Temperature behavior of damage in sapphire implanted with light ions

In this study, we compare and discuss the defect behavior of sapphire single crystals implanted with different fluences (1 × 10¹⁶–1 × 10¹⁷ cm^?2) of carbon and nitrogen with 150 keV. The implantation temperatures were RT, 500 °C and 1000 °C to study the influence of temperature on the defect structures. For all the ions the Rutherford backscattering-channeling (RBS-C) results indicate a surface region with low residual disorder in the Al-sublattice. Near the end of range the channeled spectrum almost reaches the random indicating a high damage level for fluences of 1 × 10¹⁷ cm^?2. The transmission electron microscopy (TEM) photographs show a layered contrast feature for the C implanted sample where a buried amorphous region is present. For the N implanted sample the Electron Energy Loss Spectroscopy (EELS) elemental mapping give evidence for the presence of a buried damage layer decorated with bubbles. Samples implanted at high temperatures (500 °C and 1000 °C) show a strong contrast fluctuation indicating a defective crystalline structure of sapphire.     

Neutronic-thermohydraulic oscillatory instability in modern PWRs due to high concentrations of boron in the water

Conspicuous amounts of boric acid are normally dissolved into the moderator of a modern PWR, especially in BOL operative conditions. If the concentration of such a neutronic poison attains certain limits, the nuclear temperature coefficient of the moderator, which is highly negative in the absence of boron, may turn about and reach positive values, due to the strong thermal expansion of the water. A dynamical model of a PWR system is presented, facilitating a quick stability analysis related to the co-ordination of boric acid solution in the water and control-rod insertion in the core.     

Formation of Au nanoparticles in sapphire by using Ar ion implantation and thermal annealing

In this paper, we present results of the synthesis of gold nanoclusters in sapphire, using Ar ion implantation and annealing in air. Unlike the conventional method of Au implantation followed by thermal annealing, Au was deposited on the surface of m- and a- cut sapphire single crystal samples including those pre-implanted with Ar ions. Au atoms were brought into the substrate by subsequent implantation of Ar ions to form Au nanoparticles. Samples were finally annealed stepwisely in air at temperatures ranging from 400 to 800 °C and then studied using UV-vis absorption spectrometry, transmission electron microscopy and Rutherford backscattered spectrometry. Evidence of the formation Au nanoparticles in the sapphire can be obtained from the characteristic surface plasmon resonance (SPR) absorption band in the optical absorption spectra or directly from the transmission electron microscopy. The results of optical absorption spectra indicate that the specimen orientations and pre-implantation also influence the size and the volume fraction of Au nanoparticles formed. Theoretical calculations using Maxwell-Garnett effective medium theory supply a good interpretation of the optical absorption results.     

A study of blistering in Mylar due to H+ ion implantation

A comprehensive study of blistering of Mylar under 100 and 250 keV H⁺ -ion implantation has been made. The conversion of blisters to “Karnavalaya” and vice-versa due to electron beam excitation during scanning electron microscopy of the implanted sample is found to be an analog of similar events taking place during ion implantatio. This results in the development of complex blisters. The critical dose for blistering is found to be dependent on the residual surface stresses. It is higher when the stresses are compressive as when tensile. The gases emitted during ion implantation were analysed by a residual gas analyser. CO and CO₂ are found to be the main components resulting from radiolysis of the Mylar due to H⁺ ion implantation. However, the emission of H₂ in this case remains inconclusive due to various experimental limitations. Various observed features on the Mylar surface after ion implantation were successfully explained on the basis of a proposed model of blistering based on the micro-structure of semicrystalline polymers.     

Electrical and optical surface degradation of silica due to superficial He implantation

Different oxides will be used in ITER and future fusion reactors for electrical insulation and optical components. The vacuum face of these materials will be subjected not only to neutron and gamma irradiation, but also to particle bombardment, due mainly to ionization of the residual gas and acceleration of the resulting ions by local electric fields. Previous work showed that silica suffers electrical and optical degradation when subjected to He bombardment with energies from 300 keV down to 27 keV. As the He ion energy may extend down to some few keV, or less, further work has been performed to study possible degradation for energies from 21 keV down to 5 keV. The results show that both surface and optical degradation occur at these low energies, more rapidly for the lowest energy (5 keV) ions. They also suggest that the superficial narrow implanted He profile plays an important role in the surface degradation.     

BNCT中载能粒子对肿瘤细胞损伤效应的Monte Carlo模拟及分析

为更好地了解硼中子俘获治疗(Boron neutron capture therapy,BNCT)中载能粒子穿过单个细胞引起的生物学效应以及理解在细胞或亚细胞水平上的微观剂量分布,采用Monte Carlo程序模拟载能粒子在人体细胞中的输运过程,给出了α粒子垂直细胞表面入射时的射程分布、径迹结构及靶损伤情况,同时模拟分析了^10B位于细胞中不同位置时主要载能粒子(α粒子和^7Li离子)在细胞中能量沉积情况和相应的细胞损伤与存活情况,为BNCT的微剂量研究提供了初步的理论依据。     

Temperature and dose dependences of nitrogen implantation into aluminium

It has been established that nitrogen implantation into metals can alter their surface properties such as friction, wear, corrosion, etc. Recent studies have shown that nitrogen implantation into aluminium leads to the formation of aluminium nitride which has interesting tribological, electronic and optical properties. For a given implantation energy, the characteristics of the nitrogen profile, e.g. thickness, shape and concentration, depend strongly on the experimental conditions during the implantation. In order to study the influence of the implantation parameters, aluminium samples have been bombarded with ¹⁵N⁺₂ of 100 keV to different doses ((1–20) × 10¹⁷ N⁺/cm²), at several temperatures (25–300° C). Distributions of the implanted species were investigated by nuclear reaction analysis (NRA) and by Rutherford backscattering spectroscopy (RBS). The chemical bonds of aluminium in the matrix were studied by using low-energy electron-induced X-ray spectroscopy (LEEIXS). It is shown that aluminium nitride is formed and that the nitrogen distribution presents a surface peak when the implantation temperature is higher than 200° C.     

Prediction of pressure tube fretting-wear damage due to fuel vibration

Fretting marks between fuel bundle bearing pads and pressure tubes have been observed at the inlet end of some Darlington Nuclear Generating Station (NGS) and Bruce NGS fuel channels. The excitation mechanisms that lead to fretting are not fully understood. In this paper, the possibility of bearing pad-to-pressure tube fretting due to turbulence-induced motion of the fuel element is investigated. Numerical simulations indicate that this mechanism by itself is not likely to cause the level of fretting experienced in Darlington and Bruce NGSs.     

Cross-section transmission electron microscopy of the ion implantation damage in annealed diamond

It has formerly been shown that low-damage levels, produced during the implantation doping of diamond as a semiconductor, anneal easily while high levels “graphitize” (above about 5.2 × 10¹⁵ ions/cm²). The difference in the defect types and their profiles, in the two cases, has never been directly observed. We have succeeded in using cross-section transmission electron microscopy to do so. The experiments were difficult because the specimens must be polished to ∼40 μm thickness, then implanted on edge and annealed, before final ion beam thinning to electron transparency. The low-damage micrographs reveal some deeply penetrating dislocations, whose existence had been predicted in earlier work.     

Length changes in β-plutonium due to self-irradiation damage at 4 K

A sample of β-plutonium, stabilised by quenching, has been held for over 3000 h in liquid helium. The resulting contraction fits closely to an exponential equation of the usual form: Δl/l = 4.90 × 10⁻³ (1 − exp (−2.35 × 10⁻⁴t)). It is suggested that the contraction is due to the formation under irradiation of an amorphous form of plutonium.     

Effects on channeling of radiation damage due to 28 GeV protons

For an irradiation of 2.9 × 10¹⁹ protons at a beam energy of 28 GeV, the channeling minimum yield in a silicon single crystal increased from 2.3% to 4.1%. The radiation damage occurred with a proton fluence of (4.1 ± 1.4) × 10²⁰/cm². The degradation was measured with MeV-range He ions using Rutherford backscattering. The relevance to bent crystal extraction of TeV beams is discussed.     

Simulation of damage induced by ion implantation in Lithium Niobate

A simulation tool has been developed to engineer the damage formation in Lithium Niobate by ion irradiation with any atomic number and energy. Both nuclear and electronic processes were considered and, in particular, the dependence on the ion velocity of the electronic excitation damage efficiency has been taken into account. By using this tool it is possible both to draw damage nomograms, useful to qualitatively foresee the result of a given process, and to perform reliable simulations of the defect depth profiles, as demonstrated by the good agreement with the experimental data available in the literature.     

Change in flow stress and ductility of δ-phase Pu-Ga alloys due to self-irradiation damage

An internal state variable model for the mechanical behavior of aged Pu-Ga alloys is developed and used to predict the change of the material with accumulated self-irradiation damage or age. The material model incorporates microstructural data such as the primary irradiation-induced defect density from cascades, the density and average size of helium bubbles, the initial dislocation density, and the initial average segment length of the dislocation density as input parameters, and then evaluates the stress-strain response of a representative volume element of the material. Given this response at a single material point, the deformation behavior of tensile specimens is predicted, and it forecasts increased strength, decreased strain hardening, and more strain localization with aging. Although the material point behavior showed some slight strain softening, this strain softening is expected to be masked by statistical variations of different volume elements and by the strain rate sensitivity of the material. Hence, it is not expected to appear in the stress-strain response of macroscopic tensile specimens, and only the increase in flow stress will be measured.     

Structure and optical properties of sapphire implanted with boron at room temperature and 1000 °C

Many previous studies of ion-implanted sapphire have used gas-forming light ions or heavier metallic cations. In this study, boron (10¹⁷ cm⁻², 150 keV) was implanted in c-axis crystals at room temperature, 500 and 1000 °C as part of a continuing study of cascade density and “chemical” effects on the structure of sapphire. Rutherford backscattering-ion channeling (RBS-C) of the RT samples indicated little residual disorder in the Al-sublattice to a depth of 50–75 nm but almost random scattering at the depth of peak damage energy deposition. The transmission electron micrographs contain “black-spot” damage features. The residual disorder is much less at all depths for samples implanted at 1000 °C. The TEM photographs show a coarse “black-spot damage” microstructure. The optical absorption at 205 nm is much greater than for samples implanted with C, N, or Fe under similar conditions.     

Ion implantation in TiO2: damage production and recovery,lattice site location and electrical conductivity

Ions with different oxidation states were implanted in TiO₂ (rutile). The lattice disorder as well as the lattice site location of the implanted ions were determined using Rutherford Backscattering and Channeling (RBS-C) spectrometry. The production of disorder as a function of dose and temperature, and its recovery was studied in detail. Important results are the observation of dynamic recovery at 293 K and above, and one isothermal at 77 K and two thermal recovery stages between 170 and 210 K and between 260 and 293 K. The recovery at 77 K is proportional to ln t, indicating that the activation energy increases with decreasing disorder density. The results concerning the lattice site location of 14 ion species reveal that 13 ions occupy Ti lattice site. With increasing netcharge, the maximum soluble concentration decreases by the formation of impurity–defect complexes probably enforced by charge compensation. Directed displacements from the substitutional lattice site provide some hints on the structures of these complexes. The electrical conductivity of the implanted samples increased by about 12 orders of magnitude irrespective of the oxidation state of the implanted species. From the temperature and dose dependence of the electrical conductivity as well as from its similar behaviour for noble-gas ions and other species it is concluded, that the carrier transport occurs by single energy states excitation at low doses and by variable range hopping between localized states at high doses.     

Glancing incidence diffuse X-ray scattering studies of implantation damage in Si

Diffuse X-ray scattering (DXS) at glancing incidence is a potentially powerful means for elucidating damage structures in irradiated solids. Fundamental to the analysis of diffuse X-ray scattering data is a knowledge of the atomic displacement field around defects, which for implantation damage in crystals like Si has been difficult to obtain using analytical solutions of elastic continuum theory. We present a method for predicting the diffuse scattering pattern by calculating the displacement field around a defect using fully atomistic simulations and performing discrete sums for the scattering intensity. We apply the method to analyze experimental DXS results of defects produced by 4.5 keV He and 20 keV Ga irradiations of Si at temperatures of 100–300 K. The results show that the self-interstitial in ion-irradiated Si becomes mobile around 150 K, and that amorphization of silicon by light and medium-heavy projectiles occurs homogeneously through the buildup of interstitial clusters, and not within single cascade events.