Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 1987"

Improving the validity of hyperfine field distributions from magnetic alloys: Part I: Unpolarized source☆

Abstract: The evaluation of magnetic hyperfine field distributions from 57Fe Mossbauer spectra in systems with both magnetic dipole and electric quadrupole interactions is considered. This problem is treated in higher order perturbation theory, and the case of general magnetic texture and isotropic electric quadrupole texture is treated in full. Simple expressions are given enabling the calculation of all spectrum line positions, intensities and widths. It is shown that in cases where first order perturbation treatments yield low-field artifacts in the magnetic hyperfine distribution, this procedure yields much more reliable results. It is also shown that the calculated magnetic spin texture is much more reliable. The asymmetry effects from the electric quadrupole interaction and from a correlation between this and a dipole magnetic field are considered.

Catalyst and binder effects in the use of filamentous graphite for AMS

Abstract: Filamentous graphite produced by the reduction of carbon dioxide over a transition metal catalyst has been used as a sample material in the AMS counting of 14C for several years. While iron has been the most common catalyst used, cobalt and nickel have also been investigated. We have compared the reaction's total isotopic fractionation using these three metals in various powder forms. The graphites produced over these catalysts have been compared with respect to ion beam intensity and measured isotope ratio in our AMS system. The use of the reduction catalysts as graphite binders improved the very poor thermal conductivity of pure filamentous graphite. The relative advantages of the elements for modern contamination of the sample material were also determined.

Mechanisms and theory of physical sputtering by particle impact

Abstract: This article summarizes theoretical work on sputtering during the past decade, with the emphasis on elemental sputtering in the linear cascade and spike regimes as well as alloy sputtering. The sputtering of molecules and clusters, and electronic sputtering of insulators and biomolecular materials are discussed more briefly, and topics like charge and excitation states of sputtered particles as well as surface topography are left out, in view of contemporary summary papers in this issue and elsewhere. The paper is nontutorial and assumes some general knowledge of the field on the part of the reader, based e.g. on ref. [1] or ref. [2]. The discussion emphasizes principles and methods as well as open problems rather than quantitative results. A few general recommendations for efficient utilization of computer simulation conclude the paper.

The heavy ion storage and cooler ring project ESR at GSI

Abstract: The Experimental Storage Ring ESR [1] is designed for accumulation, storage and cooling of heavy ion beams up to uranium with energies up to 500 MeV/u either directly out of the heavy ion synchrotron SIS [2] or via a stripper target, where electron-free or electron-poor ions are produced. The intense, cold beams may be used for a large variety of experiments, preferably in-ring experiments making use of the special features of the storage ring. Due to circulation frequencies of up to 2 MHz high luminosities will be attained using thin internal gas jet or electron target. Energies can be varied over a wide range from 500 MeV/u down to 3 MeV/u. The SIS/ESR-facility will also be capable of delivering beams of unstable nuclei produced by fragmentation or fission of heavy projectiles. After preselection in a large acceptance Z / A -separator the secondary beams can be prepared for internal or external experiments by stochastic precooling, intensity accumulation and final electron cooling.

Computer simulations of sputtering

Abstract: Monte Carlo simulations have become a useful tool for studying ion radiation effects at or near surfaces or interfaces, such as sputtering, reflection, mixing, etc. The principal advantage of Monte Carlo calculations is that any physical process can be included directly. Also multielement and multilayer targets, even complex geometries, can be treated exactly in order to simulate realistic cases. The present paper will concentrate mainly on sputtering calculations with the Monte Carlo code TRIM, which treats ion and recoil transport in amorphous matter. It is based — as are analytic theories and most other Monte Carlo codes — on binary collisions with target atoms initially at rest. Over the past few years, the basic physical input has been greatly improved. Both the interatomic potentials and the electronic stopping powers proved to be of crucial importance even for the lowest energies occurring in recoil cascades. With the Kr-C or universal potential and the recent ZBL electronic stopping, which includes the Z oscillations, and a planar surface binding energy set equal to the heat of sublimation, realistic sputtering predictions could be obtained for most metals — without the use of any adjustable parameters.

Helium desorption/permeation from bubbles in silicon: A novel method of void production

Abstract: The annealing behaviour of helium bubbles formed by helium implantation into silicon has been studied using both helium desorption spectroscopy and transmission electron microscopy. The combination of these techniques has demonstrated that helium can permeate out of bubbles in silicon during annealing to leave behind empty cavities.

Computer-simulation of channeling in single-crystals

Abstract: A Monte Carlo program for the calculation of channeling phenomena is described. The program combines the binary collision model and the multistring approximation. The energy loss due to electronic excitation is taken into account, with the use of the model of Dettmann and Robinson for the inner-shell electrons and the theory of Pines for valence electrons. The output of the Monte Carlo program may be used for the determination of the impurity sites in single crystals, via a set of auxiliary programs, that enable that calculation of the impurity yield and the analysis of experimental channeling dips. As an application, the site determination of iodine in silicon is described. Another application is the simulation of RBS spectra of planar channeling ions. Simulated and experimental spectra are compared for 1 MeV ions in the (110), (111) and (100) planes of silicon. A reasonable agreement was found. The possible causes of the remaining deviations are discussed.

Shock wave mechanism for cluster emission and organic molecule desorption under heavy ion bombardment

Abstract: Mechanism for large cluster emission during sputtering of solids and bioorganic molecule desorption under ion bombardment via formation and emergence of shock waves originating from some high density energy spikes due to collision cascades or Coulomb explosions on fast heavy ion paths is proposed. The peculiarities of large cluster emission and the dependence of the bioorganic molecule desorption yield on energy loss, angle of incidence, and charge state of the primary ions are compared with the available experimental data. The energy and the angular distributions of the desorbed molecules are discussed.

Quantitative mass and energy dispersive elastic recoil spectrometry: Resolution and efficiency considerations

Abstract: A time of flight-energy recoil telescope system for mass and energy dispersive recoil spectrometry has been applied to study the formation of Mg2Si layers and depth profiling of Ga1−xAlxAs quantum well structures. Measurements of the energy (depth) dependence of the mass resolution showed that the telescope could be used over the energy range from 5 to 18 MeV to distinguish between recoils of 1 amu mass difference up to mass 28 amu. The energy dependence of the detection efficiency was found to be independent of the recoil energy for 12C and 28Si recoils and no strong evidence for a recoil species dependence of the detection efficiency for recoils heavier than 16O was found.

Ion-beam-induced crystallization and amorphization of silicon

Abstract: Thin amorphous silicon layers can be produced in crystalline silicon substrates by ion-implantation. Subsequent ion-irradiation at elevated temperatures can induce such layers to either crystallize epitaxially or increase in thickness, layer by layer. This paper examines these processes and their dependence on substrate temperature and ion-irradiation parameters. It is shown that both processes, epitaxial crystallization and layer-by-layer amorphization, are controlled by ion-beam induced defect production at, or near, the crystalline/amorphous interface. The competition between defect production (determined by the ion flux and rate of nuclear energy deposition) and dynamic defect annealing (determined by the substrate temperature) is shown to play an important role in determining whether the layer crystallizes or amorphizes. Possible models for the observed behavior are discussed.

10Be: Half-life and AMS-standards

Abstract: Absolute AMS-measurements of 10Be require reliable standards for calibration. Among the existing standards, rather large differences have been observed. These differences were found partially to be due to the different half-life values which were assumed. Also for comparison of AMS-data with activity measurements, it is necessary to know the 10Be half-life as precisely as possible. Starting with 5 ml of the standardized ORNL-MASTER solution, a working solution with a well-defined 10Be content was prepared. Its specific activity was determined by liquid scintillation counting. This measurement yielded a new value of (1.51 ± 0.06) Ma for the 10 Be half-life, which is in good agreement with the previously reported values but is almost three times more accurate. Two independent dilution series produced new AMS-standards with 10Be/9Be-ratios of the order of 10−10 and 10−11. These standards were measured at the ETH/SIN-AMS-facility with high accuracy and are compared with other available 10 Be-standards.

A gas-flow model for the sputtering of condensed gases

Abstract: The bombardment of condensed gases by low keV ions or atoms establishes a collision cascade in the solid. We argue that in cases where the deposited energy density surpasses the critical temperature of the medium, the gas formed expands into the vacuum, increasing the sputtering yield and shifting the energy spectrum to lower energies. A simple model, based on the assumption of a collision-free molecular flow, is developed to give analytical results for the sputtering yield and the axial energy spectrum. The effects of the expansion process, of energy dissipation and of recondensation on the crater walls are discussed. Good agreement with experimental sputtering yields and energy spectra is found.

Fabrication of buried layers of SiO2 and Si3N4 a using ion beam synthesis

Abstract: The properties of buried layers produced by the implantation of high doses of energetic light ions O+ and N+ are reviewed. Similarities and differences in the as implanted and annealed structures are discussed as are the effects of variations in the anneal temperature, time and ramp-rate. To assess the factors governing the interfacial displacements of the growing buried layer with respect to the silicon, experiments utilising buried marker layers and involving sequential implants of O+ or N+ ions were undertaken. These confirmed the results of 18O+ and 15N+ tracer experiments by showing that during implantation the back interface of the buried layer remains effectively fixed while the front interface is displaced towards the silicon surface. During the subsequent anneal redistribution of the implanted species occurs but again this effect is most marked at the front interface.

Applications of 129I and 36Cl in hydrology

Abstract: Since the first AMS measurements of 36Cl in 1978, this cosmogenic radionuclide has proved to be a versatile tracer of hydrologic processes in over 20 field studies. Natural 129I also appears to be useful for studying hydrologie processes although incomplete understanding of its production in nature and geochemical behavior largely limits interpretation to qualitative discussions. The range of hydrologic applications demonstrated for these radionuclides covers: estimation of residence time of water in the subsurface and net infiltration in arid soils; evaluation of ion filtration, leaching of connate water, and salt dissolution as sources of ground-water salinity; estimation of lithospheric thermal-neutron fluxes; and emanation and migration characteristics of fission-product 129I in different geochemical environments.

Measurement of 129I concentrations in the environment after the Chernobyl reactor accident

Abstract: The Chernobyl nuclear reactor accident which occurred on April 26, 1986 is known to have injected into the atmosphere a pulse of a large number of radionuclides. The activities of several radionuclides present in the subsequent fallout have been measured in different locations throughout Europe by gamma-ray and beta counting. We present here measurements of concentrations of the long-lived radionuclide 129I ( T 1 2 = 1.6 × 10 7 yr ) in environmental samples collected in Israel and Europe following the nuclear reactor accident. The measurements were performed by accelerator mass spectrometry, using the 14UD Rehovot Pelletron Accelerator. Concentrations of 129I in rainwater samples collected in the Munich (West-Germany) area and in Israel during the fallout period were measured to be 2.6 × 1010 and 1.2 × 109 atoms I respectively, while a 1982 rainwater sample from Israel shows a 129I concentration of 8.2 × 107 atoms I . Three measurements of the ratio 129I/131I gave a mean value of 21, from which an effective operating time of the reactor of 1.5 to 2 yr prior to the accident can be estimated. The possible use of anthropogenic 129I as a tracing tool for global environmental processes is discussed.

Measurements of 10Be and 26Al with a tandetron AMS facility

Abstract: The present status for measuring 10Be and 26Al at the Gif-sur-Yvette Tandetron AMS facility is reviewed. We conclude that the capabilities for these two isotopes (especially 10Be) are competitive with those currently obtained at higher energy accelerators.

MeV-energy B+, P+ and As+ ion implantation into Si

Abstract: Annealing behavior of secondary defects in 2 MeV B + and P + , and 1 MeV As + ion implanted (100) Si has been investigated in terms of their structure, nature and depth distribution. This is done mainly through cross-sectional TEM observations at doses of 2 × 10 13 , 1 × 10 14 and 5 × 10 14 ions/cm 2 . The critical dose for generating secondary defects is between 2 × 10 13 and 1 × 10 14 ions/cm 2 , independent of ion species. A characteristic of B + and P + ion implanted layers is that buried secondary defects are formed beneath the substrate surface, with their maximum densities at depths of around 3.2 μm (B) and 2.1 μm (P) below the surface. These defect peak positions in the crystal are constant under all annealing conditions (e.g., a temperature range of 700 to 1250°C, annealing time of up to 6780 min at 1000°C). In contrast, the defects in the layers implanted with As + ions have an almost constant density in their depth distribution and change their depth position with annealing time and temperature. The peaks (B and P) and the central position (As) of these secondary defects are positioned deeper than both the projected range and the primary defect peak of each ion species.

Sputtering of frozen gases

Abstract: Particle-induced erosion of frozen gases takes place as beam-induced evaporation as well as sputtering. At sufficiently low temperatures electronic or knock-on sputtering is the dominant mechanism for particle ejection. Knock-on sputtering may largely be compared to ordinary sputtering of metals, although the yields are much higher for ices than for metals because of the low surface binding energy. Electronic sputtering exhibits large differences from the solid rare gases to the solid diatomic homonuclear gases or the solid heteronuclear molecular gases.

Ion implantation in GaAs

Abstract: Recent advances in the understanding of the relationship between implanted dopant solubility and electrical activity in GaAs are reviewed and direct lattice configuration measurements explaining these results are presented. The nature of residual defects remaining after activation annealing of GaAs, in particular rapid thermal annealing, are discussed, along with a review of the application of ion beams in promoting compositional disordering of GaAs-AlAs superlattices. The outstanding problems remaining in the use of ion implantation technology for GaAs are also detailed.

Iron ion implantation effects in sapphire

Abstract: Single crystals of α-Al 2 O 3 have been implanted at room temperature with 160 or 100 keV 57 Fe + ions and doses ranging from 10 16 Up to 10 17 ions cm −2 . The valence states and the local environment of implanted atoms as well as the damage in the implanted zone have been studied with the conversion electron Mossbauer spectroscopy technique associated with channeling and transmission electron microscopy. It was found that implantation introduces iron in sapphire in three charge states: Fe 2+ , Fe 3+ and metallic precipitates. Annealings in oxidizing or reducing atmosphere at temperature up to 1500°C convert all iron into Fe 3+ or Fe 0 respectively, and the precipitations of small oxide or metallic iron particles are observed correlatively with the rearrangement of the matrix.

Ion induced adhesion via interfacial compounds

Abstract: Strong, stable adhesion of Cu thin films deposited on Al 2 O 3 (sapphire) can be obtained by presputtering the substrate surface with 500 eV Ar + ions before deposition of copper. The existence of a well-defined optimum fluence of sputtering ions suggests that the interface atomic configuration can be optimized to favor the formation of Cu-Al-O chemical bonding. The existence of a ternary bonding environment is inferred independently from a new dominant peak in the XPS spectrum from interface copper, whose occurrence is correlated with the optimized adhesion conditions.

Fractionation, precision and accuracy in 14C and 13C measurements

Abstract: During the past ten years, AMS has become a powerful tool in radiocarbon dating. In some applications, an overall accuracy comparable to that of conventional high precision low level counting is required. To achieve this accuracy, fractionation during sample preparation and measurement has to be constant. Comparison of series of 13 C 12 C ratios measured with AMS and a conventional mass spectrometer indicates that systematic errors in the carbon isotope ratio measurements are in the order of 0.1 to 0.2%. With mg size samples prepared by catalytic reduction on iron, delivering beam currents of about 12 μA for at least 1 h, statistical uncertainties for 14C of 0.3% and a total error of 0.4 to 0.5% for the 14 C 12 C ratio (including the calibration error and uncertainty in the background subtraction) for modern carbon has been obtained.

14C dating with the Gif-sur-Yvette Tandetron accelerator: Status report

Abstract: Radiocarbon dating with the Gif-sur-Yvette Tandetron accelerator has been routinely performed since the middle of 1985. Samples are prepared by direct catalytic reduction of CO2 on iron powder with typically 1 to 2 mg of carbon. 12C, 13C and 14C isotopes are measured after acceleration for the direct correction of 14C ages for natural and preparation induced fractionation. The precision and the reproducibility of the 14C activities obtained for samples younger than 5 000 yr BP is close to 1%. Our background is limited by contamination in sample preparation i.e. pretreatment and catalytic reduction. The contamination due to catalytic reduction of CO2 samples, between 1 to 2 mg of carbon has a typical value of (0.25 ± 0.11)% of modern.

Computerized microtomography using synchrotron radiation from the NSLS

Abstract: Results of microtomography experiments that employ filtered radiation from the National Synchrotron Light Source X-26 Microprobe beam line are presented. These experiments have yielded images of a freeze-dried caterpillar with a spatial resolution of the order of 30 μm and show that the limit on the spatial resolution with the present apparatus will be 1 to 10 μm. Directions for improvement in synchrotron microtomography techniques and some possible applications are discussed.

Three-dimensional distributions of ion range and damage including recoil transport

Abstract: Ion range and damage distributions — in insulators also ionization distributions — are of increasing relevance for modern semi conductor (VLSI) developments. With small dimensions also the lateral distributions need to be considered. In both longitudinal and lateral profiles, strong deviations from the commonly used Pearson IV or Gaussian distributions are found and shall be demonstrated with Monte Carlo results. In the Monte Carlo calculations the contributions of the moving recoils to ionization and damage production can be studied explicitly. In cases of heavy ions incident on a lighter target, the fluxes of recoils can be larger than the ion beam flux, and the ionization and damage profiles are drastically enhanced and shifted, due to the motion of recoils.

Different behaviours of amorphization induced by ion mixing

Abstract: Ion mixing of multiple metal layers was conducted at 77 K in five selected binary metal systems, i.e. the Ag-Cr, Co-Cu, Cu-Fe, Fe-Mo and Fe-Nd systems, which all have a nearly zero or positive heat of formation (Δ H For ). TEM examination showed a common microscopic feature of localized amorphization after high dose ion irradiation, i.e. the amorphous phase was frequently interspersed in a crystalline matrix. EDS analysis was therefore carried out to determine the actual compositions of the phases formed. It revealed that all local amorphous regions resided within rather narrow composition ranges. The narrow range found here for amorphization is probably the reason that amorphous alloy formation was difficult in these systems. After comparing these observations with those reported earlier for the some 30 systems so far studied, it is found that the behaviour of ion mixing induced amorphization, and hence the binary metal systems, can be classified into two categories. The first category contains the readily amorphous phase forming systems and usually features very limited terminal solid solubilities along with large negative heat of formation. Typical examples are the Au-Ti, Ni-Mo, Ni-Nb and Ru-Zr systems. In these systems amorphization can be achieved over a wide composition range once a minimum dose is reached. For systems in the second category, amorphization is only allowed in a rather narrow composition range, as in the systems studied here. This is also the case when a system has a large solid solubility, even though its Δ H For is rather negative, e.g. the Mo-Ru and Au-V systems. In such situations, the results of phase formation may be sensitive to the composition as well as to the ion irradiation dose. The possible mechanism responsible for the different behaviors of phase formation by ion mixing are also discussed.

Density enhancement in ion implanted polymers

Abstract: Ion beam irradiation of polystyrene and polyimide films induces a strong densification of the starting material. The density increases to about 70% of the initial value after bombardment with heavy ions (Ar, Kr, Xe etc) in the energy range 25–250 key. The density change does not depend on the chemical structure and seems to be a general property of the irradiated polymers. The energy deposited along the ion track is the key parameter of the investigated process.

Review of stopping powers in organic materials

Abstract: A survey of tests concerning Bragg's rule of stopping power additivity is updated, summarising recent evidence on chemical binding and physical state effects on stopping power in organic and related materials. A general failure of simple additivity is well-established, but magnitudes of effects are still subject to uncertainty. Phase effects in organic materials are ~ 5–10% at stopping power maximum for H and He ions, decreasing as energy increases. The stopping cross-sections of gases are greater than those of the equivalent condensed phase. Accuracy of stopping power measurements in organic compounds is often insufficient to unambiguously detect and quantify such effects. However sensitive tests of additivity can be obtained where uniform series of measurements have been carried out by one group with one method in a range of materials.

Molecular weight distribution and solubility changes in ion-bombarded polystyrene

Abstract: Among the chemical and physical modifications induced by ion bombardment on polymers, solubility changes have attracted considerable attention owing to the technological interest for microlithography purpose. The solubility changes are due to occurrence of cross-linking and scission at molecular level with heavy modification of the molecular weight distribution too. The molecular weight distribution of implanted polystyrene shows considerable changes upon bombardment. These changes have been followed on bombarded nearly-monodisperse PS samples. The use of samples with known molecular weight distribution allows to apply the Gel Theory for determining the chemical yields. This method is a direct, relatively simple tool for following the chemical modifications in bombarded polymers.

Improving the validity of hyperfine field distributions from magnetic alloys: Part II: Polarized source and spin texture

Abstract: It has been shown in Part I of this work that 57 Fe Mossbauer spectra for the case of mixed electric and magnetic interactions can be calculated with much higher accuracy by using higher order perturbation theory. This is extended for the case of polarized radiation in order to treat cases of magnetic texture in more detail. It is shown that, in this case, the line intensities can be expressed as functions of the magnetic texture including the perturbation of the electric quadrupole interaction. As in Part I, the case of an isotropic electric quadrupole texture is considered.