Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 1996"

Application of hydrogen ion beams to Silicon On Insulator material technology

Abstract: A new application for proton ion beams in the field of Silicon On Insulator material (SOI) technology is reported. In this technology, based on hydrophillic wafer bonding and referred to as “Smart-Cut”, heat treatment induces an in-depth micro-slicing of one of two bonded wafers previously implanted with hydrogen. The principle of this process involves the basic mechanisms associated with high fluence proton implantation in materials, such as blistering, flaking and exfoliation. The intrinsic properties of this process lead to very high crystalline quality of the SOI layers and very good thickness uniformity. After presentation of the process details and the underlying physical aspects, the main characteristics of the Smart-Cut technology and first physical and electrical characterizations are reported.

Track creation in SiO2 and BaFe12O19 by swift heavy ions: a thermal spike description

Abstract: The thermal spike model is used in order to calculate the track radii variation versus electronic stopping power Se in two radiolysis resistant oxides: SiO2 quartz and BaFe12O19. The mean diffusion length λ of the energy deposited on the electrons is determined by fitting latent track radii versus Se: 4.0 ± 0.3 and 8.2 ± 1.3 nm respectively for both materials. A decrease in the band gap Eg (12 and 1 eV respectively) means an increase in λ.

Silicon carbide: Synthesis and processing

Abstract: Silicon carbide with its outstanding physical properties is a material of choice for special optoelectronic and electronic devices working under extreme conditions. Synthesis as well as processing are complicated compared to other materials. The present paper summarizes some aspects of crystal growth and processing and discusses arising problems.

Influence of irradiation spectrum and implanted ions on the amorphization of ceramics

Abstract: Polycrystalline specimens of alumina (Al2O3), magnesium aluminate spinel (MgAl2O4), magnesia (MgO), silicon nitride (Si3N4) and silicon carbide (SiC) were irradiated with various ions at temperatures between 200 and 450 K, and the microstructures were examined following irradiation using cross section transmission electron microscopy. Amorphization was not observed in any of the irradiated oxide ceramics, despite damage energy densities up to ∼ 70 keV/atom (∼ 70 displacements per atom). On the other hand, SiC readily amorphized after damage levels of ∼ 0.4 dpa at room temperature. Silicon nitride exhibited intermediate behavior; irradiation with Fe ions at room temperature produced amorphization in the implanted ion region after damage levels of ∼ 1 dpa. However, irradiated regions outside of the implanted ion region did not amorphize even after damage levels in excess of 5 dpa. The amorphous layer in the Fe-implanted region of Si3N4 did not appear if the specimen was simultaneously irradiated with 1 MeV He+ ions at room temperature. By comparison with published results, it is concluded that the implantation of certain chemical species has a pronounced effect on amorphization threshold dose of all five materials. Intense ionizing radiation inhibits amorphization in Si3N4, but does not appear to significantly influence the amorphization behavior of SiC.

A 4π recoil-ion electron momentum analyzer: a high-resolution “microscope” for the investigation of the dynamics of atomic, molecular and nuclear reactions

Abstract: A high-resolution recoil-ion momentum spectrometer based on a precooled localized supersonic jet target (COLTRIMS) has been combined with a novel low-energy electron analyzer with 4π solid angle for electrons with energies E e ≤ 30 eV including E e = 0 eV. Thus, three recoil-ion momentum components, the recoil-ion charge state and three momentum components of one electron emitted in any collision-induced ionization reaction are measured simultaneously with a coincidence efficiency of 28%. In order to accept large recoil-ion longitudinal momenta (along the beam) of p R ‖ ≤ 160 a.u. and simultaneously guarantee a superior resolution in this direction (Δ P R ‖ ≤ ± 0.08 a.u.), recoil ions are extracted in the longitudinal direction different from all former concepts. Test measurements, details on the present design and results of a kinematically complete experiment for single ionization are presented and possible further improvements are discussed. The future potential of such spectrometers for the investigation of collision-induced atomic many-particle reactions, the “Coulomb-explosion” of molecules and the spectroscopy of electronic states in heavy few-electron systems is illustrated. Similar techniques might be used to measure angular correlations and even the neutrino mass in β-decay experiments.

Structural freedom, topological disorder, and the irradiation-induced amorphization of ceramic structures

Abstract: The susceptibility to irradiation-induced loss of long-range translational and orientational order during amorphization of crystalline solids, a process which usually amounts to topological disordering, can be understood on the basis of available structural freedom which depends on the redundancies present in structural connectivity. A surprisingly good correlation has been established between the critical accumulated energy deposited per atom — or displacements per atom (dpa) — required for amorphization during ion irradiation and the calculated structural freedom for the structure type irradiated over a wide range of non-metal structural types. These include AO, AO2, A2O3, ABO3, ABO4, A2B2O7 and A2BO4 compact structure types, in which weakest links can often be identified, as well as less-compact network structures. The present contribution specifically compares the order of susceptibility to amorphization — in these more complex oxide structures and in the simpler network structures Si3N4, SiC, Be2SiO4, AlPO4, Si, SiO2, P2O5 and graphite — on the basis of structural connectivity and discusses the anomalous susceptibility of SiC.

Accurate universal parameterization of absorption cross sections.

Abstract: We present a simple universal parameterization of total reaction cross sections for any system of colliding nuclei valid for the entire energy range from a few A MeV to a few A GeV. The universal picture presented here treats the proton-nucleus collision as a special case of the nucleus-nucleus collision, where the projectile has charge and mass number one. The parameters are associated with the physics of the collision system. In general terms Coulomb interaction modifies cross sections at lower energies and the effects of Pauli blocking are important at higher energies. The agreement between the calculated and experimental data is better than all earlier published results.

ETACHA: a program for calculating charge states at GANIL energies

Abstract: A computer program calculating charge state distributions of ions with up to 28 electrons distributed over n = 1, 2and3 subshells has been developed. The model is based on an independent electron model taking into account electron loss, capture and excitation from and to all the subshells. Calculated atomic cross sections are recomputed periodically to take into account their dependence with the projectile energy and its mean charge state when they vary as a function of target thickness.

Comparison of TOF-ERDA and nuclear resonance reaction techniques for range profile measurements of keV energy implants

Abstract: A comparative study on the range measurements of keV energy implants by the Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) and conventionally used nuclear resonance reaction methods has been performed for 20–100 keV 15N+ ions implanted into crystalline silicon. Range profiles of 15N atoms were chosen because they can be measured accurately using a very strong and narrow resonance at Ep = 429.6 keV in the reaction 15N(p,αγ)12C which provides a challenging test for other methods. The measured range profiles were simulated by molecular dynamics calculations where the interatomic NSi pair potential is deduced from first principles calculations. The electronic stopping power for 20–100 keV nitrogen ions in silicon is deduced from the comparison of the measured and simulated range profiles. The results are discussed in the framework of the applicability of the TOF-ERDA technique for keV energy ion range measurements.

Novel SANS instrument using Neutron Spin Echo

Abstract: Following a recent report by Keller et al. in Neutron News 6, no. 3 (1995), the application of neutron spin echo as a small angle neutron scattering instrument (SESANS) using dc magnetic fields is discussed. The principle is based on the difference in Larmor precession angle in a coil with changing transmission angle, caused by scattering from a sample. A scattering object between two identical coils with opposite precessions causes depolarisation. The latter as a function of the precession field yields a real space correlation function. A modification of this principle of SESANS is introduced enabling to suppress the contribution of inelastic scattering, which changes the precession angle too. The application of this technique is very promising for systems with large correlation lengths and may enable one to measure time dependent effects in correlation lengths with a time resolution of a few seconds. The latter corresponds to the time needed to measure a complete spectrum from which a correlation function in real space is derived.

A new cascade impactor for aerosol sampling with subsequent PIXE analysis

Abstract: A small deposit area low pressure impactor (abbreviated to SDI) has been developed and tested. The device has been designed specifically to collect size-fractionated aerosol samples in remote locations for subsequent chemical analysis by PIXE. The SDI is a 12-stage, multinozzle device, but the deposit for each stage remains confined to an area with diameter less than 8 mm. It operates at a flow rate of 11 L/min and accepts the same, 25 mm diameter substrate rings as the PIXE International cascade impactor. The experimental cut-points for stages 12 through 1 are 8.50, 4.08, 2.68, 1.66, 1.06, 0.796, 0.591, 0.343, 0.231, 0.153, 0.086 and 0.045 μm equivalent aerodynamic diameter. The SDI has been tested in (and employed for) size-fractionated aerosol sampling in the Finnish Arctic and at Summit in Greenland. The data show that the SDI gives results very similar to those obtained with the PIXE International impactor, but with detection limits that are much lower. This suggests that the SDI can be used with shorter sampling times or in areas where concentrations are smaller to obtain reliable size distribution data. The results also suggest that data for a greater number of elements can be obtained with the SDI.

Accelerator mass spectrometry of plutonium isotopes

Abstract: The feasibility of measuring plutonium isotope ratios by accelerator mass spectrometry has been demonstrated. Measurements on a test sample of known composition and on a blank showed that isotope ratios could be determined quantitatively, and that the present limit of detection by AMS is ∼ 106 atoms of plutonium. For 239Pu, this limit is at least two orders of magnitude lower than that practicable by alpha-spectrometry. In addition, 240Pu239Pu ratios were measured for four samples for which the combined activity of the two isotopes had been determined previously by alpha-counting. All measurements of plutonium isotope ratios entailed injection of PuO− into the 14UD accelerator operating at 3.5 MV, gas stripping, and analysis of the 7+ charge state after acceleration. Plutonium ions at 28 MeV were detected in a longitudinal-field ionisation chamber with an energy resolution of 3%. Using uranium oxide as a surrogate for plutonium oxide, it was shown that UO− was the predominant negative ion and that the probability for its formation and extraction was 0.3%.

Pore geometry of etched ion tracks in polyimide

Abstract: Tracks of energetic heavy ions in the polyimide Kapton were etched in a NaOCl solution. It was found that the pH value of the etchant plays a crucial role for the selectivity of track etching. The bulk etching rate νb increased exponentially with pH. From the temperature dependence of the bulk etching rate an activation energy of Ea = 0.74 eV was deduced. In contrast to νb, only a slight and linear increase with pH value was observed for the track etching rate νt. As a consequence, the etch ratio ν t ν b can be adjusted over one order of magnitude by controlled varying the pH of the etchant. This is a new way to tailor the pore geometry from nearly cylindrical to funnel-shaped pores.

A perfect crystal X-ray analyser with 1.5 meV energy resolution

Abstract: A new spherical silicon crystal analyser, operated in backscattering geometry at the Si(11 11 11) reflection, allows to perform inelastic X-ray scattering experiments with a total energy resolution of 1.5 ± 0.2 meV full-width-half-maximum (FWHM) at 22 keV incident photon energy. This analyser was constructed by improving a procedure recently developed.

Carbonaceous clusters in irradiated polymers as revealed by small angle X-ray scattering and ESR

Abstract: Carbonaceous clusters are supposed to be carriers of electric conductivity in ion-irradiated polymers. They also influence the optical properties of such materials. Several experimental approaches exist to study those clusters, among which is small angle X-ray scattering, SAXS. Results obtained by this technique are presented here. According to them, the clusters result from electronic, and not from collisional energy transfer processes. Two distinctly different cluster types exist — small spherical clusters around 5–7 nm radius, and large-size fractal objects > 100 nm. It appears that the formation of both these types is closely related to the transferred energy density per track and to the degree of track overlapping: apparently small clusters emerge already within single-ion tracks of energetic heavy ions, whereas light ion irradiation requires ion track overlapping to form large-size fractal objects. The latter ones exhibit slightly ellipsoidal shapes, aligned along the ion track direction. Comparison with recent results gained by UV-Vis spectrometry shows that cluster sizes as obtained by SAXS are considerably larger. Possible reasons for this disagreement are discussed. Finally, new results of recent electron spin resonance measurements on irradiated polymers are presented and discussed in the framework of carbonaceous clusters and polymeric conductivity.

Charge-changing nuclear reactions of relativistic light-ion beams (5 ≤ Z ≤ 10) passing through thick absorbers☆

Abstract: Light-ion beams of 20 Ne, 19 Ne, 18 F, 16 O, 15 O, 14 N, 12 C and 10 B in the energy range between 200 and 670 MeV/u were fragmented in thick targets of water, carbon, lucite, polyethylene, and aluminum. The nuclear charge composition of the fragmented beam was measured via energy loss in a large-area ionisation chamber. Using a water absorber of variable thickness up to 25 cm the elemental fragment yields were measured down to Z = 5. Their build-up and decay characteristics are described by a system of differential equations. From an analysis of the depth distributions of the surviving projectiles and the lower- Z projectile fragments, both total and partial charge-changing cross sections were obtained.

The “ Q factor” method: quantitative microPIXE analysis using RBS normalisation

Abstract: The main uncertainties in the absolute quantitation of PIXE elemental concentrations are the total deposited beam charge and the local matrix composition. This is particularly true in the case of microbeam PIXE analysis, where local inhomogeneities in the sample may affect both the secondary electron emission characteristics (and hence the recorded charge) and the PIXE yield. The use of simultaneous RBS analysis can solve this problem in many cases. Provided the RBS spectrum can be well modelled, its shape gives information on the local matrix composition, while the total area depends on the total beam charge. The ratio between the true charge and the measured charge (the “ q factor”) can then be used to normalise the PIXE data. Once the system is calibrated, standarless accuracy of 5–10% can be achieved in most cases. The method is described together with its limitations. Examples of the use of the method are presented.

Etching threshold for ion tracks in polyimide

Abstract: Tracks of various heavy ions with energies up to 13 MeV/u were studied using the technique of selective chemical etching. It was found that for homogeneous track etching the energy loss of the ions has to surpass a threshold of about 450 eV/A. In a transition regime between 180 and 450 eV/A etching was possible but the mean diameter of the resulting pores showed a wider distribution than pores at higher energy losses. In order to describe this observation, the radial dose distribution was calculated using a Monte Carlo simulation code. Inhomogeneous etching is interpreted as due to the spatial fluctuations of the deposited energy along the ion path.

Elemental analysis by PIXE and other IBA techniques and their application to source fingerprinting of atmospheric fine particle pollution

Abstract: The PIXE technique in conjunction with PIGME, PESA and RBS has been used to routinely measure over 20 different elements present in fine particle atmospheric samples. PIXE provided data for selected elements from Al to U while the other techniques provided information on elements lighter than Al such as H, C, N, O, F and Na. Detection limits for the ion beam techniques on Teflon filter papers were typically between 0.02 and 0.2 μg/cm2 for a few minutes of accelerator running time. The multi-elemental capability of PIXE enabled us to use these 20 or so different elemental measurements to define fingerprints for various fine particle sources. These fingerprints included anthropogenic sources such as motor vehicles, industry and coal combustion as well as natural sources such as seaspray and soil.

Simple parametrization of cross-sections for nuclear transport studies up to the GeV range

Abstract: Simple and accurate parametrizations of nucleon-nucleon and pion-nucleon cross-sections used in nuclear transport codes up to the 1 GeV per nucleon range are presented.

Tracks of high energy heavy ions in solids

Abstract: The observation of tracks of high energy heavy ions produced in some materials was made with electron microscope in order to clarify the governing factors and mechanisms for track formation. Irradiation experiments were performed with specified and well collimated ion beams of 100 MeV class obtained from the tandem accelerator. Target specimens were evaporated thin films of germanium, silicon and titanium, and crystals of MoO3 and MoS2. The present experiments showed that there were threshold values in the energy deposition of ion to specimens for the track formations and that the size of the tracks depended on the energies deposited into the specimens. The correlations between the sizes of the tracks and the energy depositions of incident ions were examined for the tracks produced in the evaporated films of germanium, and the results were interpreted in terms of a time dependent line source model of thermal spike. Even in such a high energy region, tracks produced through nuclear collision process were observed under some conditions.

The new ATOMKI scanning proton microprobe

Abstract: The scanning proton microprobe of the institute of nuclear research (ATOMKI) in Debrecen is based on an Oxford Microbeams coupled quadrupole doublet focussing system built up to the 0° beam line of the 5 MV single ended Van de Graaff accelerator. With the setup described here PIXE analysis, Rutherford backscattering spectrometry and detection of backscattered secondary electrons can be carried out simultaneously. A modified version of the PIXYKLM computer program developed in the institute is used for the evaluation of the PIXE spectra. Applications discussed are related to investigations performed on metal archaeological samples and individual aerosol particles.

Gamma-ray attenuation coefficients in some heavy metal oxide borate glasses at 662 keV

Abstract: The linear attenuation coefficient (μ) and mass attenuation coefficients ( μ ϱ ) of glasses in three systems: xPbO(1 − x)B2O3, 0.25PbO · xCdO(0.75 − x)B2O3 and xBi2O3(1 − x)B2O3 were measured at 662 keV. Appreciable variations were noted in the attenuation coefficients due to changes in the chemical composition of glasses. In addition to this, absorption cross-sections per atom were also calculated. A comparison of shielding properties of these glasses with standard shielding materials like lead, lead glass and concrete has proven that these glasses have a potential application as transparent radiation shielding.

Algorithm for fitting XRF, SEM and PIXE X-ray spectra backgrounds

Abstract: Existing background fitting of energy dispersive X-ray spectra (X-ray, electron or proton excited) algorithms (e.g. parabolas or polynomials) are not universally applicable. Various limitations exist tying each algorithm to specific spectral shapes. This project aimed to find a generic algorithm which could reliably fit background independent of X-ray spectra source and form. A frequency differentiated non-linear digital filter “rolling ball” was chosen as a promising possibility. The algorithm is based on a square wave with an energy variable width as the structuring function, operated on by a gray scale morphological transformation. A wide range of spectra types have been successfully fitted. This algorithm behaves equivalently to traditional polynomial backgrounds in simple spectra e.g. PIXE aerosol spectra with funny filter, and is considerably more robust for multiple overlapping peaks, rapidly varying backgrounds, and is independent of X-ray energy filter or excitation mode.

Beams of short lived nuclei produced by selective laser ionization in a gas cell

Abstract: An on-line laser ion source has been developed for the production of elemental and isobaric pure beams of radioactive ions. It is based on selective resonant laser ionization of nuclear reaction products thermalized and neutralized in a noble gas at high pressure. The ion source has been tested in a wide range of recoil energies going from 1.3 MeV to ∼ 90 MeV. Efficient schemes of two step laser ionization through autoionizing states have been found for nickel, cobalt and rhodium. Residence times of the reaction products in a gas cell have been measured for helium and argon as buffer gas. Elementally pure beams of 54,55Ni and 54Co, produced in a light-ion induced fusion-evaporation reaction, and of 113Rh, produced in proton-induced fission of 238U, were obtained. An efficiency of the ion source of 6.6% for fusion reactions and of 0.22% for fission reactions has been obtained. A selectivity of the ion source of 300 for fusion and 50 for fission reactions has been achieved.

Electronic excitation and structural stability of solids

Abstract: Passing through solid matter swift heavy ions generate a high density of electronic excitations. Thus a large fraction of the valence electrons are removed from valence band states of insulators or semiconductors and put into conduction band states. Typically, the valence band is of bonding character and the conduction band has antibonding character; then the electronic excitations weaken the covalent bonds and cause a repulsive force between the atoms. An instability of atomic structure thus arises, which occurs on a femtosecond timescale. The atoms reach high kinetic energies and the atomic structure is destroyed. This mechanism is distinct and complementary to the Coulomb explosion and thermal spike models for the damage done by swift heavy ions. It should be especially important for materials with a competing high pressure phase or where excitons induce lattice defects.

The irradiation-induced crystalline-to-amorphous phase transition in α-SiC

Abstract: The ion-beam-induced crystalline-to-amorphous phase transition in single crystal α-SiC has been studied as a function of irradiation temperature. The evolution of the amorphous state has been followed in situ in specimens irradiated with 1.5 MeV Xe+ ions over the temperature range from 20 to 475 K using the HVEM-tandem facility at Argonne National Laboratory. Specimens also have been irradiated at 170 and 370 K with 360 keV Ar+ ions, and the damage accumulation process followed in situ by Rutherford backscattering spectroscopy in channeling geometry using dual-beam facilities at Los Alamos National Laboratory. The displacement dose for complete amorphization in α-SiC at 20 K is 0.25 dpa and increases with temperature in two stages. The simultaneous recovery process associated with the high-temperature stage (above 100 K) has an activation energy of 0.12±0.02 eV. The critical temperature above which amorphization does not occur is 485 K under these irradiation conditions. Dechanneling results are consistent with disordering occurring by the local accumulation of defects and show a decrease in disordering rate with increasing temperature.

The formation of clusters during ion induced sputtering of metals

Abstract: The formation of neutral clusters during sputtering of polycrystalline Ag, Al, Nb and Ta surfaces was studied experimentally by non-resonant single photon post-ionization using a UV or VUV laser beam and time-of-flight mass spectrometry. From the laser intensity dependence of the resulting mass spectra, photoionization cross-sections as well as the yields of sputtered clusters are determined as a function of the cluster size. It is shown that in all cases investigated so far the cluster yields roughly exhibit a power law dependence on the cluster size, the exponent of which is found to be inversely correlated with the sputtering yield of the sample. This finding is of particular importance, since it rules out simple statistical combination models to describe the formation of large sputtered clusters. From the yield distributions it is inferred that — depending on the sputtering conditions — up to 46% of the sputtered atoms may be emitted in a bound state. The experimental results are briefly compared with theoretical model descriptions of the cluster emission/formation process available in the literature. It is found that only Molecular Dynamics computer simulations using modern many body potentials are able to reproduce the experimental findings.

Monte Carlo simulation of 0.1–100 keV electron and positron transport in solids using optical data and partial wave methods

Abstract: A new Monte Carlo code for the detailed simulation of the transport of low-energy electrons and positrons in solids is presented, including a critical discussion of concepts and approximations in the scattering model. Inelastic scattering is calculated using a Bethe surface model based on optical and photoelectric data for the solid, making possible a good accuracy at low energies, and a high resolution (∼1 eV) in simulated energy loss spectra. Exchange corrections for electrons and relativistic corrections for energies up to ∼100 keV are included. Elastic scattering is calculated by means of a differential cross section obtained by relativistic partial wave analysis for an exchange corrected muffin-tin Dirac-Hartree-Slater atomic potential. In the simulation, no adjustments of parameters to empirical scattering data are made. For comparison, measurements have been made of the characteristic low energy loss spectrum of 100 keV electrons through a thin silicon film. Simulated results for electrons and positrons are also compared with other available experimental data, in particular at low (a few keV) energies. In general, very good agreement is obtained.