and reflectivity spectra yielded good agreement and allowed for assignment of the experimentally observed features, including those induced by difference between the Cd 4d and Zn 3d states. In addition, the variations in the electron density distributions related to two crystallographically in equivalent oxygen positions were revealed.

Growth and optical properties of Cr3+-doped CdWO4 single crystals

A high-quality Cr3+:CdWO4 single crystal at a size of approximately \Phi 25 \times 80 mm is grown using the Bridgman method with CdO, WO3, and Cr2O3 as raw materials and their molar ratio of 100:100:0.5. The temperature gradient of solid-liquid interface at growth is approximately 50 oC/cm and the growth rate is 0.05 mm/h. The X-ray diffraction (XRD), absorption, excitation, and emission spectra of different parts of the as-grown and O2-annealed crystals are investigated. Two strong broad optical absorption bands of about 472 and 708 nm are observed, and they are associated with the transitions 4A2->4T1 and 4A2->4T2. The weak 4T2->2E transition (the R-line) at 632 nm is also observed. The crystal-field parameter Dq and the Racah parameters B and C are estimated to be 1 412.4, 776.8, and 3 427.6 cm-1, respectively, according to the absorption spectra and crystal-splitting theory. A broadband fluorescence at about 1 000 nm due to 4T2->4A2 transition is produced by exciting the samples at 675 nm. After being annealed in an O2 atmosphere, the crystals become more transparent, while the effective light absorption of Cr3+ ions is evidently enhanced and the emission intensity is also strengthened due to the reduction of oxygen vacancies in the CdWO4 crystal after annealing.

Luminescence of Bi-doped CdWO4 single crystals

The bismuth-doped CdWO4 single crystals are prepared by the Bridgman method. The absorption spectra, emission spectra and X-Ray photoelectron spectroscopy (XPS) of various parts of as-grown Bi: CdWO4 crystal are investigated. The absorption edge of CdWO4 crystal is red-shifted from 345nm to 399nm due to the introdution of Bi into crystal. The four emission peaks at 470, 528, and 1078 and a weak peak at 1504 nm are observed under the excitation of light beams at 311, 373, 808, and 980 nm. From the X-Ray photoelectron spectroscopic measurements of the Bi: CdWO4 single crystals, Bi2O3 (Bi3+) and NaBiO3 (Bi5+) samples, we infer that there coexit Bi3+ and Bi5+ in Bi: CdWO4 crystals. The fluorescence emissions of visible bands at 470 and 528 nm result from both WO6-6 in Bi: CdWO4 and Bi3+ ions doped in the lattice, while the fluorescence emission of the near infrared band at 1078 nm is due to the Bi5+ luminescence. The analysis result of XPS is consistent with the change of the fluorescence intensity: along the growing direction the emission intensity at 1078 nm and the content of Bi5+ ion both decrease gradually, while the intensity at 528 nm and the number of Bi3+ both increase gradually.

CdWO4 crystal growth and production of a spectrometric detection unit with a large-volume (V ≅ 350 cm3) crystal

Large-volume (V ≅ 350 cm3) CdWO4 single crystals of high optical quality have been grown by the Czochralski method. A scintillation detection unit based on a large CdWO4 crystal has been produced and its characteristics have been studied.

Radiation Damage in CWO Scintillation Crystals With Different Defects

Results are reported on the effects of gamma-radiation upon scintillation characteristics of CdWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CWO) single crystals with different defects. The defect structure before irradiation determines the radiation-induced changes of the CWO crystals. In some CWO samples, the dose of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Gy caused destruction of defects responsible for induced absorption. Doping by Li leads to reduction of defects responsible for color and trapping centers in CWO crystals. The annealing in a reductive medium (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) or doping with Me <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> significantly increases radiation stability of CWO.

The growth of ZnWO4 and CdWO4 single crystals from melt by the low thermal gradient Czochralski technique

The features of growth of single crystals of cadmium and zinc tungstates by the low thermal gradient Czochralski technique have been investigated. The effect of change in the growth conditions on the crystal morphology and the capture of macroinclusions is studied. Crystals 45 mm in diameter and up to 150 mm long (cylindrical part) are obtained. Their optical and scintillation characteristics are measured.

Extrapure WO3 for the preparation of CdWO4 single crystals

A process has been developed for the preparation of extrapure WO suitable for crystal growth. The method involves thermal decomposition of ammonium paratungstate (APT) powder at 230–250° C, resulting in the formation of soluble ammonium metatungstate (AMT). The AMT solution is then purified and hydrolyzed. The resultant purified APT is converted to WO3. The reaction yield is at a level of 95%. The contents of metallic impurities, especially those of polyvalent impurities (Fe, Mn, Cu, Pb, V, Cr, and others), capable of influencing the optical properties of crystals, was comparable to their detection limit: 10−6 to 10−5 wt %. The purification process was tested using a pilot-scale system. The purity was confirmed by the synthesis of high-quality CdWO4 crystals.

The deformation and fracture behavior of the scintillating crystal cadmium tungstate

Cadmium tungstate single crystals are excellent scintillators and have vast potential as γ- and x-ray detectors. However, they are brittle and as a result, are difficult to manufacture. An understanding of deformation and fracture behavior is necessary in order to improve the ability to fabricate these crystals. Thus, this paper explores the deformation and fracture behavior of CdWO4 single crystals using nano-and microindentation techniques.

Structural Defects in Czochralski-Grown CdWO4 Single Crystals

Structural imperfections (dislocations, pores, and twins) in CdWO4 single crystals are investigated by microstructural analysis and x-ray diffraction. The radial dislocation distribution in the crystals is determined. The origin of pores and ways of eliminating them from CdWO4 crystals are discussed.

Production of the high-quality CdWO4 single crystals for application in CT and radiometric monitoring

Effects of the intrinsic and dopant-induced defects in cadmium tungstate (CWO) crystals upon their luminescence and functional characteristics have been studied. It has been shown that oxygen vacancies and complexes with trivalent dopants lead to formation of deep charge carrier traps that are responsible for afterglow in the millisecond range. These defects also give rise to color centers that cause absorption in the intrinsic emission band, thus worsening scintillation parameters of the crystals. On the basis of the results obtained, an improved technological process has been developed, ensuring stable technological yield of high-quality crystals. This allowed development of scintillation elements for computer tomography with afterglow of less than 100ppm after 20ms and light yield of not less than 19500 photon/MeV. Ways and advantages of the use of high-quality CWO crystals in scintillation detectors and spectrometric detection blocks are discussed.

Synthesis of Eu-doped (Gd,Y)2O3 transparent optical ceramic scintillator

A novel process for transparent oxide ceramic scintillator with a composition of Gd1.94-x Yx Eu0.06O3 was developed. The process consists of a glycine–nitrate combustion synthesis of nano-sized starting powder and subsequent controlled sintering and annealing steps. The organic molecules remaining in the as-combusted powder were efficiently removed by the combined heat-treatment at vacuum and air atmospheres. Hot-pressed ceramic scintillators show transparent optical state and high light output. Transparent optical ceramic scintillator with a high content of Gd (up to 80 mol%) was fabricated by the process. The measured light output of Gd1.54Y0.4Eu0.06O3 ceramic scintillator was about two times higher that that of CdWO4 single crystal.