Argon Projectiles Research Articles

Mechanisms of particle ejection from free-standing two-layered graphene stimulated by keV argon gas cluster projectile bombardment – Molecular dynamics study

Molecular dynamics computer simulations are employed to investigate processes leading to particle ejection from free-standing two-layered graphene irradiated by keV argon gas cluster projectiles. The effect of the primary kinetic energy and the projectile size on the ejection process is investigated. It has been found that both these parameters have a pronounced influence on the emission of particles. The interaction between argon projectiles and graphene is strong regardless of graphene's minimal thickness. A significant portion of the primary kinetic energy is deposited into the sample. Part of this energy is used for particle emission, which is substantial. As a result, circular nanopores of various dimensions are created depending on the bombardment conditions. A major part of the deposited energy is also dispersed in a form of acoustic waves. Different mechanisms leading to particle ejection and defect formation are identified depending on the projectile energy per atom. The implications of the results to a novel analytical approach in Secondary Ion Mass Spectrometry based on ultrathin free-standing graphene substrates and a transmission geometry are discussed.

Multiwalled carbon nanotubes as masks against carbon and argon irradiation. A molecular dynamics study

Experiments showed that multiwalled carbon nanotubes (MWCNT) can be used as masks against irradiation to create metallic nanowires in a substrate. In order to understand the limitations of this application, it is interesting to know the energy and number of carbon atoms emerging from the MWCNT after the irradiation and how the structure of the MWCNT is modified. Using a molecular dynamics code that we have previously developed, we have simulated the continuous irradiation of MWCNT with carbon and argon projectiles. We have obtained that the use of carbon instead of argon to irradiate the MWCNT increases de effectiveness of the MWCNTs as masks, due to the ability of the carbon projectiles to be part of the MWCNT structure and partially mend the damage produced during irradiation.We have analyzed the number, energy, and spatial distribution of the recoils generated during irradiation and the change of the MWCNT structure as a function of the incident energy (100 and 500eV), fluence (up to 4.5·1015ions/cm2), and number of shells (up to 5-shells) of the MWCNT. These results determine the effectiveness of MWCNT as a mask, being useful to understand whether the atoms emerging from the MWCNT produce damage in the substrate or not.We find that for carbon projectiles the efficiency of MWCNT as masks does not depend much on the fluence, but on the number of nanotube shells and projectile incident energy. On the other hand, for a given nanotube and fluence, we observe a threshold incident energy below which the nanotube acts as a perfect mask.

Using a current method for measuring ion-induced electron emission from LiF

The interaction of ions with an insulating substrate leads to surface charging. For intense ion beams these charges are not sufficiently fast removed between successive ion impact events. As a result the trajectories of the incident beam and the electron emission yield may be altered in a hardly predictable way. We demonstrate, that by heating an alkali-halide sample to a sufficiently high temperature, this macroscopic charging can be avoided and a simple current method can be used to study electron emission from such a surface. Measured electron emission yields from a LiF(1 0 0) single crystal due to impact of singly and multiply charged heavy noble gas ions (argon and xenon) are presented. For argon projectiles our results compare well with previous data, which were obtained using a more sophisticated electron statistics method and six orders of magnitude less intense ion beams.

Ion-induced erosion of tungsten surfaces studied by a sensitive quartz-crystal-microbalance technique

A highly sensitive quartz-crystal-microbalance (QCM) technique was used to study erosion of polycrystalline tungsten films due to impact of deuterium, carbon and argon ions, as well as retention of deuterium in these films. Polycrystalline tungsten films coated onto a SC-cut quartz crystal were bombarded by ions with impact energies from 100 eV up to a few keV and the frequency change due to mass loss (sputtering, desorption) or mass gain (implantation, adsorption) during bombardment was determined. Our setup was capable of detecting mass-changes as small as 10 −5 μg/s, which corresponds to a removal (or deposition) of only 10 −4 W monolayers/s. While our total sputtering yields for deuterium and argon projectiles compare well with the results of previous work, we derive new data on sputtering of tungsten by carbon ions. In addition we demonstrate that our setup is well suited for determining deuterium retention rates in tungsten.

Collision-Induced Dissociation of Water into Ions

A modification of the central force model (CFM) that describes the dissociation of water molecules into OH- and H+ ions is proposed for molecular dynamics simulations of energetic particle bombardment of water ice. The model keeps all the properties of the CFM but permits charge exchange between oxygen and hydrogen atoms when the water molecule starts to dissociate after collision with an energetic projectile. The reaction products, therefore, have the correct integer charges, -1 and +1 for hydroxyl and a proton. The threshold for the ionic dissociation is corrected to be at the right value, 17.2 eV, in a vacuum. Using the proposed model, total cross-sections for ionic dissociation as functions of the projectile energy are estimated for Ar and C projectiles colliding with water molecules in a vacuum and water ice. Carbon projectiles are demonstrated to produce more dissociated ions at energies lower than 300 eV. Argon projectiles are more effective in breaking the molecules at higher energies.

Activation of thick targets by energetic heavy ions and the resultant radiation levels.

This study completes data collected for thick targets exposed to carbon and oxygen ions accelerated at 86 MeV/u. The radioactivity induced in carbon and tungsten targets bombarded by argon projectiles at 95 MeV/u has been studied in order to assess the relative contributions of the incoming heavy ion and the mass number of the bombarded nuclei to the consequent radiation hazards related to the production of radioactive ion beams. Induced radioactivity measurements are only rarely made under controlled irradiation conditions, in order to derive from the measured activities the dose rates after beam bombardment and a prediction of radiation protection constraints.

Binary encounter electron emission in collisions of highly charged ions with helium gas targets

The energy and angular distribution of δ-electrons produced in collisions of bare argon projectiles with helium gas targets are reported. Double differential electron emission cross sections, d 2 σ dE e dΩ e , have been measured as a function of the electron emission energy, E e, and the polar electron emission angle, V e. The major interest was directed upon the angular dependence of the integrated single differential emission cross sections of the binary encounter (BE) electrons, dσ(Θ e ) dΩ e . The cross sections are compared with electron argon scattering data (calculated and tabulated in Phys. Rev. A 15 (1977) 2173 [1]) as well as experimental data derived from light ion impact on helium (ionic charge q≤9+). The theoretical results are calculated with the impulse approximation (IA) model developed by Lee et al. (Phys. Rev. A 41 (1990) 4816 [2]). Even for higher ionic charge states the measured BE electron emission cross sections are well described by the IA model and are found to scale quadratically with the ionic charge state, q 2, and the projectile energy, E p 2.

Measurement of emission-angle-resolved time-of-flight distributions of sputtered Mg + ions

We describe an experiment designed to perform emission-angle-resolved time-of-flight (TOF) analysis of sputter-ejected secondary ions as well as of laser-postionized sputtered neutral particles. The apparatus includes a rotatable linear TOF mass spectrometer and an electron-cyclotron-resonance (ECR) plasma ion source for the bombardment of the target with a mass-filtered Ar + ion beam pulse in the keV energy region (ion pulse width about 30 ns). We present emission-angle-resolved time-of-flight spectra of Mg + secondary ions sputter-ejected from a polycrystalline 24Mg target bombarded at 45° incidence. These TOF spectra also contain additional information on the emission-angle dependence of high energy argon projectiles scattered from the target surface.

Optical model analyses of 1.65 A GeV argon fragmentation: cross sections and momentum distributions.

An optical potential fragmentation model capable of predicting fragmentation cross sections and fragment momentum distributions is used to analyze recent measurements of 1.65 A GeV argon projectiles fragmenting in carbon and potassium-chloride targets obtained with the Heavy Ion Spectrometer System (HISS) at the Lawrence Berkeley Laboratory Bevalac. The theoretical model uses an abrasion-ablation-FSI (frictional spectator interaction) collision formalism to estimate elemental and isotopic production cross sections for comparison with the measured values. The collision momentum transfer model is incorporated into a Goldhaber formalism to analyze measured transverse and longitudinal distributions of the projectile fragments. Good agreement between theory and experiment is obtained for all observables.

Velocity dependence of electron-capture partial cross sections and alignment in low-energy collisions of Ne8+ and Ar8+ with atomic Na.

We have measured the intensities and polarizations of fine-structure lines in the N=9-8 manifolds of Ne viii and Ar viii spectra produced in collisions of ${\mathrm{Ne}}^{8+}$ and ${\mathrm{Ar}}^{8+}$ (v=0.05--0.15 a.u.) with atomic sodium, and determined alignment and partial cross sections for electron capture as functions of projectile velocity. At low velocity the partial cross section for capture into the S levels is significantly higher than for capture into higher L levels, and at higher velocities the higher-L partial cross sections increase for Ar viii and dominate for Ne viii. The low-L-level cross sections are in fair agreement with multichannel-Landau-Zener calculations, while the high-L-level cross sections are in qualitative disagreement. For the neon projectiles, the polarization of the Bohr line is approximately 0.3 and independent of velocity; for argon projectiles, the polarization is approximately 0.15 and increases with increasing velocity. A qualitative theoretical interpretation is offered.

Deexcitation of nuclei formed near the instability temperature.

Fusion-like reactions induced on medium mass targets by 27 MeV per nucleon argon projectiles were studied. The properties of evaporation residues and binary fission fragments, both cold remnants of fusion nuclei, show that highly excited nuclei were produced, near the temperature of instability of nuclear matter. Fission-evaporation competition in the deexcitation of these nuclei is reflected in the ratio of fission and residue cross sections, which provides a way of studying the role of prefission evaporation and fission barriers in the deexcitation process.

An experimental apparatus for translational energy spectroscopy for low energy X q+ on atomic hydrogen

The design and performance of a system for performing translational energy spectroscopy on atomic hydrogen targets is presented. Low energy highly-charged ions are produced in a recoil ion source through collisions between target gases and a heavy, fast pump beam from the EN tandem accelerator. These ions are directed through a double-focusing magnetic spectrometer which selects the desired projectile charge state. An atomic hydrogen target is produced in a thermal dissociation oven. The energy and final charge states of the product ions are determined by a double-focusing, hemispherical electrostatic analyzer. The current work concentrates on the change in energy of the projectile ions accompanying electron capture for 250 eV per charge argon projectiles.

Shape of the electron capture to the continuum cusps for H, H2, and He targets in the velocity range 6.3-18.0 a.u.

A general purpose projectile-frame expansion fitting method is used to characterize the shape of the electron capture to the continuum (ECC) cusp for bare carbon, oxygen, neon, and argon projectiles, incident on atomic and molecular hydrogen, and helium targets, over the velocity range 6--18 a.u. The method used to fit the ECC cusp shape allows the direct comparison of spectra taken under differing instrumental conditions in an apparatus-independent manner. The results reveal a substantial similarity in the ECC cusp shape for a wide variety of projectile Z, projectile velocity, and target combinations which has not yet been explained by current theories which predict the shape of the ECC cusp.

Channelling of 2.4 GeV Ar ions in a germanium crystal

We present the first experimental channelling study with very energetic heavy ions. This work, performed with 60 MeV/u argon projectiles bombarding a 100 μm thick germanium crystal, aimed at determining the experimental conditions and techniques suitable to the observation of channelling effects with such projectiles. We show that the detection of forward emitted gamma-rays, neutrons and beam fragmentation products allows us to get the crystal aligned and to measure the channelling parameters, χmin and ψ1/2, which we find to be in reasonable agreement with theoretical estimates.

Sharing of vacancies between closely matchedKandLshells

The sharing of vacancies in the $3d\ensuremath{\sigma}$ molecular orbital between the $K$ shells of chlorine and argon projectiles and $2{p}_{\frac{3}{2}}$ and $2{p}_{\frac{1}{2}}$ $L$ subshells of tin, antimony, and xenon targets has been studied. Projectile energies varied from 1 to 16 MeV. The vacancy-sharing ratios deduced from experimental results are interpreted in terms of a long-range exponential-coupling model.

Elastic and electronic stopping cross-sections for sodium and argon projectiles in carbon

Abstract The mean and the most probable stopping cross-section for 50- to 150-: eV sodium and 50- to 300-keV argon projectiles in carbon have been measured as functions of the foil thickness. The results are interpreted on the basis of results from a recent simulation study by Skoog. The various methods for separating the experimental stopping cross-section into an elastic and an electronic part are discussed. Both stopping cross-sections have been deduced from the experimental results.

Characteristic x-ray production by argon projectiles moving in graphite targets between 30 and 200 keV

Thick-target yields and x-ray spectra have been measured for argon ions impinging on graphite targets at energies between 30 and 200 keV. The thick-target yields of carbon $K$-shell and argon $L$-shell x rays are analyzed on the basis of a simple model involving multiple collisions and molecular-orbital promotions. The analysis results in average argon $L$-shell fluorescence yields and total decay rates for argon ions traversing the solid. These values are found to be substantially lower than those appropriate to argon ions traversing a gas target. An explanation for the differences is proposed, and found to be corroborated by analysis of the observed x-ray spectra. An approximate determination of the argon $M$-shell vacancy distribution is obtained and compared with beam-foil charge equilibrium measurements.