Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 1998"

Radiation-induced point defects in simple oxides

Abstract: We present a survey of recent theoretical studies of radiation-induced point defects in simple oxides with emphasis on highly ionic MgO, partly-covalent corundum (Al2O3) and ferroelectric KNbO3. The atomic and electronic structure of the electronic (F a and F centers) and hole centers, as well as interstitial atoms therein are discussed in light of the available experimental data. Results for defect diAusion and photo-stimulated F a fi F center conversion are also ana

Amorphization of SiC under ion and neutron irradiation

Abstract: This paper presents results on the microstructure and physical properties of SiC amorphized by both ion and neutron irradiation. Specifically, 0.56 MeV Si ions have been implanted in single crystal 6H–SiC from ambient through >200°C and the critical threshold for amorphization was measured as a function of the irradiation temperature. From a high resolution transmission electron microscopy (HRTEM) study of the crystalline to amorphous transition region in these materials, elongated pockets of amorphous material oriented parallel to the free surface are observed. Single crystal 6H–SiC and hot pressed and sintered 6H and 3C SiC were neutron irradiated at approximately 70°C to a dose of ∼2.56 dpa causing complete amorphization. Property changes resulting from the crystal to amorphous transition in SiC include a density decrease of 10.8%, a hardness decrease from 38.7 to 21.0 GPa, and a decrease in elastic modulus from 528 to 292 GPa. Recrystallization of the amorphized, single crystal 6H–SiC appears to occur in two stages. In the temperature range of ∼800–1000°C, crystallites nucleate and slowly grow. In the temperature range of 1125–1150°C spontaneous nucleation and rapid growth of crystallites occur. It is further noted that amorphized 6H (alpha) SiC recrystallizes to highly faulted fcc (beta) SiC.

Ion-beam induced damage and annealing behaviour in SiC

Abstract: The paper presents the damage accumulation in silicon carbide (SiC) as a function of the ion mass, the ion energy and the implantation temperature. A defect-interaction and amorphization model is used to analyse the dose dependence of defect production, as obtained by the various methods. The temperature dependence of the amorphization dose can be represented assuming a thermally enhanced annealing within the primary collision cascades. On the basis of such a model, a critical implantation temperature is obtained, which was found to vary with the ion mass and the implantation energy. The concurrent influence of implantation temperature and ion fluence on the resulting damage distribution in SiC is demonstrated. The damage annealing of ion implanted SiC is investigated for low, medium and high damage concentrations. The effect of the implantation temperature and the concentration of implanted atoms, both influencing the kind of defects obtained after implantation, on the annealing behaviour is analysed.

Charge states of relativistic heavy ions in matter

Abstract: Experimental and theoretical results on charge-exchange cross-sections and charge-state distributions of relativistic heavy ions penetrating through matter are presented. The data were taken at the Lawrence Berkeley Laboratory’s BEVALAC accelerator and at the heavy-ion synchrotron SIS of GSI in Darmstadt in the energy range 80‐1000 MeV/u. Beams from Xe to U impinging on solid and gaseous targets between Be and U were used. Theoretical models for the charge-state evolution inside matter for a given initial charge state are presented. For this purpose, computer codes have been developed, which are briefly described. Examples are given which show the successes and limitations of the models. ” 1998 Elsevier Science B.V. All rights reserved.

Efficiency of thermoluminescent detectors to heavy charged particles

Abstract: The efficiency of TLD-700 thermoluminescent detectors is studied for various ions as a function of energy. A new model for calculation of efficiency has been developed, based on the detector response to reference radiation, and the radial dose distribution of heavy ions only. No free parameters have to be used to calculate the thermoluminescence detector (TLD) efficiency as a function of ion species and energy. Comparison between model calculations and experimental results will be presented.

The influence of surface roughness on the angular dependence of the sputter yield

Abstract: A new approach to study the influence of target surface roughness on sputter yields is explored. The method is based on the experimental determination of the surface topography and the subsequent evaluation of a distribution of local angles of incidence for the incident ions. This distribution is used as input to the Monte Carlo program TRIM.SP for the calculation of the sputter yield of the target with the rough surface. Additionally, the redeposition of sputtered target atoms on the rough surface is calculated. The resulting net sputter yields determined with this procedure are in satisfactory agreement with experimental data.

Transition radiation and diffraction radiation. Similarities and differences

Abstract: The characteristics of diffraction radiation (DR), i.e. radiation of the charged particle moving near conducting target have been considered for an ultrarelativistic case. The simple expressions for DR fields for the semi-infinite ideal conducting target have been derived. The close connection between transition radiation (TR) and DR has been shown. The effect of finite transversal sizes of target on TR characteristics has been evaluated.

Encapsulated semiconductor nanocrystals formed in insulators by ion beam synthesis

Abstract: Both elemental and compound semiconductor nanocrystals have been formed in insulators by ion beam synthesis. Si nanocrystals in SiO2 give rise to strong optical absorption and intense photoluminescence (PL). The dose dependence of optical absorption provides evidence for size dependent changes in the Si nanocrystal bandgap due to quantum confinement, but the PL results suggest that surface or defect states play an important role in PL. CdS and CdSe nanocrystals have been formed in SiO2 and in Al2O3. Their structure, size, and optical properties are discussed.

Evaluation of non-Rutherford proton elastic scattering cross-section for magnesium

Abstract: The experimental data available for magnesium (p,p) elastic scattering cross-section at angles and energies suitable for Ion Beam Analysis have been evaluated using the theoretical model approach together with additional measurements and benchmark experiments. The results obtained provide the evaluated differential cross-sections for magnesium (p,p) elastic scattering in the energy region up to 2.7 MeV.

Track size and track structure in polymer irradiated by heavy ions

Abstract: The structure of latent tracks in polyethylene terephthalate (PET) was studied using chemical etching combined with a conductometric technique. Polymer samples were irradiated with Ar, Kr, Xe, Au, and U ions with energies in the range of 1 to 11.6 MeV/u. The etching kinetics of the tracks was investigated in the radii range 0–100 nm. The highly damaged track core manifests itself on the etching curves as a zone where the etch rate changes dramatically and reaches its minimum at a radius of a few nm. It was found that the track core radius is approximately proportional to (dE/dx)0.55. The track core is surrounded by a halo. In the track halo the etching proceeds at a rate that slowly increases approaching a constant value. Cross linking of macromolecules causes reduction of the etch rate in the halo which extends up to distances exceeding 100 nm in the case of the heaviest ions. Measurable change of the etch rate at such large radii could not be predicted from the shape of the calculated spatial distributions of energy dissipated in tracks. Obviously, formation of the extended track halo is influenced by the diffusion of active intermediates from the track core to the polymer bulk.

Measurements of internal radioactive contamination in samples of Roman lead to be used in experiments on rare events

Abstract: Improved results of a series of measurements carried out on two diAerent types of Roman lead to be used in shields for experiments on rare events are reported. The chemical impurities in both samples have been determined by neutron activation. Underground measurements based on c-spectroscopy on large masses of the two types of lead, show the absence in both samples of radioactive contamination from 214 Bi and 232 Th, in secular equilibrium, with upper limits of a few tenths of a mBq kg ˇ1 . The contamination from 40 K is less than a few mBq kg ˇ1 . Much care has been addressed to the contamination due to 210 Pb which breaks secular equilibrium and which contributes to most of the background in experiments searching low energy events like direct interactions of Weakly Interacting Massive Particles (WIMPS). We have applied to this problem the technique of cryogenic detection and found for the two samples upper limits for contamination of 210 Pb of 4 and 7 mBq kg ˇ1 , the lowest ever determined for any type of lead. ” 1998 Published by Else

Latent track formation in silicon irradiated by 30 MeV fullerenes

Abstract: It is now well accepted that electronic excitation and ionisation arising from the slowing-down of swift heavy ions can lead to structural modifications in most targets. It is shown here that new effects take place during irradiations with high energy fullerene beams. Electron microscopy observations were performed at room temperature on prethinned monocrystalline silicon samples after irradiation with 30 MeV fullerene ions. The observed damage is continuous and confined around the projectile paths. The tracks consist of amorphous material as shown by high resolution transmission electron microscopy observations. These tracks recrystallize very rapidly in the electron microscope during the observations in high resolution conditions. Furthermore, a decrease of the track diameter is observed as the cluster ions penetrate deeper inside the target, which is related to an angular scattering of the cluster constituents. At large penetration depths, before disappearing completely, the tracks end as aligned damaged regions of decreasing diameters. Finally, strong sputtering effects occur on the target surfaces, so that craters are generated at the impacts of the projectiles. The fact that amorphous tracks are generated in crystalline silicon following heavy cluster bombardment can be attributed to the strong localization of the deposited energy during the slowing-down process of rather slow projectiles.

On the condensed history technique for electron transport

Abstract: In this report we discuss the theory of the “condensed history technique”, an approximate solution to the Boltzmann transport equation that sums the effect of up to thousands of discrete, small momentum transfer elastic and inelastic collisions into single larger-effect quasi-events. This technique saves much calculational effort at the expense of introducing errors that are now understood quantitatively in terms of the development presented herein. We apply our analysis to modern realizations of the condensed history method, namely those of EGS/PRESTA, ETRAN/TLC, FLUKA, PENELOPE, and LLCA. We have also constructed an algorithm that exhibits smaller large step size instabilities than all of these methods and give several examples.

Positron collisions with one- and two-electron atoms

Abstract: The current status of coupled-state results for positron scattering by atomic hydrogen and the alkali metals is briefly reviewed The extension of the coupled-state approximation to “two-electron” targets, ie, He and the alkaline earths, is outlined and new calculations for positron scattering by He are presented

Computer simulation of a 10 keV Si displacement cascade in SiC

Abstract: The threshold energy for atomic displacement and the evolution of high-energy displacement cascades in β -SiC have been examined using molecular dynamics simulations. A modified form of the Tersoff potential was used in combination with a repulsive potential obtained from density functional theory to describe the interactions between the atoms. The evolution of lattice damage in a 10 keV Si cascade was studied for about 10 ps. The system size varied from 8000 atoms for the displacement energy calculations to 192,000 atoms for the cascade simulations. The results indicate that the minimum displacement energy is about 36 eV for Si and 28 eV for C. The cascade lifetime was found to be of the order of 0.1 ps, and the surviving C vacancies and interstitials outnumbered the corresponding Si defects by a factor of about 3. These results are discussed in light of previous theoretical and experimental studies of radiation damage in SiC.

Electron beam facility in polymer research: radiation induced functionalization of polytetrafluoroethylene

Abstract: The main field of electron irradiation processing applications is polymeric material. A universal electron beam irradiation facility for research and service in the institute of polymer research was demonstrated. The industrial electron beam accelerator works in the medium energy range from 0.6 to 1.5 MeV. One of our research activities using the electron beam accelerator is the radiation-induced functionalization of polytetrafluoroethylene (PTFE). The changes in chemical structure of PTFE irradiated in ammonia, air, and vacuum were investigated by infrared spectrometry.

A review of mixing processes in Ta/Fe and Mo/Fe systems treated by high current electron beams

Abstract: The review deals with a brief analysis of results on mixing in Ta/Fe and Mo/Fe systems using high current electron beams. Theoretical models of mass-transfer, which the authors used for the interpretation of experimental results obtained after pulsed beam treatment, have been considered. The formation of amorphous phase, intermetalloid phases Fe2Ta, as well as the formation of high dislocation density up to 5×1010 cm−2 have been shown. A non-equilibrium metastable phase Fe4Mo(Fe80Mo20) and, possibly Fe96Mo4, is formed as a result of irradiation of Mo/Fe system. The formation of a surface complex structure and near surface layers result in an enhancement of corrosion resistance, an increased depth of the hardened layer and a higher resistance to brittle damaging.

Dynamics of viscoelastic flow in ion tracks: origin of plastic deformation of amorphous materials

Abstract: In the present paper, track formation and plastic deformation (creep and anisotropic growth) of amorphous solids under heavy ion bombardment are discussed in terms of viscoelastic shear stress relaxation in thermal spike regions and the subsequent freezing-in of the associated strain increments. The resulting strained tracks are considered to represent the (symmetry-breaking) mesoscopic defects responsible for anisotropic macroscopic deformation. Recent theoretical approaches to the problem are critically reviewed. A system of viscoelastic continuum equations is proposed which includes the recently used “effective flow temperature approach” as a limiting case. General solutions of this system are presented and discussed. These solutions, completed by a model for the ion beam induced reduction of the viscosity, appears to be suited to fully describe the deformation characteristics of amorphous materials including the threshold behaviour with respect to electronic stopping power and ion fluence (“incubation dose”) as well as the target temperature dependence. An evaluation of appropriate experimental studies on the basis of the presented theory is expected to provide valuable information on the (ion beam reduced) viscosity of amorphous materials around ambient and thermal spike temperatures.

Radiation Damage Effects in Cubic-Stabilized Zirconia Irradiated with 72 MeV I+ Ions.

Abstract: Cubic-stabilized zirconia single crystals were irradiated using 72 MeV I + ions in the TASSC accelerator facility at Chalk River Laboratory (to simulate a typical U or Pu fission fragment). Irradiations were performed over the fluence range 1 × 10 18 –5 × 10 19 ions/m 2 , at temperatures of 300, 770, and 1170 K. Damage accumulation was monitored using Rutherford Backscattering Spectroscopy and ion-channeling (RBS/C) techniques. At ambient temperature and at the highest I + fluence used in these experiments (5 × 10 19 I + /m 2 ), RBS/C measurements revealed a rather high degree of lattice disorder. Specifically, the dechanneling parameter χ min varied from 80% to greater than 90% over the depth probed by RBS/C (∼1 μm). Nano-indentation measurements on the same sample indicated decreases in elastic modulus, E , and hardness, H (both by about 9%). These results suggest that an alteration in structure beyond simple defect accumulation occurs under these irradiation conditions. However, transmission electronmicroscopy (TEM) observations and in particular microdiffraction measurements failed to reveal any structural transformations in the irradiated material.

On the representation of electron multiple elastic-scattering distributions for Monte Carlo calculations

Abstract: A new representation of elastic electron–nucleus (Coulomb) multiple-scattering distributions is developed. Using the screened Rutherford cross section with the Moliere screening parameter as an example, a simple analytic angular transformation of the Goudsmit–Saunderson multiple-scattering distribution accounts for most of the structure of the angular distribution leaving a residual 3-parameter (path-length, transformed angle and screening parameter) function that is reasonably slowly varying and suitable for rapid, accurate interpolation in a computer-intensive algorithm. The residual function is calculated numerically for a wide range of Moliere screening parameters and path-lengths suitable for use in a general-purpose condensed-history Monte Carlo code. Additionally, techniques are developed that allow the distributions to be scaled to account for energy loss. This new representation allows “on-the-fly” sampling of Goudsmit–Saunderson angular distributions in a screened Rutherford approximation suitable for Class II condensed-history Monte Carlo codes.

Radiation induced paramagnetic centres in nuclear glasses by EPR spectroscopy

Abstract: An electron paramagenetic resonance (EPR) investigation of a series of glasses irradiated at room temperature with β or X radiation sources has been made in order to predict the long term behavior of glasses used in the nuclear waste disposal. EPR spectra of the irradiated glasses have been measured and treated using computer simulations. Simulation of the g factor and hyperfine interaction distributions have led to the identification of different paramagnetic centers in these irradiated glasses. The following paramagnetic defects were identified: boron oxygen hole center (BOHC), HC1 center, two different hole centers, peroxy radical (Oxy defect), E′ centers, and electron trapping processes for iron and zirconium ions: Zr4++ e−→ Zr3+ and Fe3+ + e−→ Fe2+. In particular, no paramagnetic defects associated with aluminum ions are detected in these irradiated glasses.

Stopping power in perspective

Abstract: After a brief historical survey this paper describes some of Jens Lindhard's contributions to the theory of particle penetration with the emphasis on stopping power and related problems. Main application areas are mentioned and needs for future research outlined. Stopping theory is a mature field with a certain amount of inertia. A few areas are identified where a moderate change in priorities could better serve the needs of the user community.

Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime

Abstract: Silicon targets of (1 0 0) orientation were prepared for transmission electron microscopy observations and then irradiated with either 30 MeV C602+ or 40 MeV C603+ at normal incidence. All the irradiations were performed at room temperature, up to fluences of a few 109 clusters cm−2. The incident electronic stopping powers were 48 and 57 keV nm−1 for C602+ and C603+ projectiles, respectively. High resolution observations at normal incidence evidenced amorphous zones of circular shape at each projectile impact. The track diameters near the target surface were 8.4 and 10.5 nm for irradiations at 30 and 40 MeV, respectively. This effect, which was never observed in silicon single crystals bombarded with swift heavy ions, was ascribed to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. Observations at conventional resolution of samples tilted in the microscope allowed to follow the depth evolution of the entire tracks. The damage extends from the surface to a maximum depth L which depends on the incident energy of the clusters (L=160 nm and L=190 nm for irradiations at 30 and 40 MeV, respectively). This progressive extinction of the radiation-induced disorder was linked to the decorrelation process of the C60 ions during their slowing-down in the target.

Characterisation of atmospheric fine particles using IBA techniques

Abstract: The study of fine particles in the atmosphere with diameters less than 2.5 μm has recently become of considerable interest to a broad range of researchers. Often these fine particles are collected on thin filters by samplers sucking air through them. These thin filters are ideal targets for analysis and characterisation by accelerator based ion beam analysis (IBA) methods. The IBA methods can analyse each of these filters, non-destructively, in a few minutes of accelerator running time for up to 30 different elements from hydrogen to uranium. This ability to readily characterise the fine particle composition on hundreds of filters means that statistical methods such as principal components analysis (PCA) and chemical mass balance (CMB) techniques can be used to determine the source fingerprints and source contributions for these large area sampling networks.

Energy loss and straggling of H and He ions of keV energies in Si and C

Abstract: The first two energy-loss moments of 1H, 2H, 3He and 4He ions in Si and C have been measured over the energy range of about 1–1000 keV/amu by ion backscattering. These results are compared with recent experimental data and theoretical predictions. Target-electron motion is found to strongly modify energy-transfer kinematics over most of the investigated energy range.

The Berlin time-of-flight ERDA setupe

Abstract: The new mass and energy dispersive Elastic Recoil Detection Analysis (ERDA) spectrometer located at a high-energy target position of the heavy-ion-beam laboratory (ISL) of the Hahn-Meitner-Institut Berlin is presented. Many different projectile ions from hydrogen to xenon with variable energies up to several MeV/u are available. The recoil identification is done by means of a time-of-flight (TOF) energy setup with a relatively large solid angle of 1.57 msr. Due to the long flight path of 123 cm and a time resolution of about 180 ps, a good mass and depth resolution can be achieved.

Formation and development of disordered networks in Si-based ceramics under ion bombardment

Abstract: In the present paper the results of an extended study on the disordering of Si, SiC, Si 3 N 4 and SiO 2 by ion bombardment will be reviewed, with respect to both long and short range order. It was found that amorphization occurs by nucleation and growth of defect agglomerates in the still crystalline matrix, until a critical damage density is achieved and a transition between the ordered and the disordered networks occurs. In SiO 2 , Si 3 N 4 and Si the disordered phase consists of a random network of [SiX 4 ]-tetrahedrons (X = O, N, Si), which conserved the chemical short range order of the crystalline materials. In SiC first a highly disordered network of [SiC 4 ]-tetrahedra forms, which is however not as random as in the other materials, since a correlation between the orientation of neighboring tetrahedra exists. Further bombardment of this network then results in complete destruction of the initial chemical short range order and the formation of Si–Si and C–C bonds. The results are compared with theoretical predictions of the amorphizability of the different compounds and the microstructure of the disordered phases.

High resolution TEM observation and density estimation of Xe bubbles in high burnup UO2 fuels

Abstract: High resolution transmission electron microscopy (TEM) observations, nano-diffraction and nano-area energy dispersive X-ray spectroscopy (EDX) analyses were carried out in the outer region of high burnup UO 2 pellets. Samples were prepared from the outer low temperature region of UO 2 pellets, which had been irradiated to the pellet average burnups of 30 and 49 GWd/t in BWRs and 83 GWd/t in a test reactor under a pressurized water reactor (PWR) condition (1 GWd/t=2.5 × 10 19 fissions/cm 3 ). Lattice images and nano-area EDX results indicate the presence of 4–10 nm size Xe bubbles with five-metal particles of Mo–Tc–Ru–Rh–Pd. Nano-diffraction patterns from bubbles show two different new patterns other than that of matrix UO 2 . From the Xe/U proportion obtained by the EDX peak and nano-diffraction patterns, it was suggested that Xe in the small bubbles existed as a solid or in a near solid state at very high pressure.

Fullerene nanotubes can be used when transporting gamma-quanta, neutrons, ion beams and radiation from relativistic particles

Abstract: The nanotube system is shown to be very effective when transporting the beams of neutral and charged particles. Also, it can be used for intensive radiation of relativistic leptons which is similar to channeling radiation in crystals. Now it is possible to make nanotubes of 100–300 μm length and of 1–50 nm diameter.

Radiation damage in yttria-stabilized zirconia under Xe ion irradiation

Abstract: Single crystals of yttria-stabilized zirconia were irradiated with 60 keV Xe ions in an electron microscope at several temperatures in the range 300–1473 K and the process of damage evolution was studied. Amorphization did not occur in the zirconia irradiated with 60 keV Xe ions up to the fluence of 1.8×1016 Xe/cm2 at all irradiation temperatures. In the specimen irradiated with Xe ions below 1073 K, defect clusters are formed first and then bubbles are formed. On the contrary, in the specimen irradiated above 1073 K bubbles are formed first and then defect clusters are formed. From the observation of bubble sizes and densities, temperature and dose dependence of bubble swelling were estimated. With increase in fluence or irradiation temperature the average size of the bubble increased resulting in greater values of swelling.