Electron paramagnetic resonance spectroscopy of Cr 3+ in hexagonal Cs 2 NaGaF 6 crystals

Powder samples of hydrothermally grown Cr 3+ -doped Cs 2 NaGaF 6 crystals have been investigated with electron paramagnetic resonance spectroscopy at X - (9.5 v GHz) and Q -band (34 v GHz). Analysis of the spectra clearly demonstrates that there are two distinct Cr 3+ centres in the Cs 2 NaGaF 6 crystal, having nearly identical g factors, but differing largely from the viewpoint of their zero field splitting. By using the 53 Cr hyperfine spectra observed with electron nuclear double resonance spectroscopy, it is deduced that these centres have opposite signs for the zero field splitting. The spectroscopic properties of the Cr 3+ centres in the isostructural Cs 2 NaGaF 6 and Cs 2 NaAlF 6 crystals are compared and discussed.     

Photostimulated detection of radiation defects produced by VUV light in BaF2

Production of radiation defects in a widely used scintillation material BaF₂ has been studied by means of a combination spectroscopy of synchrotron radiation (SR) and laser, in which defects produced by SR irradiation are sensitively detected by observing the luminescence stimulated by laser light. The photostimulated luminescence arises from the recombination of self-trapped holes (V_K centers) with electrons released from trapped centers by laser light. The obtained result reveals that the production efficiency of radiation defects is drastically dependent on the excitation photon energy of valence or core excitons.     

Optical and luminescence characteristics of BaF2 and BaF2:Tm in vacuum UV and UV spectral regions

The spectral characteristics of BaF₂ and BaF₂:Tm crystals and ceramics are compared. The vacuum UV (VUV) and UV absorption spectra and excitation spectra of core-valence and exciton luminescence are investigated. The main exciton and electron-hole processes responsible for the structure of recorded spectra are considered.     

Study of VUV emission and γ-ray responses of Nd:BaF2 scintillaotor

Nd³⁺ 1%, 5% and 10% doped BaF₂ single crystals were grown by the micro-pulling down method. Photoluminescence properties, including excitation and emission spectra and luminescence decay were measured under synchrotron radiation excitation at the Superlumi station in HASYLAB at DESY (Hamburg, Germany). The Nd³⁺ related 5d-4f emission lines peaking around 180 nm, 230 nm, and 260 nm, identified as the 5d–⁴I_j, 5d–⁴F_j, and 5d–²G_j transitions, were observed under 140–168 nm excitation. In photoluminescence decay under the 160 nm excitation, the dominant component decay time is about 12, 2.5 and 1.2 ns for Nd³⁺ 1%, 5% and 10% concentration, respectively. The decay time shortening is explained by the concentration quenching effect. Transmittance of Nd1% sample is about 80% for wavelengths above 185 nm. Finally, gamma-ray responses, non-proportionality and energy resolution of Nd1% sample were compared with the undoped BaF₂ scintillator. The light yield of the Nd1%:BaF₂ is about 93% of that of undoped BaF₂. ^©2009 Elsevier Ltd. All rights reserved.     

钠铝氟化物包覆BaF2材料上转换发光性能

采用水热法合成了一种非稀土上转换发光材料,由钠铝氟化物和BaF2组成。采用X射线衍射(XRD)、傅里叶红外光谱(FTIR)和扫描电镜(SEM)对其进行了表征,分析表明,钠铝氟化物主要以NaF和Na3AlF6两种晶态存在于BaF2晶体表面,形成化学复合,呈现包覆结构。荧光光谱测试表明,样品在583nm可见光和863nm近红外光室温激发下,均能产生304和324nm的宽波段紫外光发射。时间分辨光谱表明,发光均有一个上升和衰减过程,衰减大约自15ms开始,磷光寿命大于18.4ms。借助上转换发光能量转换过程对发光机理进行了研究,分析表明,发光源于量子限制效应的带间光吸收和界面缺陷的光吸收,发光机制属于量子限制效应-界面发光中心复合发光。     

Effects of pre-irradiation treatment on EPR,optical absorption and TL of albite

In the present work pre-irradiation annealing effect was investigated in albite, NaAlSi 3 O 8 , a silicate crystal. Albite powder samples were heated at 500, 600, 700, 800 and 900 v C for 40 minutes, and then submitted to n -irradiation. Results show that first TL peak (around 160 v C) decreases with annealing temperature, while higher temperature peaks strongly increase. EPR signal around g factor 2, attributed to Al-O m -Al centers, increase with annealing temperature too, as well as optical absorption bands that appear in the crystal after relatively strong n -dose irradiation (of some kGys).     

Cross luminescence of BaF2 crystal: Ab initio calculation

Using ab initio methods and taking into account the lattice relaxation and polarization caused by the occurrence of the core hole, we have studied theoretically the cross luminescence in barium fluoride crystals in terms of the embedded-cluster approach. Two schemes of modeling of the core hole have been performed—in the form of an additional point charge and in the form of the 5p state of the barium ion. Calculations have been done both by the Hartree-Fock method and by the density functional method. We have showed that the deformation of the lattice caused by the occurrence of the core hole leads to states localized on fluorine ions of the nearest environment splitting off from the valence band of the BaF₂ crystal. The cross-luminescence bands at 5.7, 6.3, and 7.1 eV are caused by transitions from these localized states. We have also showed that the low-energy edge of the cross luminescence is formed by transitions from states that are localized on ions of the second coordination sphere.     

Thermally activated muonium formation in Al2O3 and BaF2

Direct evidence is presented for thermally activated muonium formation in Al₂O₃ and BaF₂. In each case the signature of the transition is a transverse field depolarization rate of the prompt diamagnetic component which is much too large to be attributable to muon-nuclear dipolar couplings. Longitudinal field decoupling measurements in Al₂O₃ support the interpretation that the initial diamagnetic fraction converts into a state with a stongly coupled electron —-i.e., muonium.     

He wall bombardment and wall erosion in fusion devices

Abstract

A small fraction of the α-particles produced in a future (D, T)-fusion plasma will escape directly and hit some areas of the surrounding solid walls with their full energy, while the majority will be confined in the magnetic field, slowed down in the plasma and reach the first wall with about the same energy distribution as the other plasma particles.

During bombardment of a solid by energetic α-particles the implanted surface layers undergo several modifications leading finally to a rough surface with a spongy microstructure. Some of these modifications, i.e. blistering and flaking, are only observed if the critical gas concentration builds up first at some depth below the surface. This condition is met only if simultaneous erosion by sputtering stays below a certain limit. In fusion devices with loss fractions of ? 1% high energy α-particles and a plasma temperature near the first wall above some 10 eV, blistering and flaking are likely to be avoided.     

Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Multiphoton excitation of visible and VUV luminescence in MgO crystals

Irradiation of pure MgO crystals with intense XeCl laser radiation yields VUV emission associated with the decay of bi-excitons. This emission exhibits a 4-photon dependence on incident laser intensity. Collisional excitation of luminescence from Cr³⁺ and Ni²⁺ impurities has been detected in doped samples. This emission also shows a multiphoton dependence on incident laser intensity.     

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract

The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F⁺- centres in pure Al₂O₃ are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.     

Intrinsic luminescence in oriented BeO crystals under VUV and inner-shell excitation

Beryllium oxide (BeO) crystals were investigated by time-resolved low temperature VUV-spectroscopy at the SUPERLUMI station and BW3 beam line of HASYLAB (DESY, Hamburg). Photoluminescence spectra (3–10.5 eV), luminescence decay kinetics upon selective photoexcitation, as well as luminescence excitation (50–650 eV) and reflectivity (9–35 eV) spectra were measured and analyzed for oriented BeO crystals. It was shown that study of oriented crystals makes the traditional time-resolved spectroscopy method essentially more informative. Formation of the self-trapped exciton excited states of different multiplicity was found to sensitively depend on excitation energy and mutual orientation of the crystal's C optical axis and electric vector E of exciting polarized synchrotron radiation.     

Photostimulated luminescence in BaX 2 :Eu 2+ (X=Br,Cl) X-ray storage phosphors

The photostimulated luminescence (PSL) effect in BaX 2 :Eu 2+ (X=Br, Cl) is comparable to that observed in BaFBr:Eu 2+ which is used in commercial X-ray storage phosphor screens. After X-irradiation the PSL stimulation spectra of BaX 2 :Eu 2+ (X=Br, Cl) single crystals are identical to the F centre absorption spectra, i.e. the F centres are the PSL-active electron trap centres. The nature of the hole centres is still unknown. The PSL response time of about 0.70 v s is within experimental error of 0.02 v s identical to the Eu 2+ radiative lifetime, whereas in BaCl 2 :Eu 2+ the PSL response time is 0.60 v s, and thus longer than the Eu 2+ radiative lifetime of 0.47 v s.     

火焰原子吸收法测定选矿过程中重晶石中的钡

研究了火焰原子吸收法测定重晶石矿物中钡的方法。确定了狭缝宽度、燃烧器高度、溶液介质、酸度等参数，并确定了样品分解的方法，获得了较好的方法精密度和准确度     

Electromagnetic properties and microwave absorbing characteristics of doped barium hexaferrite

M-type barium hexaferrite BaFe_12−x(Mn_0.5Cu_0.5Ti)_x/2O₁₉ (x varying from 0 to 3 in steps of 1) have been synthesized by the usual ceramic sintering method. The ferrite powders possess hexagonal shape and are well separated from one another. The powder of these ferrites were mixed with polyvinylchloride plasticizer to be converted in to a microwave absorbing composite. X-ray diffraction (XRD), scanning electron microscope (SEM), ac susceptometer, vibrating sample magnetometer, and vector network analyzer were used to analyze its structure, electromagnetic and microwave absorption properties. The results showed that, the magnetoplumbite structures for all the samples have been formed. The sample having higher magnetic susceptibility and coercivity exhibits a larger microwave absorbing ability. Also, the present investigation demonstrates that microwave absorber using BaFe_12−x (Mn_0.5Cu_0.5Ti)_x/2O₁₉ (x=2$x=2$ and 3)/polyvinylchloride can be fabricated for the applications over 15 GHz, with reflection loss more than −25 dB for specific frequencies, by controlling the molar ratio of the substituted ions.     

Inter- and intraconfigurational luminescence of Er 3+ ions in BaY 2 F 8 under VUV excitation

Using energy- and time-resolved spectroscopy the luminescence properties of Er 3+ doped BaY 2 F 8 crystals were investigated at 10 v K under VUV synchrotron radiation excitation. Radiative intraconfigurational f - f and interconfigurational d - f transitions in Er 3+ ions were observed under f - d excitation. Whereas the onset of 4 S 3/2 population via f - d excitation starts at 59 900 v cm m 1 , efficient excitation of emissions arising from the 2 P 3/2 state begins only above 67 000 v cm m 1 in VUV region. Such behaviour can be explained by a cross-relaxation process of the type ( 2 F(2) 5/2 , 4 I 15/2 ) M ( 2 P 3/2 , 2 P 3/2 ) taking place within f -states of Er 3+ ions finally populating the emitting 2 P 3/2 state.     

火焰原子吸收光谱法测定光学新材料钛酸钡锶中钡和锶

本文介绍了火焰原子吸收光谱法测定光学新材料钛酸钡锶中钒和锶，研究样品制备、测试条件和共存元素干扰，本法的准确度和精密度均满意。     

Effect of hydrostatic and chemical pressure on BaF2 and BaF2: Eu2+ crystals

The effect of hydrostatic pressure on a BaF₂ crystal was studied within the shell model in the pair-wise potential approximation. The structural phase transition from the cubic to orthorhombic phase was simulated. The behavior of the unit-cell parameters of the α-and β-BaF₂ phases under hydrostatic pressure (from 0 to 12 GPa) was investigated. The fundamental vibration frequencies of BaF₂ under hydrostatic pressure (0–3.5 GPa) were calculated for both phases. The effect of chemical pressure on the BaF₂ crystal was studied by simulating Ba_1?x Me_xF₂ mixed crystals (Me=Ca, Sr). It was shown that at impurity concentrations up to 15–20 at. % the lattice constant varies in the same way as it does when hydrostatic pressure increases to P _c , which corresponds to a phase transition to the orthorhombic phase. The effect of chemical and hydrostatic pressure on BaF₂: Eu²⁺ doped crystals was also studied. The dependence of the absorption and luminescence zero-phonon line shift on the Eu²⁺-ligand distance was calculated.     

Defect cluster formation in M2O3-doped CeO2

Atomistic simulation calculations based on energy minimisation techniques have been used to study the energetics associated with M₂O₃ solution in CeO₂. Results show that the binding energy of an oxygen vacancy to one or two substitutional cations is a strong function of dopant cation radius; small dopant ions prefer to occupy first neighbour sites, large dopant ions prefer second neighbour sites. The crossover occurs at approximately Gd³⁺, which also exhibits the smallest binding energy. These results are used to predict lattice parameter as a function of defect concentration and comparison is made to experimental values.