Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 2002"

Morphology of ion-sputtered surfaces

Abstract: We derive a stochastic nonlinear continuum equation to describe the morphological evolution of amorphous surfaces eroded by ion bombardment. Starting from Sigmunds theory of sputter erosion, we calculate the coefficients appearing in the continuum equation in terms of the physical parameters characterizing the sputtering process. We analyze the morphological features predicted by the continuum theory, comparing them with the experimentally reported morphologies. We show that for short time scales, where the effect of nonlinear terms is negligible, the continuum theory predicts ripple formation. We demonstrate that in addition to relaxation by thermal surface diffusion, the sputtering process can also contribute to the smoothing mechanisms shaping the surface morphology. We explicitly calculate an effective surface diffusion constant characterizing this smoothing effect and show that it is responsible for the low temperature ripple formation observed in various experiments. At long time scales the nonlinear terms dominate the evolution of the surface morphology. The nonlinear terms lead to the stabilization of the ripple wavelength and we show that, depending on the experimental parameters, such as angle of incidence and ion energy, different morphologies can be observed: asymptotically, sputter eroded surfaces could undergo kinetic roughening, or can display novel ordered structures with rotated ripples. Finally, we discuss in detail the existing experimental support for the proposed theory and uncover novel features of the surface morphology and evolution, that could be directly tested experimentally. � 2002 Published by Elsevier Science B.V.

Gamma-ray attenuation coefficients in bismuth borate glasses

Abstract: Mass attenuation coefficients of glasses in the system: xBi2O3(1−x)B2O3 (x=0.30, 0.35, 0.40, 0.45 and 0.55) were determined at 356, 662, 1173 and 1332 keV photon energies using a narrow beam transmission method. Appreciable variations were observed in these coefficients due to changes in the chemical composition of glasses. These coefficients were then used to determine effective atomic numbers of glass samples, which were found to be constant with bismuth concentration and energy.

Ion beam analysis of rough thin films

Abstract: The influence of surface roughness on Rutherford backscattering spectroscopy (RBS) spectra has been studied experimentally and by computer simulation with the SIMNRA code Rough thin films are described by a distribution of film thicknesses, while rough substrates are approximated by a distribution of local inclination angles Correlation effects of surface roughness are neglected Rough film effects can be calculated for RBS including non-Rutherford scattering, nuclear reaction analysis and elastic recoil detection analysis The results of simulation calculations show good agreement with experimental data For thin films of high Z elements on rough substrates additionally plural scattering plays an important role

Use of radiation in biomaterials science

Abstract: Radiation is widely used in the biomaterials science for surface modification, sterilization and to improve bulk properties. Radiation is also used to design of biochips, and in situ photopolymerizable of bioadhesives. The energy sources most commonly used in the irradiation of biomaterials are high-energy electrons, gamma radiation, ultraviolet (UV) and visible light. Surface modification involves placement of selective chemical moieties on the surface of a material by chemical reactions to improve biointeraction for cell adhesion and proliferation, hemocompatibility and water absorption. The exposure of a polymer to radiation, especially ionizing radiation, can lead to chain scission or crosslinking with changes in bulk and surface properties. Sterilization by irradiation is designed to inactivate most pathogens from the surface of biomedical devices. An overview of the use of gamma and UV radiation to improve surface tissue compatibility, bulk properties and surface properties for wear resistance, formation of hydrogels and curing dental sealants and bone adhesives is presented. Gamma and vacuum ultraviolet (VUV) irradiated ultrahigh molecular weight polyethylene (UHMWPE) exhibit improvement in surface modulus and hardness. The surface modulus and hardness of UHMWPE showed a dependence on type of radiation, dosage and processing. VUV surface modified e-PTFE vascular grafts exhibit increases in hydrophilicity and improvement towards adhesion of fibrin glue.

Binary theory of electronic stopping

Abstract: Binary stopping theory has been developed to characterize the electronic stopping of swift heavy ions in matter. It is an extension of Bohr’s classical theory of 1913 incorporating screening, higher-order-Z1 and shell corrections, high-speed quantum and relativity corrections as well as projectile excitation and ionization. The main numerical input comes from optical properties. The computation of shell corrections involves orbital velocity distributions of target and projectile electrons. Calculated stopping parameters depend on ion charge. Equilibrium stopping forces may be computed by adoption of a suitable model for the equilibrium charge state. This paper summarizes the current status of the theory, in particular the sensitivity of its predictions to pertinent input. Charge-dependent stopping forces have been calculated for selected systems and compared to experimental results. Equilibrium stopping forces calculated for a wide variety of ion–target combinations are compared with experimental data from the literature spanning over 6 decades in ion energy.

On the conflicting roles of ionizing radiation in ceramics

Abstract: Ionizing radiation can produce competing effects in ceramic materials. It is well established that ionizing radiation can produce displacement damage via radiolysis in alkali halides and some other ceramics. At high stopping powers (electronic d E /d x >5–50 keV/nm, depending on the material), additional displacement damage via inelastic collision processes can also be created in the vicinity of the ion track (swift-heavy-ion displacement damage). On the other hand, ionizing radiation can promote the recovery of displacement damage in many ceramic insulators by enhancing the mobility of point defects (ionization-induced diffusion). Therefore, under different irradiation conditions (electronic stopping powers), ionizing radiation can lead to either a substantial enhancement or suppression of radiation resistance in ceramics. The microstructures of SiC, Al 2 O 3 , MgO, MgAl 2 O 4 , Si 3 N 4 and AlN were examined by transmission electron microscopy (TEM) following irradiation with ion beams ranging from 1 MeV H + to 710 MeV Bi + . The oxides and Si 3 N 4 were found to be susceptible to ionization-induced diffusion. In these materials, high fluxes of ionizing radiation produced coarsening of dislocation loops and cavities, and inhibited low-temperature amorphization. At high electronic stopping powers, displacement damage was produced in the ion tracks in the oxides and Si 3 N 4 that could not be attributed to normal elastic collision processes. The amorphous ion track diameter in Si 3 N 4 associated with 710 MeV Bi ion irradiation was 3.5 nm. AlN and SiC were resistant to swift-heavy-ion-track displacement damage up to electronic stopping powers of 34 keV/nm.

Stability of polymers under ionising radiation: The many faces of radiation interactions with polymers

Abstract: Ionising radiations have dramatic effects on the properties of polymers commonly used in hard radiation environments. From the early radiolysis studies, polymers have been classified into those undergoing mainly chain scissions and those being cross-linked. Real situations are however far more complex due to the semi-crystalline organisation of most polymers implying transition temperatures (glass transition, fusion temperature of the crystalline fraction), oxygen diffusion driven formation of oxidised species and effects of the polymer formulation (anti-oxidants, influence of the fillers). Progress has been made in the understanding of such phenomena and different examples have been considered illustrating the different faces of the polymer evolution under ionising radiation. Particular emphasis has been given to the changes of the final polymer properties in their conditions of use. Two examples of materials of widespread importance in nuclear technology: elastomers and epoxy resins, will be discussed.

Relation between microscopic defects and macroscopic changes in silicon detector properties after hadron irradiation

Abstract: Silicon detectors produced from materials with different resistivities and oxygen concentrations have been irradiated with energetic neutrons, protons and pions. Isothermal annealing studies have shown correlation between microscopic defect evolution and the macroscopic detector performance. It was found that the annealing behavior of the electron traps attributed to the single and double charged divacancy is strongly related to the current related damage parameter α. In both cases the isothermal evolution is independent of the oxygen and doping concentration in the material under investigation ( 2×10 14 O ] 18 cm −3 and 10 12 P ] 13 cm −3 ) and the absolute values do not depend on the particles used for the irradiation provided the fluence is properly normalized by the nonionizing energy loss (NIEL). In contrast to this result the introduction rates of the observed point defects VOi and CiCs were however found to depend on the particle type. Thus clear indication is given that the generation of point defects does not scale with NIEL. Compared to neutron irradiated samples the introduction rate after irradiation with charged hadrons was found to be higher by a factor around 2.

Defect Production, Multiple Ion-Solid Interactions and Amorphization in SiC

Abstract: Recent progress in the atomic-scale simulations of fundamental damage production processes in SiC is reviewed, which includes the displacement threshold energy surface, the primary damage state and statistics of defect production, multiple ion–solid collision events and structural evolution in SiC. The threshold energy surface, E d , appears to be highly anisotropic, and the results of molecular dynamics (MD) simulations, in conjunction with experimental studies, suggest that E d values of 20 eV for C and 35 eV for Si should be used in Kinchin–Pease calculations. The Si displacement cascades with energies up to 50 keV show that the surviving defects are dominated by C interstitials and vacancies, consistent with experimental observations. The defect production efficiency decreases with increasing recoil energy, but the number and size of clusters or complex domains formed at the end of cascades are very small, independent of cascade energy. A large number of 10 keV displacement cascades were randomly generated in a model crystal to simulate multiple ion–solid interaction and damage accumulation. The coalescence of clusters represents an important mechanism leading to the complete amorphization of SiC, and the relative disorder and swelling behavior show an excellent agreement with experimental observations. HRTEM images simulated from the MD cell reveal the microstructural evolution of multiple ion–solid collision events, and provide atomic-level interpretations of experimentally observed features in SiC.

Ion-irradiation-induced defects in bundles of carbon nanotubes

Abstract: We study the structure and formation yields of atomic-scale defects produced by low-dose Ar ion irradiation in bundles of single-wall carbon nanotubes. For this, we employ empirical potential molecular dynamics and simulate ion impact events over an energy range of 100–1000 eV. We show that the most common defects produced at all energies are vacancies on nanotube walls, which at low temperatures are metastable but long-lived defects. We further calculate the spatial distribution of the defects, which proved to be highly non-uniform. We also show that ion irradiation gives rise to the formations of inter-tube covalent bonds mediated by carbon recoils and nanotube lattice distortions due to dangling bond saturation. The number of inter-tube links, as well as the overall damage, linearly grows with the energy of incident ions.

Implantation studies of keV positive muons in thin metallic layers

Abstract: The implantation of low energy polarized positive muons (μ+) with energies between 0.5 and 30 keV in thin (<100 nm) Al, Cu, Ag, and Au films sputtered onto quartz glass substrates has been studied by using a muon spin rotation technique. The fraction of muons stopped in the metal was determined by measuring the energy dependence of the μ+ decay asymmetry which is essentially proportional to the fraction of μ+ stopping in the metal layer. From this quantity also the backscattering probability from the metallic layer can be obtained. The results are compared with predictions of implantation profiles of muons obtained with the TRIM.SP and SRIM2000 Monte Carlo codes. The applicability of these simulations to predict the interaction of low energy muons in matter and observed differences with the experimental data are discussed.

Scintillation properties of (C6H13NH3)2PbI4 : Exciton luminescence of an organic/inorganic multiple quantum well structure compound induced by 2.0 MeV protons

Abstract: We report a new type of scintillator especially suitable for pulse-radiation detection Thin films of organic/inorganic perovskite compound (n-C6H13NH3)2PbI4, which is characterized by a multiple quantum well structure, were bombarded by 20 MeV protons, and their radiation-induced emission spectra were obtained A single and sharp emission peak due to an exciton was observed at the wavelength of 524 nm This emission was clearly detected even at room temperature, and its quantum efficiency was very high The line shape of this emission did not change, retaining its sharpness, and no other emissions appeared throughout the irradiation The optical response of (n-C6H13NH3)2PbI4 is very fast (n-C6H13NH3)2PbI4 is a promising scintillator material, meeting requirements not satisfied by conventional scintillators

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements

Abstract: Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference material. The positron lifetime technique has been used to detect the presence of positron traps in all the samples. The change in the highmomentum part of the annihilation line due to positron trapping is illustrated. The measured data are in a good qualitative agreement with recent theoretical calculation and constitute the most complete measurement series, up to now, to establish a future data-base for positron annihilation spectroscopy. 2002 Elsevier Science B.V. All rights reserved.

Transmission electron microscopy observation on irradiation-induced microstructural evolution in high burn-up UO2 disk fuel

Abstract: In order to identify the conditions of the rim structure formation as a function of burn-up and temperature, and to clarify the formation mechanism of this restructuring, UO2 fuel disks were irradiated at four thermal conditions, between 400 and 1300 °C, and at four different burn-ups, between 36 and 96 MWd/kgU, without external mechanical constraint. The microstructural evolutions as a function of the irradiation parameters are observed by high resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM observations reveal the transition from original to sub-divided grains of rim structure and make clear that the burn-up threshold is between 55 and 82 MWd/kgU. The temperature threshold of this restructuring could be 1100±100 °C. Moreover, polyhedral sub-divided-grains with size ranging between 0.5 and 2 μm, not only rounded grains in the size range 150–350 nm, are clearly observed. These configurations are explained by assuming that the grain sub-divisions occurred homogeneously within the original polyhedral grains, while the existence of rounded grains might be due to free surface effects. TEM observations of re-structured samples show that most of sub-grain boundaries are low angle and are heavily decorated by fission gas bubbles in the range 3.5–8 nm. In the non-restructured samples, dislocations and small precipitates are present, and many of the bubbles form “strings” along dislocation lines. In specimens irradiated at high temperature, many dislocations seem to be anchored by fission product precipitates. These results suggest that the formation mechanism of the restructuring is based on polygonization, and the precipitates could have some “pinning effect” on dislocations and defect clusters.

Empirical potential approach for defect properties in 3C-SiC

Abstract: Defect energetics in silicon carbide (SiC) have been widely studied using Tersoff potentials, but these potentials do not provide a good description of interstitial properties. In the present work, an empirical many-body interatomic potential is developed by fitting to various equilibrium properties and stable defect configurations in bulk SiC, using a lattice relaxation fitting approach. This parameterized potential has been used to calculate defect formation energies and to determine the most stable configurations for interstitials using the molecular dynamics method. Although the formation energies of vacancies are smaller than those obtained by ab initio calculations, the properties of antisite defects and interstitials are in good agreement with the results calculated by ab initio methods. It is found that the most favorable configurations for C interstitials are 〈1 0 0〉 and 〈1 1 0〉 dumbbells on both Si and C sites, with formation energies from 3.04 to 3.95 eV. The most favorable Si interstitial is the tetrahedral interstitial site, surrounded by four C atoms, with a formation energy of 3.97 eV. The present results will be discussed and compared to those obtained by others using various empirical potentials in SiC.

Investigation of the natMo(p,x) nuclear reaction to monitor proton beams: New measurements and consequences on the earlier reported data

Abstract: The excitation function of the natMo(p,x) 96 mg Tc monitor reaction has been measured up to 38 MeV to verify published experimental cross-section data sets measured earlier by using this monitor reaction. The activation method and the stacked foil technique using high-resolution HpGe gamma spectrometry were applied to determine the excitation function. Reliable data sets were produced with the help of simultaneous measurement of the excitation functions of natTi(p,x)48V, natNi(p,x)57Ni, natCu(p,x)62Zn, natCu(p,x)63Zn and natCu(p,x)65Zn monitor reactions in the whole investigated energy range. The new cross-section values indicate the necessity of normalisation the excitation functions of about 250 of proton induced nuclear reactions by a factor of 0.8 measured earlier by Levkovski using the natMo(p,x)96mgTc process as monitor reaction.

Experimental studies of heavy-ion slowing down in matter

Abstract: Measurements of heavy-ion slowing down in matter differ in many aspects from experiments with light particles like protons and α-particles. An overview of the special experimental requirements, methods, data analysis and interpretation is presented for heavy-ion stopping powers, energy- and angular-straggling and ranges in the energy domain from keV/u up to GeV/u. Characteristic experimental results are presented and compared with theory and semiempirical predictions. New applications are outlined, which represent a challenge to continuously improve the knowledge of heavy-ion slowing down.

Structural relaxation in amorphous silicon carbide

Abstract: High purity single crystal and chemically vapor deposited (CVD) silicon carbide have been amorphized under fast neutron irradiation. The gradual transition in physical properties from the as-amorphized state to a more relaxed amorphous state prior to crystallization is studied. For the three bulk properties studied: density, electrical resistivity, and thermal conductivity, large property changes occur upon annealing between the amorphization temperature and the point of crystallization. These physical property changes occur in the absence of crystallization and are attributed to short and perhaps medium range ordering during annealing. It is demonstrated that the physical properties of amorphous SiC (a-SiC) can vary greatly and are likely a function of the irradiation state producing the amorphization. The initiation of crystallization as measured using bulk density and in situ TEM is found to be ∼875 °C, though the kinetics of crystallization above this point are seen to depend on the technique used. It is speculated that in situ TEM and other thin-film approaches to study crystallization of amorphous SiC are flawed due to thin-film effects.

Fabrication of a new generation of track-etched templates and their use for the synthesis of metallic and organic nanostructures

Abstract: This paper reports on the realisation of nanoporous supported templates by the track-etching method. Spin-coated polycarbonate films have been irradiated by energetic heavy ions (Ar9+, 220 MeV), UV irradiated and chemically etched in aqueous NaOH (2 N, 70 degreesC). The thickness of the supported templates has been varied between 200 nm and a few microns and the pore size between 15 and 100 nm. Dynamics of pore formation in these supported templates has been compared to dynamics of pore: formation in classical self-supported track-etched membranes. This new generation of templates has been successfully used to electrochemically synthesise polypyrrole nanotubes and copper (Cu) nanowires. The pore size and the nanostructure morphology, providing information on the pore shape, have been studied by high resolution electron microscopy. (C) 2002 Elsevier Science B.V. All rights reserved.

The status of theoretical L-subshell ionization cross sections for protons

Abstract: Accuracy of PIXE analyses depends on inner-shell ionization cross sections. These cross sections are often calculated in the ECPSSR theory; results of its variant (that uses DHS wavefunctions) are employed in the GUPIX package. While the ECPSSR theory agrees well with massive compilations of K x-ray production, it deviates systematically from Lsubshell data – in particular, L1 and L2 cross sections. Various modifications of the ECPSSR theory and their results are reviewed versus the empirical database from Orlic for 0.3–3.5 MeV protons on 45 6 Z2 6 92 targets. Comparison with this database does not give a clear choice between the ECPSSR and the ECPSSR with all modifications. Independently of the chosen theory and because of the increasing scatter in the data with the decreasing proton energy, PIXE packages that may rely on reference cross sections for L1 x-ray production in lanthanides by 0.2-MeV protons could err by an order of magnitude in the determination of the concentration of these elements. 2002 Elsevier Science B.V. All rights reserved.

Quantitative analysis of untreated hair samples for monitoring human exposure to heavy metals

Abstract: The method of quantitative analysis for untreated hair samples, which we developed three years ago, has proved to be quite useful for investigating environments contaminated by certain toxic elements. In the present work, the experimental conditions are improved. Loss of certain elements owing to irradiation damage, which has remained as one of the experimental uncertainties, was examined. It was found that the concentration of sulfur decreases gradually throughout the irradiation, while for the other elements, including arsenic and mercury, no changes occur under our measuring conditions. Furthermore, the degree of alteration of elemental concentration depending on the position along the hair was investigated. As a result, concentrations of some elements at different positions on a 14-cm-length hair, which was taken from a small-scale miner in the Philippines, showed some dependence on the distance from the root reflecting her history as a miner, while mercury does not show large deviation from a main trend. It was also found that concentration of mercury in hairs taken from different parts of a body does not show large difference. These results demonstrate that mercury and arsenic concentration in hairs, obtained by the present method, become a good index for an estimation of human exposure to these toxic elements. Changes of concentration of some elements depending on the way of cleaning before irradiation are studied in detail and the optimum way of washing is established.

Cold ion-cutting of hydrogen implanted Si

Abstract: The strength of the H-implanted layer has been measured in 〈1 0 0〉 , 〈1 1 1〉 and 〈1 1 0〉 oriented Si wafers using the crack opening method. The required annealing temperature for mechanical layer transfer increases in the order 〈1 0 0〉 , 〈1 1 1〉 and 〈1 1 0〉 . The damage induced by the implantation has been studied by Rutherford backscattering in the channeling mode (RBS/C). The same methods have been used to investigate the influence of boron and arsenic doping on the mechanical exfoliation. Boron doping reduces the strength of the H-implanted layer thereby enabling mechanical layer transfer at temperatures below 200 °C. We found that the exfoliation takes place closer to the wafer surface in highly boron doped Si as compared to the undoped Si. The RBS damage peak also appears to move closer to the surface when the boron concentration of the H-implanted layer is >1019 cm−3. No lowering of the exfoliation temperature was observed for compensated and arsenic doped Si layers. We suggest that the lowering of the exfoliation temperature with increasing boron doping is related to Si–H bonds associated with the neutralization of shallow acceptors by hydrogen.

Quantitative, high sensitivity, high resolution, nuclear microprobe imaging of fluids, melts and minerals

Abstract: Samples of fluids and melts trapped and preserved as inclusions in growing minerals or healed fractures provide unique windows on a range of geological processes from mantle melting and metasomatism through to economic ore formation and remobilization. Recent advances in nuclear microprobe (NMP) technology at the CSIRO provide powerful tools for the study of these inclusions and associated mineral assemblages. These tools include a new NMP designed for high resolution and high sensitivity, PIXE analytical methods for quantitative imaging and analysis, and simultaneous PIGE imaging. The quantitative imaging and analysis methods are based on the dynamic analysis approach, which generates a fast matrix transform for projection of list-mode PIXE data onto pure elemental images. Recent advances provide rapid pixel-by-pixel correction for matrix and absorption effects in different (mineral) compositions across the image area to yield true quantitative images. These methods are combined in a software package called GeoPIXE II.

Study of chemical, optical and thermal modifications induced by 100 MeV silicon ions in a polycarbonate film

Abstract: A thin film of polycarbonate was bombarded with 100 MeV Si 8+ ions Fourier transform infrared and ultraviolet/visible spectroscopic techniques were employed for studying the changes in chemical and optical properties whereas differential scanning calorimetry was used for studying the changes in the thermal properties It was observed that there was a slight shift in the optical absorption edge towards the red end of the spectrum when the ion fluence was increased It was further observed that there was a substantial chemical and thermal modification in the sample such as breaking of CO single bond and formation of phenolic bond and gradual decrease in the glass transition temperature with the increase in ion fluence

Micro-PIXE in plant sciences: Present status and perspectives

Abstract: Fundamental processes of plant physiology are affected or regulated by mineral nutrients. Hence understanding the mechanisms of nutrient uptake and their functions in plant metabolism is of fundamental importance in both basic and applied plant studies. The present knowledge of ion uptake mechanisms is based mostly on techniques for bulk analysis, including analysis of small (mg-sized) samples but without spatially resolved results. On the other hand, advanced studies of elemental transport at a cellular level are conducted using techniques with high and very high spatial resolution, but with low sensitivity for elemental analysis. Thus the results obtained are usually restricted to macronutrients or elements present in high quantities. There is a high demand for studies of the functions of trace elements. In addition, it is known that, depending on their concentrations, elements can play different roles in plant life. Studies related to elemental deficiency and toxicity, as well as environmental pollution, require accurate, fully quantitative methods with good spatial resolution. Ideally, these studies should be conducted on organs and tissues as far down as the cellular level. This is where micro-PIXE has been applied until present and can in the near future play a much more important role. Progress is subject to closer collaboration between plant biologists and the PIXE community in terms of addressing problems of specimen preparation, refinement of analytical protocols such as quantitative elemental mapping and the interpretation of results.

Positronium molecule formation, Bose–Einstein condensation and stimulated annihilation

Abstract: Low energy positron production and storage may be used to make and observe some interesting many-positron, many-electron systems both in vacuum and in the presence of ordinary matter. One of the most interesting possibilities is to make an annihilation photon laser. One might start on the path to this laser by creating a high density positron burst and forming a dense gas of positronium atoms within a cavity in a solid. This will pave the way for subsequently demonstrating the near room temperature Bose–Einstein condensation of a dense gas of positronium and establishing conditions under which stimulated emission of annihilation photons can be observed.

Modeling chemical and topological disorder in irradiation-amorphized silicon carbide

Abstract: In order to explore the relationship of chemical disorder to topological disorder during irradiation-induced amorphization of silicon carbide, a topological analysis of homonuclear bond distribution, atom coordination number and network ring size distribution has been carried out for imposed simulated disorder, equilibrated with molecular dynamics (MD) procedures utilizing a Tersoff potential. Starting configurations included random atom positions, β-SiC coordinates chemically disordered over a range of chemical disorder parameters and atom coordinates generated from earlier MD simulations of embedded collision cascades. For random starting positions in embedded simulations, the MD refinement converged to an average Si coordination of 4.3 and an average of 1.4 Si–Si and 1.0 C–C bonds per Si and C site respectively. A chemical disorder threshold was observed (χ≡NC–C/NSi–C>0.3–0.4), below which range MD equilibration resulted in crystalline behavior at all temperatures and above which a glass transition was observed. It was thus concluded that amorphization is driven by a critical concentration of homonuclear bonds. About 80% of the density change at amorphization was attributable to threshold chemical disorder, while significant topological changes occurred only for larger values of the chemical disorder parameter.

JAERI Takasaki in-air micro-PIXE system for various applications

Abstract: In JAERI Takasaki, an in-air micro-PIXE system has been developed. This system enables multi-elemental mapping of samples in atmospheric environment with spatial resolution of 1 μm. Various research programs, such as biomedical research, dental study, environmental science and geology, have been started in recent years. Several related techniques for these applications were developed. A FTP server has been operated for the remote collaborators to share experimental data over the internet. We have developed image processing methods for elemental concentration analysis and a new beam monitoring technique in thick target irradiation.

Can we measure the gravitational free fall of cold Rydberg state positronium

Abstract: In this paper we examine the possibilities for detecting the free fall of Rydberg positronium atoms. In our scheme, cold positronium atoms are emitted from a “point” source and excited to the n=25 circular Rydberg state with L=n−1. The positronium atoms are allowed to travel horizontally 10 m in a field free vacuum and focused onto a detector using an elliptical Van der Waals mirror. A free fall distance of order 50 μm and a few detected atoms per hour are anticipated. Various extraneous influences on the positronium, such as collisions with residual gas atoms, Stark mixing in stray electric and magnetic fields, photoionization due to thermal radiation, and accelerations due to patch potentials are estimated.

Characterisation of heavy metal pollutants of soils and sediments around a crude-oil production terminal using EDXRF

Abstract: The EDXRF technique was used to measure the concentration of heavy metals in soils, sediment and solid wastes (sludge and scales) around one of the major crude-oil production terminals south-south-west of the Niger Delta in Nigeria. Fourteen elements – K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Zr, Ba and Pb – were detected and their concentrations and enrichment factors determined. Very high enrichment factors were obtained for Sr, Zr, Pb, Ba and Fe in all the samples, and also for K, Ca and Mn in the solid waste samples. The high enrichment factors for heavy metals obtained in the soil and sediment samples show that there is a considerable heavy metal pollution which could be correlated with the crude-oil production industry. There is need for a close monitoring of the waste disposal in order to minimise environmental pollution.