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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]

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]

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]

Abstract: In this paper we present results of detailed spectroscopic studies of Tb3+ luminescence from BaF2:0.075mol% Tb performed at Superlumi station of Hasylab, DESY, Hamburg and at Institute of Physics, N. Copernicus University, Torun, Poland (IF UMK Torun). We have measured UV/VUV excitation spectra of the dominant blue Tb3+ luminescence and emission spectra under excitation into various bands found from the excitation spectra. The excitation spectra are dominated by the two well known broad bands due to the parity and spin-allowed (SA) 4f8 →4f75d transitions. The higher energy triple t-band and unresolved single e-band at lower energies are widely separated (10Dq about 16,500cm−1) reflecting the single d-electron energy in the high symmetry cubic crystal field with a relatively low contribution from the low symmetry component. In addition to these bands we have also detected a number of weaker bands at lower energy sides of both dominant SA bands. While the structured band with components at 252.8, 252.3, 250.4 and 248.4nm clearly corresponds to the SF (spin-forbidden) band found earlier in LiYF4:Tb, the bands at 183, 178 and 172nm have been never, to the best of our knowledge, reported before. We present arguments that these new bands are the SF counterparts of the triple SA t-band at about 158nm. The d–f exchange energies for d(e) and d(t) electrons are different at 7779 and 8245cm−1, respectively. Although the energy levels corresponding to the identified high and low spin (HS and LS) states of the 4f75d configuration are, in BaF2, well separated, there are no SA nor SF transitions generating d–f emission in BaF2:Tb. This is because the numerous 4f8 energy levels intercept the excitation energy from 4f75d levels leading to the well known blue and green Tb3+ 4f8 emissions. [Copyright &y& Elsevier]

Abstract: Barium Fluoride (BaF2), mainly used in optical applications, has similar properties to those of CaF2 but better stability under environmental conditions. In this paper, we report measurements of loss tangent and the real part of the relative permittivity ε r of single crystal BaF2 in the temperature range from 14 to 290K at frequencies of 8, 10.4, 12.1, 17.6, 21.1 and 24.4GHz. Microwave properties of BaF2 were determined by measurements of the resonance frequency and the unloaded Q-factor of TE011 and quasi TE0mn modes cylindrical cavities containing the sample under test. Two techniques namely Hakki–Coleman Dielectric Resonator and dielectric post resonators have been used for characterizing the dielectric material. Losses due to the uncalibrated cables and adaptors inside the cryocooler are accounted in the calculation of unloaded Q-factor using the Transmission Mode Q-Factor data processing technique. The permittivity of BaF2 exhibited a 5% increase from 6.9 (14K) to 7.35 (290K). The loss tangent increased from 1×10−5 to 1.1×10−4 over the measured temperature range, 14–290K at a frequency of 10.4GHz. The measured microwave properties show that BaF2 can be used in many microwave devices. [Copyright &y& Elsevier]

Open f-shell rare-earth (RE) ions in wide bandgap host materials are usually characterized by closely spaced electronic levels due to various electron configurations and charge states. These levels provide convenient luminescent transitions that can be excited by efficient recombination of charge carriers generated in the host material by ionizing radiation. Therefore, it is the area of ionizing radiation detectors, where search for new, fast and efficient scintillator materials for high-energy physics and nuclear medicine, has yielded much of the recent advances in the understanding of radioluminescence and scintillation mechanism in some solid state, UV and VUV luminescent, RE-activated materials. In this paper we shall present selected results of basic experiments such as radioluminescence, VUV spectroscopy, time profiles and thermoluminescence, on barium fluoride (activated with Ce, Pr, Nd, Tb) and two aluminum perovskites, YAlO3 and LuAlO3, activated with Ce. We shall demonstrate that these results point to consecutive carrier capture and recombination at RE ions as the basic mechanism of radioluminescence and scintillation in these materials, despite the strong self-trapping and poor charge transport properties. Consequently, various electron and/or hole traps that intercept and retain for some time the recombining charge carriers play an active role influencing both the scintillation light yield and time profiles of scintillation pulses in these and many other wide bandgap RE-activated luminescent materials. [Copyright &y& Elsevier]

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.

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]

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]

A highly efficient, reliable, and low-cost γ– γ TDPAC spectrometer, PACAr, optimized for 181Hf-implanted low-activity samples, is presented. A versatile EPROM-based routing/coincidence unit was developed and implemented to be use with the memory-card-based multichannel analyzer hosted in a personal computer. The excellent energy resolution and very good overall resolution and efficiency of PACAr are analyzed and compare with advanced and already tested fast–fast and slow–fast PAC spectrometers. [ABSTRACT FROM AUTHOR]

Crack-free PbSe on (100)-oriented Si has been obtained by a combination of liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) techniques. MBE is employed first to grow a PbSe/BaF2/CaF2 buffer structure on the (100)-oriented Si. A 2.5 μm thick PbSe layer is then grown by LPE. The LPE-grown PbSe displays excellent surface morphology and is continuous over the entire 8×8 mm2 area of growth. This result is surprising because of the large mismatch in thermal expansion coefficients between PbSe and Si. Previous attempts to grow crack-free PbSe by MBE alone using similar buffer structures on (100)-oriented Si have been unsuccessful. It is speculated that the large concentration of Se vacancies in the LPE-grown PbSe layer may allow dislocation climb along higher order slip planes, providing strain relaxation. [ABSTRACT FROM AUTHOR]

The YBCO superconductor was synthesized with BaF2. The synthesis temperature of the specimen with BaF2 was to be considerably lower than that for the unused one. The BaF2 decomposes during the heat treatment, and does not influence the structure and critical temperature. The critical current density, Jc, in the specimen is 1×108 A/m2 at 77 K and 0.5 T. However, Jc is actually lower for the QMG specimen because the Y2BaCuO5 particles are larger than those of the QMG specimen. [ABSTRACT FROM AUTHOR]