380 keV proton irradiation effects on photoluminescence of Eu-doped GaN

The effect of 380 keV proton irradiation on the photoluminescence (PL) properties has been investigated for undoped and Eu-doped GaN. As the proton irradiation exceeds , a drastic decrease of PL intensity of the near band-edge emission of undoped GaN was observed. On the other hand, for Eu-doped GaN, the PL emission corresponding to the ⁵D₀→⁷F₂ transition in Eu³⁺ kept the initial PL intensity after the proton irradiation up to . Present results, together with our previous report on electron irradiation results, suggest that Eu-doped GaN is a strong candidate for light emitting devices in high irradiation environment.     

Microdefects and 3d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques

Microdefects and 3d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3d electron signal in the spectra of all tested samples. The 3d electron signal in the spectrum of Fe₅₀Al₅₀ alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3d electrons of Fe atoms and 3p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.     

Micro-XRF and micro-XAFS studies of an Al matrix Fe–Ni composite

We report on the distribution and local coordination of Fe and Ni in an Al matrix Fe–Ni composite, by means of X-ray Fluorescence mapping (XRF), micro- (μ-) and conventional Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies. The μ-XRF maps reveal that Fe segregates and forms Fe-rich islands which are depleted of Ni. The combined μ-EXAFS and EXAFS results reveal that both metals are bonded only to Al. More specifically, the Fe-rich islands are identified as FeAl₃ microcrystallites while in the Fe-poor regions, Fe belongs to an intermetallic FeAl phase. The bonding environment of Ni is also modified due to the variations in the distribution of the metals. In the region with high Ni concentration, i.e. Fe-poor regions, the Ni atoms are bonded to 10.5 ± 1.1 Al atoms that substitute Ni in fcc Ni. On the contrary, in the Ni-poor regions, where the Fe concentration exhibits maxima, the number of Al atoms in the first nearest shell of Ni is equal to 7.8 ± 0.9, i.e. in this region, Ni forms intermetallic NiAl. Finally, the atomic percentage of the Fe and Ni atoms that belong to the Fe-rich and Fe-poor islands is determined from the Fe-K and Ni-K edge EXAFS analysis. The majority of the Fe atoms (～80 at%) belongs to the FeAl₃ microcrystallites, embedded into a FeAl matrix. On the contrary, the same atomic percentage of Ni atoms (～50 at%), occupies sites in both the NiAl regions as well as in the matrix of the (Ni, Al) solid solution.     

Fundamentals of surfactant sputtering

We introduce a new sputter technique, utilizing the steady-state coverage of a substrate surface with up to 10¹⁶ cm⁻² of foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These atoms strongly modify the substrate sputter yield on atomic to macroscopic length scales and therefore act as surfactant atoms (a blend of “surface active agent”). Depending on the surfactant-substrate combination, the novel technique allows enhanced surface smoothing, generation of novel surface patterns, shaping of surfaces and formation of ultra-thin films. Sputter yield attenuation is demonstrated for sputtering of Si and Fe substrates and different surfactant species using 5 keV Xe ions at different incidence angles and fluences up to 10¹⁷ cm⁻². Analytical approaches and Monte Carlo simulations are used to predict the sputtering yield attenuation as function of surfactant coverage. For sputtering of Si with Au surfactants we observe high sputter yields despite a steady-state surfactant coverage, which can be explained by strong ion-induced interdiffusion of substrate and surfactant atoms and the formation of a buried Au_xSi surfactant layer in dynamic equilibrium.     

Irradiation damage in proton irradiated palladium-iron solid solutions

Transmission electron microscopy was used to investigate the irradiation damage, in particular irradiation induced precipitation (IIP), in Pd-base alloys containing 2, 8, 12 and 18 at % Fe. The specimens were irradiated mainly using 400 keV protons at a current density of 0.16 μA/mm² over the temperature range 110 to 750°C. A few samples containing 2 and 8% Fe were also irradiated using 3 MeV NiP⁺ ions. The irradiation microstructure of the proton irradiated alloys consists of dislocation loops over the temperature range 110 to 550°C and voids up to 650°C in all the alloys. IIP of Pd₃Fe was observed only in the Pd-18% Fe alloy between 110 and 500°C, irradiated to a dose of 0.9 dpa. Pd₃Fe was associated with dislocation loops, voids and grain boundaries. IIP was not observed in the Pd-2,8 and 12% Fe alloys proton irradiated to the same dose, nor to a higher dose of 1.5 dpa. It was also not observed in the 2 and 8% Fe alloys irradiated at 600 and 700°C by 3 MeV Ni⁺ ions.The absence of IIP in the more dilute alloys is attributed to the fast back diffusion of Fe atoms, which is due to the high mobility of vacancies in these alloys. This causes the Fe concentration at the sinks to remain below the solubility limit. Therefore, even though Fe is an undersized solute, the size effect alone is not sufficient for the production of IIP at point defect sinks in most Pd-Fe alloys. It is proposed that IIP can occur only when the alloy concentration is high enough to minimize the rate of back diffusion, which depends not only on the vacancy mobility but also on the concentration gradient near point defect sinks.     

Damage creation in ion irradiated Si1−xGex/Si structures

Strained SiGe/Si structures have been proposed as substrates for fabrication of high speed metal oxide semiconductor transistors. However, influence of strain and/or presence of Ge atoms on damage creation during ion irradiation have not been explored to a significant extent. In this study, Rutherford backscattering spectrometry (RBS) was used to characterize Si_1−xGe_x/Si structures irradiated by 140 keV He⁺ ions at room temperature. When compared with pure Si, strained samples show enhanced damage accumulation as a function of He fluence. Channeling angular scans did not reveal any specific configuration of displacements. Possible mechanisms for enhanced damage in strained Si are discussed.     

X-ray absorption spectroscopy and X-ray photoelectron spectroscopy studies of CaSO4:Dy thermoluminescent phosphors

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO₄:Dy phosphor samples at the Dy L₃ edge with synchrotron radiation. Measurements were carried out on a set of samples which were subjected to post-preparation annealing at different temperatures and for different cycles. The EXAFS data have been analysed to find the Dy-S and Dy-O bond lengths in the neighbourhood of the Dy atoms in a CaSO₄ matrix. The observations from EXAFS measurements were verified with XANES and XPS techniques. On the basis of these measurements, efforts were made to explain the loss of thermoluminescence sensitivity of CaSO₄:Dy phosphors after repeated cycles of annealing at 400 °C in air for 1 h.     

The influence of relative humidity on iron corrosion under proton irradiation

With regard to the storage for high-level radioactive waste and the reversible period of a geological repository, the influence of proton irradiation on the indoor atmospheric corrosion of iron has been investigated in relation to the relative humidity (RH) in the atmosphere. Irradiation experiments were performed using a 3-MeV extracted proton beam. Relative humidity varies from 0% to 85%. Before and after each irradiation, the surfaces of the sample were characterised by Rutherford backscattering spectrometry in order to determine oxygen concentrations in the metal. The maximum oxidation rate was observed for 45% RH in air under proton irradiation and was compared with literature data without irradiation where the maximum oxidation rate was observed at 95% RH. The experimental results are discussed on the basis of the Langmuir-Hinshelwood (LH) model: they are explained by the contrast between the adsorption of O₂ and H₂O species on the active cathodic sites of the iron surface and by the formation of H⁺(H₂O)_n.     

PIXE and RBS analysis of Tl-1223 superconducting phase substituted by scandium

Ion beam analysis techniques (IBA) were performed to determine the elemental stoichiometry of superconducting samples of type TlBa₂Ca_2−xSc_xCu₃O_9−δ, with 0 ? x ? 0.6, prepared via solid-state reaction technique. By combining particle induced X-ray emission (PIXE) with Rutherford backscattering spectrometry (RBS), the stoichiometry of the samples is determined. However, the oxygen content is obtained by using non-Rutherford backscattering cross-section at 3 MeV proton beam. Furthermore, the prepared samples were also characterized using X-ray powder diffraction (XRD) and electrical resistivity measurements. The X-ray data indicate that the partial substitution of Ca²⁺ by Sc³⁺ ions does not affect the tetragonal structure of Tl-1223 superconducting phase. The superconducting transition temperatures T_c, determined from electrical resistivity measurements, was found to be highly correlated to the Sc-content.     

Displacement per atom calculation in YBCO superconductors through Monte Carlo simulation

The results of the calculations of the displacements per atom distribution induced by the gamma irradiation up to 15 MeV on YBa₂Cu₃O_7−x superconducting slabs are presented. Firstly, a calculation procedure for the displacement cross sections and the displacement per atom distributions was applied using the Monte Carlo simulation through the MCNPX code system. Then, based on this algorithm, the displacement per atom in-depth distributions were calculated starting from the energy flux distributions obtained from the simulation process, taking into account the contribution from each atom, obtaining a predominance of the Cu-O₂ planar sites over yttrium and barium atoms and more specifically the oxygen atoms predominate at low energies and the copper atoms at higher energies. Finally, the linear correlation observed between the displacement per atom distributions and energy deposition profiles at each incident energy was analyzed.     

First-principles investigation of site preference and bonding properties of neutral H in 3C-SiC

SiC can be extensively used in a space environment and other extreme conditions because of its superiority on radiation tolerance. H (proton) that generally exists in the space has large effects on the structure and properties of space materials such as SiC. We have performed the first-principles calculations based on density functional theory to investigate the site preference and bonding properties of neutral H in 3C-SiC. Spin polarization effect for H is taken into account. We show that the supercell should be large enough to diminish the H-H interaction due to the periodic boundary condition. Based on a series of calculations with different exchange-correlation schemes and potentials, we are able to determine the relative stability for different H configurations in SiC. The AB_C (anti-bond of C) configuration is shown to be the most energetically favorable, while the BC (bond center) and T_Si (tetrahedral interstitial of Si) configurations are less stable. We demonstrate that H prefers to form a stronger bond with C rather than Si, particularly in the BC configuration, in contrast to the previous study. Our results will provide a useful reference to the application of 3C-SiC in a space environment.     

Proton beam induced luminescence of silicon dioxide implanted with silicon

Light emission from a silicon dioxide layer enriched with silicon has been studied. Samples used had structures made on thermally oxidized silicon substrate wafers. Excess silicon atoms were introduced into a 250-nm-thick silicon dioxide layer via implantation of 60 keV Si⁺ ions up to a fluence of 2 × 10¹⁷ cm⁻². A 15-nm-thick Au layer was used as a top semitransparent electrode. Continuous blue light emission was observed under DC polarization of the structure at 8-12 MV/cm. The blue light emission from the structures was also observed in an ionoluminescence experiment, in which the light emission was caused by irradiation with a H₂⁺ ion beam of energy between 22 and 100 keV. In the case of H₂⁺, on entering the material the ions dissociated into two protons, each carrying on average half of the incident ion energy. The spectra of the emitted light and the dependence of ionoluminescence on proton energy were analyzed and the results were correlated with the concentration profile of implanted silicon atoms.     

Morphological change in FePt nanogranular thin films induced by swift heavy ion irradiation

We have investigated morphology change of FePt nanogranular films (FePt)₄₇(Al₂O₃)₅₃ under irradiation with 210 MeV Xe ions. Here, electron tomography technique was extensively employed to clarify three-dimensional (3D) structure in irradiated specimens, in addition to conventional transmission electron microscopy (TEM) techniques such as bright-field observation and scanning TEM energy dispersive X-ray spectroscopy (STEM-EDX) analysis. The ion irradiation induces the coarsening of FePt nanoparticles with elongation along the beam direction. Electron tomography 3D reconstructed images clearly demonstrated that when the fluence achieves 5.0 × 10¹⁴ ions/cm², well-coarsened FePt balls have been formed on the irradiated surface, and the particles in the film interior have been deformed into rods along the ion trajectory. The alloy particles become inhomogeneous in composition after prolonged irradiation up to 1.0 × 10¹⁵ Xe ions/cm². The particle center is enriched with Pt, while Fe is slightly redistributed to the periphery.     

Effects of Fe-He potential on primary damage formation in Fe-1%He

The effects of different Fe-He interatomic potentials on primary damage formation in Fe-1%He are investigated using molecular dynamics (MD) methods. Simulations of cascades produced by primary knock-on atoms (PKA) of energy E_p = 0.5-10 keV were performed at an irradiation temperature of 100 K. It is found that the Fe-He potentials have significant effects on the point defect creation and the formation of Fe-He interstitial clusters, whereas small effects on the formation of He-vacancy clusters.     

Sputtering of clusters from copper-gold alloys

Polycrystalline Cu, Cu₂₀Au₈₀, Cu₄₀Au₆₀, Cu₈₀Au₂₀ and Au samples were bombarded with 15 keV Ar⁺, and the resulting secondary neutral yield distribution was studied by non-resonant laser post-ionisation mass spectrometry. Neutral clusters containing up to 15 atoms were observed for the targets. The yield of neutral clusters, Cu_mAu_n−m, containing n atoms, Y_n, was found to follow a power in n, i.e. Y_n∝n^-δ, where the exponent δ varied from 5.2 to 10.1. For a fixed n, the cluster yields showed a variation with number of copper atoms, m, much greater than expected for a binomial distribution suggesting that the clusters are not formed randomly above the surface and a component of preformed cluster emission occurs. In addition, the cluster compositions from the sputtered alloys were indicative of sputtering from a copper rich surface.     

Study on effects of swift heavy ion irradiation on the crystal structure in CeO2 doped with Gd2O3

To simulate the effects of Gd₂O₃-doping and high-energy fission products in UO₂, Gd₂O₃-doped CeO₂ pellets were irradiated with 200-MeV Xe¹⁴⁺ ions. Doping and irradiation effects were analyzed using X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS). The lattice constant of CeO₂ decreases and the local structure is disordered with increased doping levels. However, the irradiation induces an expansion of the lattice and a disordering of atomic arrangement near the Gd atoms. The effects of the irradiation become more pronounced with increasing Gd₂O₃-dopant levels. Our results are compared with those of a study involving Er₂O₃-doped CeO₂.     

Computer simulation of high-energy-beam irradiation of single-crystalline silicon

The structural relaxation caused by the high-energy-beam irradiation of single-crystalline silicon was simulated by the molecular dynamics method. As the initial condition, high thermal energy was supplied to the individual silicon atoms within a cylindrical region of nanometer-order radius located in the center of the specimen. The Tersoff potential was assumed as the interaction mechanism between silicon atoms. The supplied thermal energy was first spent to change the crystal structure into an amorphous one within a short period of about 0.3 ps; then it diffused in the specimen by an ordinary thermal dissipation process. The amorphized track radius R_a was determined as a function of the energy density of the thermalized region. It was found that the relationship between R_a and the effective stopping power gS_e follows the relation , which is similar to the formula derived on the basis of the thermal spike model [G. Szenes, J. Nucl. Mater. 336 (2005) 81]. It was also found that the mechanism of structural transition changes at the thermalized region of 1 nm radius.     

Interdiffusion of polydisperse Mw distributions subsequent to proton irradiation of PS melts

Under exposure to ionizing radiation the molecular weights of polymeric matter changes via formation of crosslinks and scissions of chains. As a result the molecular weight (M_w) distribution of a polymer depends on the applied ion fluence and differs from the initial one, which in our case is of the Schulz-Zimm type. Statistical theories can be used to estimate the M_w distribution.We study M_w distributions in proton irradiated polystyrene (PS) thin films by diffusion measurements. As chains of different degrees of polymerization diffuse unequally fast diffusion depth profiles contain information about the involved M_w distribution. Both irradiation and analysis with nuclear reaction analysis (NRA) were made at the Freiburg van de Graaff accelerator.As comparison gel permeation chromatography (GPC) measurements were conducted. However, the use of GPC is limited by the minimal amount of material needed and also does not work for very short chains in our case.Here we concentrate on the M_w distributions, the measurements and the fitting procedure will be presented elsewhere.     

Study of the magnetic modifications of Fe3O4 ferrite thin films induced by 2.03 GeV Kr ions irradiation

The effect of 2.03 GeV Kr²⁶⁺ ions irradiation on antiphase boundaries (APBs) of polycrystalline Fe₃O₄ ferrite thin films has been investigated. The structure, magnetic and electrical transport properties of samples were characterized. The initial crystallographic structure of the Fe₃O₄ remains unaffected after Kr-ion irradiation, but the magnetic and electrical transports properties are sensitive to swift heavy ions (SHI) irradiation and exhibit different behaviors depending on the Kr-ion fluence range. The energy deposition is mainly due to the electronic energy loss (S_e) and the large value of energy transferred induces an unusual density of defects, stress and heat annealing effect in the samples, which can affect on the arrangement of magnetic moments and APBs density strongly. On the basis of our observations we conclude that the production, accumulation and free of the defects and stress induced by SHI irradiation is more dominant in the case of the magnetic and electrical transport properties modifications of the Fe₃O₄ thin films.     

Structural-phase transformations in near-surface layers of alumina-zirconium ceramics induced by low-energy high-current electron beams

The effect of low-energy high-current electron beams (LEHCEBs) at E = 15 keV on mechanical characteristics of near-surface layers of alumina-zirconium ceramics (AZC) of the 20Al₂O₃-80ZrO₂(Y) composition (in mass %) is investigated by the method of dynamic indentation. It is shown that electron-beam treatment reduces Young’s modulus and nanohardness of this ceramic material. The action of LEHCEBs on the structural-phase state of modified subsurface layers of AZC specimens is analyzed. Their elemental and phase compositions are examined using several techniques: electron-probe X-ray spectral microanalysis (EPMA), secondary-ion mass spectrometry (SIMS), and X-ray diffractometry. It is found out that irradiation of the ceramic material by LEHCEBs stimulates the transitions of an m-phase of zirconium dioxide into a tetragonal modification, and results in a considerable decrease in the alumina phase. Based on the analysis of the data obtained, a conclusion is drawn that it is the processes resulting from irradiation and giving rise to formation of a subsurface layer with a phase composition different from that of the bulk material, which are responsible for the effect of modification of the material mechanical property.