采用EPR谱和TEM对YSZ单晶辐照损伤的研究

 利用电子顺磁共振(EPR)谱和透射电子显微镜(TEM)研究了YSZ单晶的辐照效应。200 keV的Xe和400 keV的Cs离子注入［111］取向的YSZ单晶中,注量均为5×10¹⁶ cm^-2。EPR结果表明辐照产生了共振吸收位置g‖=1.989 和 g⊥=1.869、对称轴为［111］的六配位Zr³⁺顺磁缺陷。Cs辐照产生了比Xe 离子辐照多约150倍的六配位Zr³⁺顺磁缺陷。两种样品的剖面电子显微分析表明没有发现非晶化转变,但是Cs离子辐照的样品在损伤集中区域产生了密度较高的缺陷。因此,EPR谱和电子显微观察均说明在相同离位损伤(约160 dpa)的情况下,Cs离子辐照比Xe 离子辐照产生了更多的缺陷。造成这一现象的原因是Cs离子是化学活性的而Xe 离子却是惰性的。     

单晶YSZ的Xe+辐照损伤的电子显微分析

 不同注量200keV Xe⁺ 注入YSZ单晶样品的电子显微分析结果表明，随着辐照注量的增加，缺陷簇的密度增大，在1×10¹⁵~1×10¹⁶cm^-2Xe⁺注量，缺陷簇密度迅速增大，形成间隙型位错环；当Xe⁺注量增大到1×10¹⁷cm^-2，缺陷簇密度的增加变得缓慢，并且有直径为2～4nm的Xe气泡析出。选区电子衍射花样表明YSZ样品没有产生非晶化转变。在Xe⁺辐照的离位率高达约350dpa的情况下，YSZ晶体没有非晶化，其原因主要是由于注入的Xe⁺以气泡形式析出。     

Radiation induced colour centers and damage in YAlO 3 :Ce and YAlO 3 :Ce,Zr scintillators

Radiation induced absorption spectra (irradiation up to 500 v Gy by 60 Co) and TSL of a set of YAP:Ce crystals co-doped by Zr 4+ were investigated. Positive effect of Zr 4+ co-doping was found, namely reduction of both the defect creation and presence of TSL active traps. Up to three absorption bands were observed in the radiation induced absorption spectra. They probably have different origin - from colour centers in the green part of the spectra to change of the Ce 3+ valency in the near UV.     

Effect of electron and proton irradiation on recombination centers in GaAsN grown by chemical beam epitaxy

Deep level transient spectroscopy (DLTS) was deployed to study the evolution, upon electron irradiation and hydrogenation of GaAsN grown by chemical beam epitaxy, of the main nitrogen-related nonradiative recombination center (E1), localized at 0.33 eV below the bottom edge of the conduction band of the alloy. On one hand, the electron irradiation was found to enhance the density of E1 depending on the fluence dose. On the other hand, the hydrogenation was found to passivate completely E1. Furthermore, two new lattice defects were only observed in hydrogenated GaAsN films and were suggested to be in relationship with the origin of E1. The first defect was an electron trap at average thermal activation energy of 0.41 eV below the CBM of GaAsN and was identified to be the EL5-type native defect in GaAs, originating from interstitial arsenic (As_i). The second energy level was a hole trap, newly observed at average thermal activation energy of 0.11 eV above the valence band maximum of the alloy and its origin was tentatively suggested to be in relationship with the monohydrogen–nitrogen (N–H) complex. As the possible origin of E1 was tentatively associated with the split interstitial formed from one N atom and one As atom in single V-site [(N–As)_As], we strongly suggested that the new hole trap took place after the dissociation of E1 and the formation of N–H complex.     

Irradiation effect of yttria-stabilized zirconia by high dose dual ion beam irradiation

Yttria-stabilized zirconia （YSZ） is irradiated with 2.0-MeV Au2＋ ions and 30-keV He＋ ions. Three types of He, Au, Au ＋ He （successively） ion irradiation are performed. The maximum damage level of a sequential dual ion beam implanted sample is smaller than single Au ion implanted sample. A comparable volume swelling is found in a sequential dual ion beam irradiated sample and it is also found in a single Au ion implanted sample. Both effects can be explained by the partial reorganization of the dislocation network into weakly damaged regions in the dual ion beam implanted YSZ. A vacancy-assisted helium trapping/diffusion mechanism in the dual ion beam irradiated condition is discussed. No phase transformation or amorphization behavior happens in all types of ion irradiated YSZ.     

Phase change in CoTi2 induced by MeV electron irradiation

The phase change induced by MeV electron irradiation in the intermetallic compound E9₃–CoTi₂ was investigated using high-voltage electron microscopy. Under MeV electron irradiation, CoTi₂ was first transformed into an amorphous phase and, with continued irradiation, crystallite formation in the amorphous phase (i.e. formation of crystallites of a solid-solution phase within the amorphous phase) was induced. The critical temperature for amorphisation was around 250 K. The total dose (dpa) required for crystallite formation (i.e. that required for partial crystallisation) was high (i.e. 27–80 dpa) and, even after prolonged irradiation, the amorphous phase was retained in the irradiated sample. Such partial crystallisation behaviour of amorphous Co₃₃Ti₆₇ was clearly different from the crystallisation behaviour (i.e. amorphous-to-solid solution, polymorphous transformation) of amorphous Cr₆₇Ti₃₃ reported in the literature. A possible cause of the difference is discussed.     

Identification of F+ centers in hafnia and zirconia nanopowders

Luminescence properties of undoped hafnia and zirconia nanopowders prepared by solution combustion synthesis were investigated under photo- and electron-beam excitation in 10–400 K temperature range. Along with the main luminescence band revealed in investigated materials at low temperatures at 4.2–4.3 eV and ascribed to the emission of self-trapped excitons, there are luminescence bands due to defects and impurities introduced during sample preparation. At room temperature the latter emissions dominate in the luminescence spectra as the intrinsic self-trapped exciton emission is quenched. Analysis of decay kinetics of defect centers allowed identification of F⁺ centers emission at 2.8 eV with lifetimes ∼3–6 ns in hafnia and zirconia under intra-center excitation.     

Determination of the electron relaxation rates in paramagnetic metal complexes: applicability of available NMR methods

Four different approaches for determining the electron relaxation rates in paramagnetic metallo-proteins are investigated, using a paramagnetic Ni2+ complex of a protein as an example. All four approaches rely on the determination of the longitudinal paramagnetic relaxation enhancements, R1p, of the 1H nuclei and the backbone 15N nuclei. Three of the methods utilize the field dependence of the R1p rates. It is found that the applicability of each of these methods depends on whether the fast-motion condition, omegaS2tau2<1, applies to the electron relaxation, omegaS being the Larmor frequency of the electron spin S and tau the correlation time of the electron relaxation. If the fast-motion condition is fulfilled, the electron relaxation rate can be obtained from the ratio of the R1p rates of one or more protons at two magnetic field strengths (method A). On the other hand, if the fast-motion condition does not apply, more elaborate methods must be used that, in general, require a determination of the R1p rates over a larger range of magnetic field strengths (method C). However, in the case of paramagnetic metal ions with relatively slow electron relaxation rates only two magnetic field strengths suffice, if the R1p rates of a hetero nucleus are included in the analysis (method B). In the fourth method (method D), the electron relaxation is estimated as a parameter in a structure calculation, using distance constraints derived from proton R1p rates at only one magnetic field strength. In general, only methods B and C give unambiguous electron relaxation rates.     

Identification of paramagnetic centers and the dating of cave dripstones by electron paramagnetic resonance

Dating of samples of cave dripstones collected from the Jaskinia Czarna (the Polish Tatra Mountains) and fossil bones was carried out by means of electron paramagnetic resonance (EPR). By this method the age of the sample is found as the ratio of the total dose (TD) to the annual dose rate (D _a). TD is established on the basis of EPR spectra, whileD _a is the sum of the internal dose rate, calculated on the basis of the concentration and activity of radioactive nuclides in the sample, and the external dose rate measured at the site of the sample collection by a calibrated γ-radiation probe. Two models of uranium accumulation were used: the model of linear accumulation and the model on the basis of the assumption of a disturbed radiation equilibrium in the uranium series. The results were compared with the age determined by the uranium-thorium method. Moreover, by the computer resolution enhancement method the dynamics of paramagnetic centers in calcite annealed in nitrogen atmosphere was studied in the range of 283–873 K, and the results were compared with earlier results for calcite annealed in the air. On the basis of the study of paramagnetic center dynamics in dripstone calcite it was established that the radicals responsible for the EPR signals were CO₂⁻ in different symmetries of the crystalline field, CO₃³⁻ in the axial symmetry, CO₃⁻ in the axial and isotropic symmetry and HCO₃²⁻. The results implied that samples to be subjected to EPR dating should be kept at room temperature.     

Phase change of CoTi and Co3Ti induced by MeV-scale electron irradiation

This study investigates equilibrium-to-nonequilibrium solid phase transitions induced by MeV-scale electron irradiation in B2-CoTi and L1₂-Co₃Ti intermetallic compounds by means of high-voltage electron microscopy. Under MeV-scale electron irradiation, B2-CoTi transforms into a body-centered cubic solid solution through chemical disordering and eventually transforms into an amorphous phase. The critical temperature for amorphisation is found to be 110 K. L1₂-Co₃Ti also exhibits chemical disordering at temperatures below 700 K. However, its amorphisation does not occur even at a low temperature of 20 K. The dominant factor in these solid phase transitions is discussed in terms of the Gibbs free energy.     

Colour centre production in yttria-stabilized zirconia by X-ray and electron irradiations: effect of yttria content

Yttria-stabilized zirconia (YSZ) single crystals (for 9.5 and 18 mol% yttria) were irradiated at room temperature (RT) by X-rays (W white spectrum) and 2.5-MeV electrons. The growth curves of the so-called T-centre (for trigonal centre, i.e. Zr³⁺ sitting in a trigonal symmetry site) were studied as a function of absorbed dose, or irradiation time, by UV–visible optical absorption (OA) spectroscopy and X-band electron paramagnetic resonance spectroscopy. The defect concentration at saturation and the production rate are increased by a factor around two for 18 mol% yttria with respect to 9.5 mol%. Defect decay was then followed after irradiation by OA spectroscopy as a function of ageing time at RT. Growth and decay curves of the T-centre are modelled on the basis of rate equations of charge-exchange reactions with the zirconium lattice ions. Increase in yttrium content is thought to decrease hole trapping on Zr³⁺ ions, thereby enhancing T-centre formation.     

On the production of paramagnetic defects in silicon by electron irradiation

Monocrystalline silicon samples of different impurity contents have been irradiated with 1.5 MeV electrons in order to produce divacancies in their negative charge state. In these samples different combinations of defects have been observed with electron paramagnetic resonance. The conditions for production and observation of these defects are compared. For two new EPR spectra, labelled (Si-) NL11 and (Si-) NL12, the spin Hamiltonian parameters are reported. For NL11, which arises from an S = 1 spin state, the obvious identification with the neutral charge state of the divacancy can not be confirmed.     

Microstructure and thermal stability of Fe,Ti, and Ag implanted yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) was implanted with 15 keV Fe or Ti ions up to a dose of 8×10¹⁶ at cm^–2. The resulting dopant concentrations exceeded the concentrations corresponding to the equilibrium solid solubility of Fe₂O₃ or TiO₂ in YSZ. During oxidation in air at 400° C, the Fe and Ti concentration in the outermost surface layer increased even further until a surface layer was formed of mainly Fe₂O₃ and TiO₂, as shown by XPS and ISS measurements. From the time dependence of the Fe and Ti depth profiles during anneal treatments, diffusion coefficients were calculated. From those values it was estimated that the maximum temperature at which the Fe- and Ti-implanted layers can be operated without changes in the dopant concentration profiles was 700 and 800° C, respectively. The high-dose implanted layer was completely amorphous even after annealing up to 1100° C, as shown by scanning transmission electron microscopy. Preliminary measurements on 50 keV Ag implanted YSZ indicate that in this case the amorphous layer recrystallizes into fine grained cubic YSZ at a temperature of about 1000° C. The average grain diameter was estimated at 20 nm, whereas the original grain size of YSZ before implantation was 400 nm. This result implies that the grain size in the surface of a ceramic material can be decreased by ion beam amorphisation and subsequent recrystallisation at elevated temperatures.     

Surface free energy of ultra-high molecular weight polyethylene modified by electron and gamma irradiation

Surface free energy of biocompatible polymers is important factor which affects the surface properties such as wetting, adhesion and biocompatibility. In the present work, the change in the surface free energy of ultra-high molecular weight polyethylene (UHMWPE) samples, which is produced by electron beam and gamma ray irradiation were, investigated. Mechanism of the changes in surface free energy induced by irradiations of doses ranging from 25 to 500 kGy was studied. FTIR technique was applied for sample analysis. Contact angle measurements showed that wettability and surface free energy of samples have increased with increasing the irradiation dose, where the values of droplet contact angle of the samples decrease gradually with increasing the radiation dose. The increase in the wettability and surface free energy of the irradiated samples are attributed to formation of hydrophilic groups on the polymer surface by the oxidation, which apparently occurs by exposure of irradiated samples to the air.     

Spectral analysis of paramagnetic centers induced in human tooth enamel by x-rays and gamma radiation

Based on study of spectral and relaxation characteristics, we have established that paramagnetic centers induced in tooth enamel by x-rays and gamma radiation are identical in nature. We show that for the same exposure dose, the intensity of the electron paramagnetic resonance (EPR) signal induced by x-radiation with effective energy 34 keV is about an order of magnitude higher than the amplitude of the signal induced by gamma radiation. We have identified a three-fold attenuation of the EPR signal along the path of the x-radiation from the buccal to the lingual side of a tooth, which is evidence that the individual had undergone diagnostic x-ray examination of the dentition or skull. We have shown that the x-ray exposure doses reconstructed from the EPR spectra are an order of magnitude higher than the applied doses, while the dose loads due to gamma radiation are equal to the applied doses. The data obtained indicate that for adequate reconstruction of individual absorbed doses from EPR spectra of tooth enamel in the population subjected to the combined effect of x-radiation and accidental external gamma radiation as a result of the disaster at the Chernobyl nuclear power plant, we need to take into account the contribution to the dose load from diagnostic x-rays in examination of the teeth, jaw, or skull.     

Modification of Lexan polycarbonate induced by electron irradiation

An 8 MeV electron-induced modification of Lexan polycarbonate (Lexan) films has been studied systematically using UV–visible spectroscopy, LCR meter, X-ray diffractogram (XRD) and differential scanning calorimetry (DSC) techniques. The optical properties of the Lexan films showed a decrease in the optical energy gap with an increase in the electron dose. The AC conductivity and dielectric constant were found to change significantly due to irradiation, and the dielectric constant was found to obey the universal law of dielectric constant. The XRD results show that the crystallite size and the percentage of crystallinity of films decrease after irradiation. The decrease in glass transition temperature shown by DSC studies reveals that the polymeric system has changed towards a more disordered state.     

Study of size dependent features of swift heavy ion irradiation in nanosized zinc ferrite

In the present work zinc ferrite nanoparticles of different crystallite size were irradiated with 200 MeV Ag¹⁵⁺ ion beam. The structural and magnetic characterization performed for these samples indicate the presence of size dependent irradiation induced changes in the nanoparticles. The superparamgnetic nanoparticles do not alter their behavior after irradiation; however paramagnetic samples exhibit weak ferrimagnetism in the irradiated specimen. Results obtained from these measurements are in agreement with results obtained from the electron paramagnetic resonance spectroscopy.     

Depth distribution profiles of colour centers produced in alkali halides by electron irradiation

Depth distributions of colour centers produced in 〈100〉 KCl crystals by irradiation with 1.0 and 2.0 MeV electrons were measured. A distinct anisotropy in the depth distributions of the optical colouration density as well in the F- and M-center density was observed.     

电子辐照能量对Kapton/Al热控涂层光学性能的影响

 研究了电子辐照时，电子能量与累积通量对Kapton/Al热控涂层光学性能的影响。采用原位测量的手段记录了辐照前后的光谱反射系数。试验结果表明，电子辐照后Kapton/Al热控涂层的反射性能,在太阳光谱辐射强度较大的300~1 200nm波长区间产生较大程度退化。在电子辐照作用下，作为离子导电型聚合物的 Kapton薄膜表面没有发现辐照充电效应。辐照后涂层材料存在“退火效应”，或称“漂白效应”。Kapton/Al涂层太阳吸收比的变化量与电子辐照累积通量的变化关系成幂函数形式,其系数与指数的极大值与极小值分别出现在电子能量为50keV附近。在辐照累积通量相同时，该变化量随辐照电子能量的提高而增大。