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We, at the ab initio level, simulated the rearrangement magnitudes of the adjacent neighboring ions, surrounding the (100) surface F-center in ABO3 perovskite matrixes. They are noticeably greater than the respective ionic shift magnitudes of the adjacent neighboring ions surrounding the bulk F-center. In ABO3 perovskites, the electron charge is noticeably better bounded on the inside of the bulk oxygen vacancy, as interior the respective (100) surface vacancy. The oxygen vacancy formation energy, located on the (100) surface of ABO3 perovskites, as a rule, is smaller as in the bulk. This slight energy distinction encourages the oxygen vacancy segregation from the ABO3 perovskite bulk to their (100) surfaces. The ABO3 complex oxide (100) surface F-center generated defect levels are positioned nearer to the (100) surface CB bottom than the bulk F-center generated respective defect levels. In contrary, the BaF2, SrF2 and CaF2, both, surface and bulk F-center charges are well localized inside the fluorine vacancy. The ionic rearrangement magnitudes of the adjacent neighboring ions, surrounding the surface and bulk F-centers in BaF2, SrF2 and CaF2 matrixes, are much smaller regarding the respective situation in ABO3 perovskites. [ABSTRACT FROM AUTHOR]

BaF 2 ceramics were prepared using a one-step cold sintering process with an ultra-low sintering temperature of 150 °C and uniaxial pressures ranging from 450 to 900 MPa. The relative density and microstructure improved steadily with the increasing pressure, and a fully densified microstructure with a relative density of 97.2% was achieved at 900 MPa. For BaF 2 ceramics with a thickness of 1 mm, the optimum in-line transmittance in the visible light region (58.5%) was achieved at a wavelength of 720 nm, and the maximum value (65.3%) was obtained at 1864 nm. The permittivity of the ceramics increased gradually from 6.18 to 7.09 with increasing pressure, and the dielectric loss was optimized from 0.01 to 0.003. Additionally, the mechanical properties improved continuously with the increasing pressure, and the optimal compressive strength (257 MPa), hardness (2.01 GPa), and Young's modulus (54.8 GPa) were achieved when cold sintered at 900 MPa. [ABSTRACT FROM AUTHOR]

Ion beam technology is considered as one of the most effective and unique techniques for materials engineering. Here, we show that the selective chemical etching is used successfully for the fabrication of pyramidal etch pits by revealing the ion-induced damage in the surface and below surface regions of barium fluoride single crystals. The size of ion-induced pits is increasing by etching the ion impacted zones for longer time. Moreover, the types of the induced defects are probed using optical absorption spectroscopy. The presented findings are discussed in terms of the peculiarities of ion energy deposition as well as the comparison with the induced structures using different type of ions, namely slow highly charged ions. Furthermore, they might be of importance in today’s nano and microelectronics applications, where the fabrication of size-controlled surface structures is of high necessity.

The refractive index change of semiconductor nanocrystals must be known when designing precision optical components based on semiconductor nanocrystals. In this study, Kramers-Kronig transformation has been used to compute the linear refractive index change of cadmium selenide (CdSe) nanocrystals embedded in glass. Classical theory and numerical computation of Kramers-Kronig relation have been given and explained in detail. CdSe nanocrystals have been grown in borosilicate glass by using melting, rapid-cooling, and heat treatment procedure. Compared to the production studies of semiconductor nanocrystals in borosilicate glass in the literature, the growth temperature of CdSe nanocrystals in glass has been decreased and their growth time has been extended. In other words, a slow growth method has been performed to form pure CdSe nanocrystals in glass. The quantum size effect has been observed at their optical absorption spectra. Surface and longitudinal optical phonon modes of CdSe nanocrystals have been measured by using Raman spectroscopy. The refractive index change has been found as ~1.36×10-4 for CdSe nanocrystals with a diameter of ~3.93 nm. [ABSTRACT FROM AUTHOR]

The effect of BaF2, MgF2, and AlF3 on the structural and luminescent properties of gallo-germanate glass (BGG) doped with erbium ions was investigated. A detailed analysis of infrared and Raman spectra shows that the local environment of erbium ions in the glass was influenced mainly by [GeO]4 and [GeO]6 units. Moreover, the highest number of non-bridging oxygens was found in the network of the BGG glass modified by MgF2. The 27Al MAS NMR spectrum of BGG glass with AlF3 suggests the presence of aluminum in tetra-, penta-, and octahedral coordination geometry. Therefore, the probability of the 4I13/2→4I15/2 transition of Er3+ ions increases in the BGG + MgF2 glass system. On the other hand, the luminescence spectra showed that the fluoride modifiers lead to an enhancement in the emission of each analyzed transition when different excitation sources are employed (808 nm and 980 nm). The analysis of energy transfer mechanisms shows that the fluoride compounds promote the emission intensity in different channels. These results represent a strong base for designing glasses with unique luminescent properties. [ABSTRACT FROM AUTHOR]

Radiation resistant inorganic materials emitting cross-luminescence are one of the most prospective candidates for new generation ultrafast detectors for medical tomography. Radiative transitions leading to cross-luminescence occur between valence and core states, and therefore calculations of the electronic structure of doped materials can explain ultrafast transitions and predict new cross-luminescent materials. In current work we demonstrate results of ab initio calculations of undoped and doped BaF 2 by means of hybrid density functional theory. As a result of the work, the density of states (DOS) for nominally pure BaF 2 and a whole series of BaF 2 doped with various trivalent ions were obtained. The positions of the core energy levels of dopant ions lying between the Ba(5p) zone and the F(2s) zone, as well local geometries and formation energies were calculated. Our calculations show that the 5p states of impurity ions can be located below the 5p zone of barium by several eV. This opens up opportunities for transitions from the core 5p Ba zone to impurity 5p states, which might be involved in experimentally observed appearance of an ultrafast component in doped BaF 2. [Display omitted] • Formation energies and electronic structures of 14 dopant complexes in BaF 2 were calculated • Explanation for nature of fast luminescence component in La-doped BaF 2 is proposed • Bi 3 + and In 3 + dopants are shown as candidates for further CL studies in BaF 2 [ABSTRACT FROM AUTHOR]

Group-II fluorides (BaF 2 and MgF 2) insulators are deposited on hydrogenated (C–H) diamonds by E-beam evaporator at room temperature. The bandgaps, chemical bonds and band offsets of fluorides are investigated by the high-resolution X-ray photoelectron spectroscopy (XPS) measurements. The XPS results show bandgap energies of 9.4 ± 0.2 eV for BaF 2 and 9.7 ± 0.2 eV for MgF 2 by F 1s energy loss spectra. The valence band offsets (ΔE V 's) of BaF 2 /MgF 2 on the C–H diamond are determined as 5.2 ± 0.2 and 4.8 ± 0.2 eV, respectively. There are both type II energy band configuration for BaF 2 and MgF 2 on the C–H diamond with conduction band offsets of 1.3 ± 0.2 eV and 0.6 ± 0.2 eV, respectively. The relatively oxygen-weak or oxygen-free fluoride/C–H diamond interface and large ΔE V suggest that fluorides are an excellent gate dielectric for the C–H diamond field-effect transistors with high hole mobility and low gate leakage. [ABSTRACT FROM AUTHOR]

In the present research work, we have theoretically analyzed the photonic band-gap properties of one-dimensional photonic structures composed of polycarbonate and non-glass materials. These photonic structures, PC1, PC2, PC3 and PC4, are composed of alternating layers of polycarbonate/Al2O3, polycarbonate/MgF2, polycarbonate/BaF2 and polycarbonate/TiO2 materials, respectively. The period of each photonic structure is made up of a thin non-glass material layer sandwiched between two identical polycarbonate layers. The transfer matrix method has been used to investigate the transmission properties of PC1 to PC4. The comparison between the transmission spectra of PC1 to PC4 shows that the polycarbonate and TiO2-based photonic structure (PC4) possess three PBGs of zero transmission located at far-ultraviolet, visible and near-infrared regions of the electromagnetic spectrum at normal and oblique incidence (θ0 = 55°), both corresponding to TE wave only. The index of refraction of all five materials used in this study was obtained by applying the Sellmeier-type dispersion relationship to ensure accuracy in the results. The purpose of selecting polycarbonate along with Al2O3, TiO2, MgF2 or BaF2 as constituent materials of these photonic structures is due to the heat resistance properties of polycarbonate and the unique optical properties of oxide and fluoride materials with wide transparency from the ultraviolet to the near-infrared regions of the electromagnetic spectrum. The proposed work can be used to design some influential wavelength-selective reflectors composed of 1D PCs behind the active region of the solar cells for improving the photovoltaic performance of solar panels. This study can further be utilized for the fabrication of advanced solar cell designs consisting of 1D photonic mirror-based luminescence and reflection concentrators. The low temperature problem which arises in satellites may also be overcome with the help of smart windows based on the proposed multilayer structures. [ABSTRACT FROM AUTHOR]

High performance normally-OFF hydrogenated diamond (C-H diamond) metal–insulator–semiconductor (MIS) field-effect transistors (FETs) have been achieved with Al/BaF2 gate materials. The 15-nm BaF2 film was formed by electron beam evaporation at room temperature. The flat-band voltage and hysteresis voltage extracted from the capacitance–voltage characteristics of the Al/BaF2/ C-H diamond MIS diode indicate low positive fixed charges and trapped charges in the BaF2 film. The 4- $\mu \text{m}$ FET device with a threshold voltage (${V}_{\text {TH}}$) of −0.90 V exhibits distinct pinch-off characteristics. The enhancement mode could be due to primarily the Schottky barrier depletion effect of the gate metal on the channel, and secondarily the positive fixed charges present in the BaF2 layer. The maximum transconductance and maximum saturated drain current are 30 mS/mm and −96.5 mA/mm, respectively. The device performance benefits from the decent conductivity at the BaF2/C-H diamond interface with an almost constant effective hole mobility of about 225.0 cm2/ $\text{V}\cdot \text{s}$ for −2.1 V $\le {V}_{\text {GS}}$ – ${V}_{\text {TH}} \le -4.1$ V. [ABSTRACT FROM AUTHOR]

Featured Application: laser surface-processing. Monocrystalline barium fluoride (BaF2) slab targets were irradiated by focused 46.9-nm laser radiation at various fluence levels above the ablation threshold. Well-developed ablation patterns with sharp edges were studied by AFM (atomic force microscopy). Their inner surfaces were uniformly covered by periodic structures. The spatial period of the ripples depends on the laser fluence. When the sample is rotated by 45°, the orientation of the grating-like structure changes accordingly. Thus, the grating vector of the periodic structure seems to be coupled to the crystallographic planes of the single crystal. This means that the XUV-laser induced ripples reported here differ from LIPSS (laser-induced periodic surface structures) associated with interference phenomena occurring on illuminated surfaces. Therefore, other mechanisms are discussed to explain the formation of the periodic nanostructures reported in this article. [ABSTRACT FROM AUTHOR]

Site-selective time-resolved laser spectroscopy techniques, which provide high spectral resolution at low temperatures (4.2 K and 6.5 K), were used to clarify the formation mechanism, spectral and kinetic properties of the Nd3+ ion single-site optical centers formed in the fluorite-type BaF 2 single crystals doped with Nd3+. This formation occurs by heterovalent substitution of Ba2+ host cations with additional charge compensation by F− anions. To determine the lifetimes of the Stark 4F 3/2 (1) levels of the Nd3+ impurity centers, the fluorescence decay kinetics was measured using the gated photon counting technique. The data from spectroscopic experiments were combined with density functional theory (DFT) calculations to identify and analyze the local structure of Nd3+ cations in the Nd3+ ions doped BaF 2 crystal. Our studies have shown that several types of single-site Nd3+ optical centers can be distinguished in BaF 2 :Nd3+. We confirmed the existence of an optical center with high C 4v site symmetry and a long radiative lifetime of the 4F 3/2 (1) state τ = 14.3 ms. We also describe two other configurations of centers with local structure characterized by low site symmetry of C s , which were identified and described for the first time. The decrease in the tetragonal site symmetry from C 4v to C s was explained by the fact that the Nd3+ dopants, distributed over the equivalent cationic positions of the fluorite-type crystal lattice, coordinate to anionic motifs that essentially differ in the position of the additional charge compensator F−. This structural variability induced in the local coordination environment of the optical center affects its spectroscopic properties through a shift in the energy of the 4F 3/2 (1) Stark level of the Nd3+ ion and results in a different radiative lifetime τ. • Nd3+ ion single-site optical centers in BaF 2 crystals were studied. • DFT analysis showed four configurations of the Nd3+ ion sites in BaF 2. • Local geometries of the Nd3+ ion single-sites in BaF 2 were determined. • The existence of Nd3+ site in BaF 2 with high C 4v site symmetry was confirmed. • Two configurations of the Nd3+ sites with low symmetry of C s were revealed. [ABSTRACT FROM AUTHOR]

• GeO 2 -TiO 2 -BaO-BaF 2 -Ga 2 O 3 system doped with Er3+, Tm3+, Ho3+ were synthesized. • The extremely low IR absorption coefficients were achieved. • Enhanced emission at 1.5 µm, 1.8µm and 2.0 µm by introducing BaF 2 in the composition was observed. • The luminescence lifetime significantly increased in the function BaF 2. • Optimal BaF 2 content in titanate-germanate glass was evaluated. A novel TiO 2 -GeO 2 -BaO-BaF 2 -Ga 2 O 3 glasses doped with rare-earth were synthesized, and the impact of the substitution of BaF 2 for BaO on properties has been investigated by luminescence spectroscopy. The IR absorption coefficient is extremely low, which is attractive for optical fibers emitting NIR radiation. Near-IR luminescence associated to the Er3+:4I 13/2 → 4I 15/2 (FWHM = 108 nm), Tm3+:3F 4 → 3H 6 (FWHM = 244 nm) and Ho3+:5I 7 → 5I 8 (FWHM = 179 nm) transitions are enhanced in the presence of BaF 2. Luminescence lifetimes increase with increasing content of BaF 2 in the glass composition. Fabricated optical materials with relatively high BaF 2 content represent a good starting point for further investigations in the field of glass technology and modern photonics. [ABSTRACT FROM AUTHOR]

The effects of substitution of BaF2 for BaO on physical properties and 1. 8 μm emission have been systematically investigated to improve spectroscopic properties in Tm3+ doped gallium tellurite glasses for efficient 2.0 μm fiber laser. It is found that refractive index and density gradually decrease with increasing BaF2 content from 0 to 9 mol.%, due to the generation of more non-bridging oxygens. Furthermore, OH− absorption coefficient (αOH) reduces monotonically from 3.4 to 2.2 cm−1 and thus emission intensity near 1.8 μm in gallium tellurite glass with 9 mol.% BaF2 is 1.6 times as large as that without BaF2 while the lifetime becomes 1.7 times as long as the one without BaF2. Relative energy transfer mechanism is proposed. The maximum emission cross section and gain coefficient at around 1.8 μm of gallium tellurite glass containing 9 mol.% BaF2 are 8.8 × 10−21 cm2 and 3.3 cm−1, respectively. These results indicate that Tm3+ doped gallium tellurite glasses containing BaF2 appear to be an excellent host material for efficient 2.0 μm fiber laser development.

In the present research work, we have theoretically analyzed the photonic band-gap properties of one-dimensional photonic structures composed of polycarbonate and non-glass materials. These photonic structures, PC1, PC2, PC3 and PC4, are composed of alternating layers of polycarbonate/Al2O3, polycarbonate/MgF2, polycarbonate/BaF2 and polycarbonate/TiO2 materials, respectively. The period of each photonic structure is made up of a thin non-glass material layer sandwiched between two identical polycarbonate layers. The transfer matrix method has been used to investigate the transmission properties of PC1 to PC4. The comparison between the transmission spectra of PC1 to PC4 shows that the polycarbonate and TiO2-based photonic structure (PC4) possess three PBGs of zero transmission located at far-ultraviolet, visible and near-infrared regions of the electromagnetic spectrum at normal and oblique incidence (θ0 = 55°), both corresponding to TE wave only. The index of refraction of all five materials used in this study was obtained by applying the Sellmeier-type dispersion relationship to ensure accuracy in the results. The purpose of selecting polycarbonate along with Al2O3, TiO2, MgF2 or BaF2 as constituent materials of these photonic structures is due to the heat resistance properties of polycarbonate and the unique optical properties of oxide and fluoride materials with wide transparency from the ultraviolet to the near-infrared regions of the electromagnetic spectrum. The proposed work can be used to design some influential wavelength-selective reflectors composed of 1D PCs behind the active region of the solar cells for improving the photovoltaic performance of solar panels. This study can further be utilized for the fabrication of advanced solar cell designs consisting of 1D photonic mirror-based luminescence and reflection concentrators. The low temperature problem which arises in satellites may also be overcome with the help of smart windows based on the proposed multilayer structures.

Monocrystalline barium fluoride (BaF2) slab targets were irradiated by focused 46.9-nm laser radiation at various fluence levels above the ablation threshold. Well-developed ablation patterns with sharp edges were studied by AFM (atomic force microscopy). Their inner surfaces were uniformly covered by periodic structures. The spatial period of the ripples depends on the laser fluence. When the sample is rotated by 45°, the orientation of the grating-like structure changes accordingly. Thus, the grating vector of the periodic structure seems to be coupled to the crystallographic planes of the single crystal. This means that the XUV-laser induced ripples reported here differ from LIPSS (laser-induced periodic surface structures) associated with interference phenomena occurring on illuminated surfaces. Therefore, other mechanisms are discussed to explain the formation of the periodic nanostructures reported in this article.

One crucial issue for the application of scintillation crystals in future high-energy physics experiments is radiation damage in a severe radiation environment, such as the high luminosity large hadron collider. While radiation damage induced by ionization dose in inorganic scintillators is well understood, investigations are ongoing to understand radiation damage induced by hadrons, including both charged hadrons and neutrons. Aiming at understanding neutron-induced radiation damage in fast inorganic scintillators, lutetium yttrium oxyorthosilicate (LYSO)/lutetium fine silicate (LFS), BaF2, and PWO crystals were irradiated at Los Alamos Neutron Science Center by a combination of particles, including neutrons, protons, and γ-rays. The results indicate that LYSO/LFS and BaF2 crystal plates are radiation hard up to a 1-MeV equivalent neutrons fluence of 9 × 1015 equivalent neutron (neq)/cm2, but not PWO, and the neutron-induced radiation damage in LYSO crystals is a factor of ten less than protons. [ABSTRACT FROM AUTHOR]

To precisely characterize the defects in crystalline solids including semiconductors and alloys, it is critical but challenging to improve the time resolution of positron annihilation lifetime (PAL) spectrometers. The effects of geometry and wrapping of scintillators on the time resolution of the PAL spectrometer were studied in detail. Geant4 program simulation was used to clarify the reason why cone-frustum-shaped scintillators have better time resolution than cylinder-shaped scintillators. A figure of merit was proposed to comprehensively evaluate the impacts of the transport and absorption of fluorescent photons on the time resolution of scintillators. Additionally, for the simulations of cone-frustum-shaped scintillators, polished reflective coatings can further improve the time resolution without losing count rate. Eventually, using three cone-frustum-shaped Ba F 2 scintillators and a double-stop setup, we significantly improved the time resolution of PAL spectrometer to a new record of 88.7 ps, the best time resolution up to date. This enables us to distinguish various vacancy types in crystalline solids more precisely in future. • Explained why cone-frustum-shaped scintillators have better time resolution. • Proposed a figure of merit to evaluate the time resolution of PAL spectrometer. • Achieved the best time resolution up to date for the PAL spectrometer. [ABSTRACT FROM AUTHOR]

The SiO2-Al2O3-BaF2-TiO2-CaF2 transparent silicate glass-ceramics containing BaF2 nanocrystals were successfully prepared by heat treatment process through conventional melting method. Effects of heat treatment processes and Yb3+ concentration on the downconversion (DC) emission of the co-doped Er3+/Yb3+ transparent silicate glass-ceramics were investigated. With the increase of temperatures and times of heat treatment process, the DC emission intensity of the co-doped Er3+/Yb3+ glass-ceramics was significantly enhanced. At the same time, with the increase of Yb3+ concentration, the value of DC intensity of Er3+/Yb3+ co-doped bands centered at 849, 883 and 1533 nm is maximized when the concentration of Yb3+ reaches 2.5 mol.%. When the concentration exceed 2.5 mol. %, the DC emission intensity of Er3+/Yb3+ co-doped bands centered at 849, 883 and 1533 nm was decreased, owing to the self-quenching effect. It’s interesting that the DC emission intensity of Er3+/Yb3+ co-doped band centered at 978 nm didn’t quench when the Yb3+ concentration exceed 2.5 mol. %. At the same time, the DC mechanism and ET processes between Yb3+ and Er3+ ions were discussed.

Abstract Powder sample of Cubic phase BaF 2 :Ce phosphor was synthesized by chemical co-precipitation method and annealed at 500 °C for 2 h. The synthesized powder of BaF 2 :Ce phosphor was characterized by XRD, which shows FCC structure with 30 nm crystallite size. TEM images show a cubic structure and FTIR contains the existence of F-Ba-F, C-H, C˭C, and O-H bonds. Glow curve of the Thermoluminescence contains two major peaks, one at 175 °C and another at 275 °C. The intensity of the major dosimetric peak (175 °C) observed to be increased linearly from 1 Gy to 3 kGy for the doses of 60Co gamma rays and then saturates. Overall, the new BaF 2 :Ce phosphor shows good material properties for the high gamma dose measurement applications. [ABSTRACT FROM AUTHOR]

Abstract Forward electron photo-transfer and reverse thermal transfer between divalent and trivalent heterogeneous lanthanides in barium fluoride crystals has been studied using optical absorption. In crystals activated by two heterogeneous lanthanides and grown in reducing conditions, one of the lanthanides becomes bivalent whereas the other lanthanide remains trivalent. Illumination of the crystal in the ultraviolet bands led to the transfer of electrons from divalent lanthalides (Eu, Yb, Sm) to trivalent ions (Ho, Nd, Dy, Tm, Sm, Yb). The thermal ionization energies to the conduction band of created lanthanides are determined from the thermal bleaching curves of absorption bands. The experimental energies are compared with the estimated energies of the Dorenbos model. Highlights • Forward electron photo-transfer and reverse thermal transfer between lanthanides in BaF 2 crystals has been studied. • Ultraviolet illumination led to the transfer of electrons from 2+ (Eu, Yb, Sm) to 3+ (Ho, Nd, Dy, Tm, Sm, Yb) lanthanides. • The thermal ionization energies to the conduction band are determined from the thermal bleaching curves of absorption bands. • The experimental energies are compared with the estimated energies of the Dorenbos model. [ABSTRACT FROM AUTHOR]

Hexagonal boron nitrite (hBN) is an attractive material for many applications such as in electronics as a complement to graphene, in anti-oxidation coatings, light emitters, etc. However, the synthesis of high-quality hBN at cost-effective conditions is still a great challenge. Thus, this work reports on the synthesis of large-area and crystalline hBN nanosheets via the modified polymer derived ceramics (PDCs) process. The addition of both the BaF2 and Li3N, as melting-point reduction and crystallization agents, respectively, led to the production of hBN powders with excellent physicochemical properties at relatively low temperatures and atmospheric pressure conditions. For instance, XRD, Raman, and XPS data revealed improved crystallinity and quality at a decreased formation temperature of 1200 °C upon the addition of 5 wt% of BaF2. Moreover, morphological determination illustrated the formation of multi-layered nanocrystalline and well-defined shaped hBN powders with crystal sizes of 2.74−8.41 ± 0.71 µm in diameter. Despite the compromised thermal stability, as shown by the ease of oxidation at high temperatures, this work paves way for the production of large-scale and high-quality hBN crystals at a relatively low temperature and atmospheric pressure conditions.

Abstract Optical spectra (absorption, luminescence, decay of emission, spectra with time resolution) of Sm2+ ions in alkaline-earth fluoride crystals (CaF 2 , SrF 2 , BaF 2) in the wavelength range 120–900 nm at 7–300 K were studied. By additive coloration of about 60% of the original Sm3+ is converted to Sm2+. One component of the decay of the Sm2+ emission was observed in BaF 2 crystals with a lifetime of 16 ms (a slow component) and in CaF 2 crystals with a lifetime of 1.3 μs (fast component) at low temperature. In SrF 2 crystals, two decay of luminescence components are found in the millisecond and nanosecond time range. The luminescence spectra of the slow and fast components in CaF 2 , SrF 2 , BaF 2 are similar, which confirms their attribution to forbidden 4f-4f and allowed 5d-4f transitions, respectively. The results obtained made it possible to clarify the position of the excited levels of Sm2+ 4f(5D 0) and 5d(e g) relative to the bottom of the conduction band of alkaline-earth fluorides. Highlights • Optical spectra (absorption, luminescence, decay of emission, spectra with time resolution) of Sm2+ ions in crystals CaF 2 , SrF 2 , BaF2 were studied. • One component of the decay of the Sm2+ emission was observed in BaF 2 crystals with a lifetime of 1 ms and in CaF 2 crystals with a time of 1 μs. • In SrF 2 crystals, two decay of luminescence components are found in the millisecond and nanosecond time range. • The results clarify the position of the levels of Sm2+ 4f(5D 0) and 5d(e g) relative to the conduction band. [ABSTRACT FROM AUTHOR]

The refractive index is a fundamental physical property of a medium, which can be used for the identification and purity issues of all media. Here we describe a refractive index measurement technique to determine simultaneously the refractive index of different solid particles by monitoring the transmittance of light from a suspension using a charge-coupled device (CCD) camera. An important feature of the measurement is the liquid evaporation process for the refractive index matching of the solid particle and the immersion liquid; this was realized by using a pair of volatile and non-volatile immersion liquids. In this study, refractive indices of calcium fluoride (CaF2) and barium fluoride (BaF2) were determined using the proposed method. [ABSTRACT FROM AUTHOR]

• Highly energetic heavy ions create localized electronic excitations in BaF 2 single crystals. • Fabrication of pyramidal pits in BaF 2 surface by ion irradiation. • Tuning the size of the created pits using selective chemical etching. • Creation of the pits is ascribed to the creation of ion-induced defects aggregates. • Evidencing the creation mechanism of the induced surface damage by fast heavy ions. Ion beam technology is considered as one of the most effective and unique techniques for materials engineering. Here, we show that the selective chemical etching is used successfully for the fabrication of pyramidal etch pits by revealing the ion-induced damage in the surface and below surface regions of barium fluoride single crystals. The size of ion-induced pits is increasing by etching the ion impacted zones for longer time. Moreover, the types of the induced defects are probed using optical absorption spectroscopy. The presented findings are discussed in terms of the peculiarities of ion energy deposition as well as the comparison with the induced structures using different type of ions, namely slow highly charged ions. Furthermore, they might be of importance in today's nano and microelectronics applications, where the fabrication of size-controlled surface structures is of high necessity. [ABSTRACT FROM AUTHOR]

Eu3+-doped transparent glass ceramics containing MF2 (M = Ba, Ca, Sr) nanocrystals were fabricated using a melt-quenching method, and the resulting structures were studied using X-ray diffraction. Levels 5D1 and 5D0 of Eu3+ ions were verified as thermally coupled levels using the fluorescence intensity ratio method. The fluorescence intensity ratios, optical temperature sensitivity and thermal quenching ratios of the transparent glass ceramics were studied as a function of temperature. With an increase in temperature, the relative sensitivity ( SR) decreased sharply at first, then slowly increased, before finally decreasing. The minimum SR values of GCBaF2 (GCB), GCCaF2 (GCC) and GCSrF2 (GCS) were 2.8 × 10−4, 0.8 × 10−4 and 1.9 × 10−4 K−1 at 360, 269 and 319 K, respectively. Glass ceramics with an intense emission intensity can be used to convert the measured spectrum into temperature and may have an important role in temperature detectors. [ABSTRACT FROM AUTHOR]

In this paper we present the prompt photofission neutron detection performance of undoped CaF2 scintillator using Threshold Activation Detection (TAD). The study is carried out in the frame of C-BORD Horizon 2020 project, during which an efficient toolbox for high volume freight non-intrusive inspection (NII) is under development. Technologies for radiation monitoring are the part of the project. Particularly, detection of various radiological threats on country borders plays an important significant role in Homeland Security applications. Detection of illegal transfer of Special Nuclear Material (SNM) - 235U, 233U and 239Pu - is particular due to the potential use for production of nuclear weapon as well as radiological dispersal device (RDD) V known also as a “dirty bomb”. This technique relies on activation of 19F nuclei in the scintillator medium by fast neutrons and registration of high-energy β particles and γ-rays from the decay of reaction products. The radiation from SNM is detected after irradiation in order to avoid detector blinding. Despite the low 19F(n,α)16N or 19F(n,p)19O reaction cross-section, the method could be a good solution for detection of shielded nuclear material. Results obtained with the CaF2 detector were compared with the previous study done for BaF2 and 3He detector. These experimental results were obtained using 252Cf source and 9 MeV Varian Linatron M9 linear accelerator (LINAC). Finally, performance of the prompt neutron detection system based on CaF2 will be validated at Rotterdam Seaport during field trails in 2018.

• Characterization of refractive materials with transparent properties able to endure the harsh environment of ITER. • BaF 2 -Parylene-C, Sapphire and YAG with Broad Band Anti-Reflective (BBAR) have been tested with neutronic and gamma radiation. • Parylene-C is not suitable with neutrons instead BBAR protection appears to be compatible with neutrons. • Parylene-C can be used up to 110° C but after gamma radiation at 332 kGy and 50° C, the optical coating is damaged too. • Sapphire and YAG are the best candidates as refractive substrates for the WAVS diagnostic. The aim is to provide an assessment of radiation damage in coated films on refractive substrates that could be used as relevant information to protect the field lenses in optical diagnostics for ITER. Radiation-resistant optical materials with transparent properties should transfer light from plasma to detectors through the interspace until the port cell area where they are located (more than 10 m from the vacuum window). These optical coatings and substrates should have resilience enough to withstand neutron and gamma irradiation without significant degradation of their transmittance. Coated sapphire (Al 2 O 3 windows), YAG with Broad Band Anti-Reflective (BBAR) and substrates of BaF 2 /CaF 2 protected with anti-humidity coating (Parylene-C) were extensively tested. After testing, transmission measurements and analysis of optical surfaces yield significant discoveries. All substrates showed good refractive performance under gamma radiation. On the contrary, Parylene-C did not resist temperatures above 100 degrees Celsius as expected, according to manufacturing specifications. In addition, it was observed that with an energy dose of 332 kGy of gamma rays and a significantly lower temperature of 50 degrees Celsius, the Parylene-C protection is also damaged. Coated sapphire had the best overall performance with respect to the neutron irradiation tests. Nevertheless, the decrease in transmission observed in the YAG and BaF2 coated samples is not significant for the expected cumulative neutron dose that these samples will receive at their location within the diagnostic. This information was considered for the selection of the best candidates as refractive lenses and optical coatings for the Preliminary Design Review (PDR) of ex-vessel components that will integrate the Wide-Angle Viewing System (WAVS) for ITER Equatorial Port 12. [ABSTRACT FROM AUTHOR]

YbF-doped BaF crystals with various concentrations of YbF (0.05, 0.1, 0.17, 0.2 mol) have been grown using the conventional Bridgman method. Dielectric relaxation is used to study the charge compensating defects in BaF crystals with low YbF concentration. The optical absorption spectra reveal the existence of both Yb (in UV domain) and Yb (in IR domain) ions. In the investigated temperature range (150-320 K) we have observed only one type of dielectric relaxation. This relaxation with activation energy eV is associated with trigonal-type () centers. The charge compensating defects are discussed while taking also into account the optical absorption spectra and the calculated number of NNN dipoles whose relaxation is observed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The 1st observation of surface nanostructures induced by irradiation of BaF2 surface with highly charged ions (HCI) at higher potential energy is shown. [•] Evidence for the significance of the deposited potential energy in the creation of surface hillocks in BaF2 is confirmed. [•] The existence of potential energy threshold for the creation of surface etch pits in CaF2 is shown. [•] Similarities and differences for the creation of HCI-induced nanohillocks and etch pits in CaF2 and BaF2 of the same crystalline structure are discussed. [•] We compare the results with modifications induced by swift heavy ions. [ABSTRACT FROM AUTHOR]

Abstract: Absorption and site-selective laser spectroscopy of BaF2 single crystals homogeneously doped with Nd3+ ions as well as heterogeneously doped with Nd3+ and either Gd3+ or Yb3+ ions, performed at 10K, is presented. In each case a single centre, assigned to be of C3v symmetry, is dominant. A heterogeneous Nd3+–RE3+ (RE being either Nd or Yb) is also present in each of the double doped crystals. Energy levels for six multiplets of the single Nd3+ centre and for three multiplets for each Nd3+–RE3+ centre are presented together with the 4F3/2 (R) multiplet fluorescence lifetime for each centre. Calculations give comparable solution energies for C3v and C4v symmetry Nd3+ centres in BaF2; the C3v symmetry configuration has lower energy for the preceding lanthanides while C4v is favoured from Sm3+ onwards. [Copyright &y& Elsevier]

Ultra Fast Timing has been applied at the IGISOL facility since more than a decade ago with the aim to systematically study nano- and subnanosecond lifetimes in the neutron-rich nuclei from the A~110 region. Over this period two generations of crystals and photomultipliers have been introduced, which allowed to study more complex level schemes populated in β decay. The IGISOL facility provides unique capabilities to study the A~110 region not matched elsewhere in the world. [ABSTRACT FROM AUTHOR]

Abstract: An ab initio model potential embedded cluster study on the and 5d manifolds of the long range compensated, cubic, substitutional Tb3+ defects of Tb-doped BaF2 is presented here. 4f–4f and 4f–5d transition energies and 4f–5d absorption electric dipole oscillator strengths for the dipole allowed transitions are reported and comparisons with emission and excitation experimental spectra are presented. We find good agreement with the energies of the experimental 4f–4f green and blue emissions. The characteristic features of the 4f–5d excitation spectrum (spin allowed vs. spin forbidden transitions) are well reproduced by the calculations. The transition energies from the ground state to the different spin nonet and spin septet levels of the 5d configuration of this ion in the center of the lanthanide series show an error of around 10%, which is in line with previous calculations on other lanthanide ions with lower spin multiplicities and simpler manifolds at the beginning and at the end of the series. The calculation supports a recent assignment of experimental excitation bands between 54,000 and 66,000cm−1 as transitions. On the other hand, a new assignment is made of the highest band in the excitation spectrum at around 76,000cm−1: it corresponds to transitions to levels of the 5d configuration with the subshell in the excited 6 P term, rather than to a Tb-bound exciton. Preliminary studies on excitonic states of Tb-doped BaF2 indicate that energy levels that can be characterized as Tb-trapped excitons are found in the same spectral region as the spin forbidden transitions at ground state Tb–F distances, and their energy lowers very much at shorter Tb–F distances. [Copyright &y& Elsevier]

Hydrogenated diamond (C H diamond) inverters have been fabricated by normally-off Al/BaF 2 /C-H diamond metal-insulator-semiconductor field effect transistors (MISFETs) and load resistors. Electron beam evaporation was used to deposit BaF 2 film on the diamond surface at room temperature to reduce thermal degradation effect on the C H bond and adsorbates during the deposition. The resulting 4-μm normally-off MISFET achieves the maximum saturation drain current, on-resistance, maximum transconductance and switching ratio of −113.4 mA/mm, 37.1 Ω·mm, 35.4 mS/mm and 108, respectively. Effective hole mobility of more than 200 cm2/V·s is achieved in a large gate voltage range (−0.35 V ≤ V GS -V TH ≤ −4.35 V). Correspondingly, based on the high-performance diamond MISFET, the diamond inverters exhibit good voltage transfer characteristics and dynamic switching characteristics at the frequency of 50 kHz. [Display omitted] • Hydrogenated diamond (C H diamond) inverters have been fabricated by Al/BaF 2 /C-H diamond MISFET. • The 4-μm device delivers excellent electrical properties (a high transconductance of 35.4 mS/mm and a maximum drain current of -113.4 mA/mm) • Effective hole mobility of more than 200 cm2/V·s is achieved in a large gate voltage range • The diamond inverters exhibit good voltage transfer characteristics and dynamic switching characteristics. [ABSTRACT FROM AUTHOR]

A new type of Tb3+-doped luminescent glass-ceramics was prepared with yellow phosphorus slag. The effects of BaF 2 and CaF 2 on the crystallization and luminescence performance of glass-ceramics were explored. Results indicated that Tb3+ was 2 wt%; as the amount of BaF 2 and CaF 2 increased, the crystallization activation energy weakly decreased, which promoted crystallization. The crystallization mechanism changed from bulk to surface with the addition of BaF 2 , but the Avrami parameters of addition with CaF 2 were all greater than 3, they were volume growth. With the increase of BaF 2 and CaF 2 , the crystal structures change from short columnar to needle-like and from crisscross short rods to islands, respectively. BaF 2 did not promote Tb3+ ion luminescence but instead reduced the fluorescence intensity. The addition of CaF 2 the luminous intensity first increased and then decreased. When 2 wt% CaF 2 was added, the luminous intensity was the highest. • Luminescent glass-ceramics were prepared from yellow phosphorus furnace slag. • The influence of the addition of CaF2 and BaF2 on glass-ceramics was explored. • It provides ideas for reducing the production cost of luminescent glass ceramics. [ABSTRACT FROM AUTHOR]

We report on a feasibility study of scintillation detectors with fast decay times which might be used to enable spectroscopy in a high-count-rate, high-energy x-ray environment, such as is used for cargo inspection. Such detectors need to have a small size and high density in order to reliably detect x rays between 1 and 9 MeV with good spatial resolution and need to be suitable for read-out with semiconductor photo-detectors. Three inorganic scintillator detectors with decay constants less than 10 ns were selected: ZnO (in the form of wafers of 4.5 \times 0.5 \times 20~mm and single crystals of 5 \times 5 \times 30~mm), BaF2~ (5 \times 5 \times 30~mm) and PbWO4~(6 \times 6\times 20~mm). ZnO has a known problem of self-absorption of the emitted scintillation light. Using the wafers to create stacks of different thicknesses, the effective attenuation length was determined to be 2.25 \pm 0.25~mm for one batch of ZnO and 1.5\pm 0.3~mm for another batch, making ZnO unlikely for the proposed use. Although the larger ZnO crystal also scintillates, light transmission was so poor that no signal was measured in tests with gamma ray sources. ZnO was thus discarded as a viable candidate for the purposes of this study. The full width at half maximum (FWHM) of the pulses produced by BaF2 was measured to be \sim 2~ns, consistent with being dominated by the response of the photomultiplier tube used in the measurement. BaF2 by itself, however, emits in the UV, which is unsuitable for use with semiconductor readout. In another series of measurements, BaF2 was coupled to fast wavelength-shifting PVT sheets with Eljen DSB1 dye, and the FWHM of the signal measured this way was \sim 4.8~ns, with a fast decay constant of \sim 11.2~ns. The long decay component appears to be suppressed somewhat by the use of the wavelength shifter. The measured FWHM of the pulses produced by PbWO4 was \sim 3~ns, with a decay constant of \sim 8.3~ns and no long component. These results suggest that dense sub-10-ns scintillators exist that can be used for high-count-rate x-ray cargo inspection with good spatial resolution. PbWO4 is very dense, has no long decay component or self-attenuation and scintillates in the visible spectrum, but due to its low light output, it needs to be read out by a silicon photomultiplier or similar device. BaF2 is a possible alternative, when used in combination with a fast wavelength shifter. [ABSTRACT FROM PUBLISHER]

We have investigated the defect formation processes of BaF2 single crystals under vacuum ultraviolet radiation with the use of combination spectroscopy of synchrotron radiation (SR) and laser. The defect formation efficiency was measured as a function of excitation photon energy of SR by means of a laser-induced luminescence method. It was found that the efficiency depends significantly on the excitation photon energy. We show that the dependence can be explained in terms of the diffusive motion of hot conduction electrons generated by the photoexcitation. The relationship between the defect formation and the energy relaxation of core excitons is also discussed. [ABSTRACT FROM AUTHOR]

Fluoride single crystals have a wide band gap. Thus, we were interested in studying the scintillation properties of fluoride single crystals. We investigated the scintillation properties of fluoride crystals grown by the micro-pulling-down and annealing methods. The time required to grow crystals using these methods is lesser than that using conventional methods such as the Czochralski and Bridgman methods. Both ZnF2 and PbF2 exhibited small light outputs of less than 1,000 photons/MeV, while BaF2 and CeF3 exhibited light outputs of \sim6,000 and \sim2,000 photons/MeV, respectively. In addition, BaF2 is a well-known fast scintillator, but it also has a slow component. We tested a BaF2 crystal with a Ce/La co-dopant and observed the suppression of the slow component. [ABSTRACT FROM PUBLISHER]

Abstract: OH− impurities located near the (111) BaF2 surface have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Twenty surface OH− configurations were studied, and the hydroxyls located on the first surface layer are the energetically most favorable configurations. For the (111) BaF2 surface atomic layers, the surface hydroxyls lead to a remarkable XY-translation and a dilating effect in the Z-direction, overcoming the surface shrinking effect in the perfect slab. Bond population analysis shows that the surface effect strengthens the covalency of surface OH− impurities. The studies on band structures and density of states (DOS) of the surface OH−-impurity systems demonstrate that there are two defect levels induced by OH− impurities. The O p x and p y orbitals form two superposed occupied O bands, located above the valence bands (VB), and the H s orbitals do the major contribution to an empty H band, located below the conduction bands (CB). Because of the surface effect, the O bands move downward, toward the VB with respect to these bands in the bulk case, and this leads to narrowing of the VB→O gap and widening of the O→H gap which corresponds to the first optical absorption. [Copyright &y& Elsevier]