Light Ion Impact Research Articles

Inner-shell ionisation of Xe[formula omitted]–Au and Pb collision systems: MO picture

The generation of collision-induced vacancies and their transfer within the electronic inner shells are the source of several unprecedented phenomena, which have been revealed in recent experimental studies conducted using state-of-the-art next-generation accelerators and detectors. Despite the multitude of studies conducted over the years, our understanding of the atomic vacancy transfer mechanisms remains exiguous. The detection and analysis of collision-induced X-rays is a widely-used powerful technique for analysing atomic-scale phenomena. However, both experimental and theoretical investigations on ion–atom collision-induced X-rays have remained mostly restricted to high-energy light-ion impact on heavy atoms. To date, only a few studies have been reported on the investigation of very-low-energy heavy-ion impact on heavy atoms using X-rays, especially M-shell X-rays of the heavy atom. This study was conducted to evaluate the inner-shell ionisation of Au (135 and 508 μg/cm2) and Pb (107, 157 and 390 μg/cm2) targets due to low-energy (2–5 MeV) 54Xeq+-ion impact. The collision-induced X-ray spectra of both the collision partners were examined, and the corresponding intensity ratios and cross-sections were compared with those calculated using two well-known ionisation theories, viz. PWBA and ECPSSR. The measured cross-sections were underestimated by these theories. Moreover, the cross-sections measured in this study were found to be several orders of magnitude higher than those obtained in other reported studies on low-energy lighter-ion-impact on Au and Pb. The significant discrepancies observed between the experimental and theoretical values have been explained qualitatively within the framework of quasi-molecular orbital formation using level correlations. The insights obtained from the results of this study can be applied to analyse the vacancy transfer mechanisms in very-heavy systems (combined atomic number > 100).

Theoretical multiple-ionization cross sections of Ne-like molecules by light-ion impact: H2O

We present a simple theoretical model to calculate multiple-ionization cross sections (MICS) of Ne and Ne-like molecules (molecules with ten electrons as ${\mathrm{H}}_{2}\mathrm{O}, \mathrm{C}{\mathrm{H}}_{4}, \mathrm{N}{\mathrm{H}}_{3}$, and HF) for proton and light-ion impact, taking into account both direct and postcollisional emissions. In this paper, we tackle the case of water molecules, relevant to investigate the radiobiological effects of ion impact on living matter. The theory is developed in the framework of the independent electron model (IEM). To keep the model as simple as possible, we describe the impact parameter dependence of the single-particle ionization probabilities required by the IEM through decreasing exponential functions for each target orbital. We obtain the parameters of the exponential functions from the total net-ionization cross sections for each orbital by applying either the continuum distorted wave-eikonal initial-state (CDW-EIS) approximation or Rudd's model. We then calculate the contribution of Auger postcollisional electron emission to MICS by using the Ne postcollisional emission probabilities. This postcollisional treatment offers a very simple alternative to the much more complex molecular evaluation of postcollisional relaxation and provides results in close agreement with experimental data for proton and other light ions. We also demonstrate the relevance of considering postcollisional emission for water molecules after the $2{a}_{1}$ ($2s$ Ne-like) orbital direct ionization.

Experimental cross sections for water ionization due to the impact of light ions—A review

This work is a short review on the experimental cross sections for water ionization due to the impact of light ions (Z ≤ 6), including related processes such as electron capture and water molecule fragmentation. These cross sections are crucial in studies dealing with the biological effects produced by the impact of ions on living beings. Physical interpretations of the experimental results are also provided.

State-selective charge transfer cross sections for light ion impact of atomic hydrogen

Owing to the utility of diagnosing plasma properties such as impurity concentration and spatial distribution, and plasma temperature and rotation, by detection of photon emission following capture of electrons from atomic hydrogen to excited states of multiply charged ions, new calculations of state-selective charge transfer involving light ions have been carried out using the atomic orbital close-coupling and the classical trajectory Monte Carlo methods. By comparing these with results of other approaches applicable in a lower impact energy regime, and by benchmarking them using key experimental data, knowledge of the cross sections can be made available across the range parameters needed by fusion plasma diagnostics.

PIXE from thin films and amorphous alloys induced by medium energy heavy ions

Characteristic X-rays emitted under impact of fast light ions with surfaces (PIXE) provide information not only on atomic excitation and further recombination processes but also on elemental composition and dynamics of restructuration of the surface.In this work radiation emitted during interaction of medium energy (∼200keV) heavy ions (Ar, N) with Si (110) surface and with Fe/Si and Fe/Cu/Si thin (1–50nm) films in grazing incidence-exit angle geometry were measured in time sequence in order to show that dynamics of selective modification of surface structure and composition can be monitored in-situ with PIXE. It is shown that surfaces of amorphous alloys are not stable against heavy ions (HI) irradiation due to preferential sputtering and implantation and that the dynamics of such modification can also be monitored with PIXE. The method is used for example to find detection limit for implanted Ar ions.

Theoretical investigations of electron emission after water vapour ionization by light ion impact

An ab initio quantum-mechanical treatment is applied for treating the ionization process of water vapour by light ions. In this theoretical model, the initial state of the system is composed of a projectile and a water target described by a plane wave and an accurate one-centre molecular wave function, respectively, whereas the final state is constituted by a slow ejected electron and a scattered projectile represented by a Coulomb wave and a plane wave, respectively. The obtained results are compared to available experimental data in terms of doubly differential cross sections ( DDCS), singly differential cross sections ( SDCS) and total cross sections ( TCS). A good agreement is generally found especially for the SDCS and the TCS.

Ion–Atom/Argon—Calculation of ionization cross sections by fast ion impact for neutral target atoms ranging from hydrogen to argon

A FORTRAN 90 program is presented which calculates the total cross sections, and the electron energy spectra of the singly and doubly differential cross sections for the single target ionization of neutral atoms ranging from hydrogen up to and including argon. The code is applicable for the case of both high and low Z projectile impact in fast ion–atom collisions. The theoretical models provided for the program user are based on two quantum mechanical approximations which have proved to be very successful in the study of ionization in ion–atom collisions. These are the continuum-distorted-wave (CDW) and continuum-distorted-wave eikonal-initial-state (CDW-EIS) approximations. The codes presented here extend previously published codes for single ionization of target hydrogen [Crothers and McCartney, Comput. Phys. Commun. 72 (1992) 288], target helium [Nesbitt, O'Rourke and Crothers, Comput. Phys. Commun. 114 (1998) 385] and target atoms ranging from lithium to neon [O'Rourke, McSherry and Crothers, Comput. Phys. Commun. 131 (2000) 129]. Cross sections for all of these target atoms may be obtained as limiting cases from the present code. Program summary Title of program: ARGON Catalogue identifier: ADSE Program summary URL: http://cpc.cs.qub.ac.uk/cpc/summaries/ADSE Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Licensing provisions: none Computer for which the program is designed and others on which it is operable: Computers: Four by 200 MHz Pro Pentium Linux server, DEC Alpha 21164; Four by 400 MHz Pentium 2 Xeon 450 Linux server, IBM SP2 and SUN Enterprise 3500 Installations: Queen's University, Belfast Operating systems under which the program has been tested: Red-hat Linux 5.2, Digital UNIX Version 4.0d, AIX, Solaris SunOS 5.7 Compilers: PGI workstations, DEC CAMPUS Programming language used: FORTRAN 90 with MPI directives No. of bits in a word: 64, except on Linux servers 32 Number of processors used: any number Has the code been vectorized or parallelized?: Parallelized using MPI No. of bytes in distributed program, including test data, etc.: 32 189 Distribution format: tar gzip file Keywords: Single ionization, cross sections, continuum-distorted-wave model, continuum-distorted-wave eikonal-initial-state model, target atoms, wave treatment Nature of physical problem: The code calculates total, and differential cross sections for the single ionization of target atoms ranging from hydrogen up to and including argon by both light and heavy ion impact. Method of solution: ARGON allows the user to calculate the cross sections using either the CDW or CDW-EIS [J. Phys. B 16 (1983) 3229] models within the wave treatment. Restrictions on the complexity of the program: Both the CDW and CDW-EIS models are two-state perturbative approximations. Typical running time: Times vary according to input data and number of processors. For one processor the test input data for double differential cross sections (40 points) took less than one second, whereas the test input for total cross sections (20 points) took 32 minutes. Unusual features of the program: none

Cw and pulsed EPR study of lithium irradiated n-type 21R SiC

The impact of light and medium mass ions in crystals in the MeV range is of particular interest in high energy implantations. In the present work, extensive continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) studies of a 21R SiC Lely platelet, after irradiation with 8 MeV 7Li2+ ions in the random direction, up to a maximum dose of approximately 1 × 1016 particles/cm2 are presented. The existence of new types of defects induced in the end-of-range region of impinging ions is discussed and analyzed. Due to the complexity of the induced structure, the technique of progressive annealing was utilized, revealing interesting features in the experimental spectra. The results are compared to known literature and an attempt is made to explain the occurring similarities. Furthermore, a new paramagnetic defect was isolated and analyzed, persisting up to 1100 °C during the annealing procedure.

Ion-induced electron emission from diamond

The electron emission yields (γ) from conductive (B doped) and undoped chemical vapor deposited diamond caused by light (protons) and heavy (argon) ion impact were measured as functions of ion dose and energy (40–300 keV). Very large values of γ are obtained for the case of B doped diamond for both ions. Whereas the emission due to Ar is found to decay very rapidly towards the low γ value measured for graphite, it remains persistently very high (γ∼25) for the case of protons, indicating possible application of diamond as a sensitive detector for light ions.

Comparison between theory and experiments for the electron emission statistics induced by protons incident on thin carbon foils

Recently, measurements on the statistics of the number of electrons emitted due to MeV light ion impact on solid targets have been fitted adequately by negative binomial distributions. We present here Monte Carlo simulations for backward and forward emissions induced by protons incident on thin carbon foils in the energy range 250 keV –2 MeV, and compare them with experimental results. A general discussion on the statistical distribution of the number of emitted electrons is given. A transport model giving the general form of the probability generating function (pgf) of the distribution of the number of emitted electrons is also proposed.

Electron emission yield for impact of swift bare light ions on metals

The electron emission yield induced by 0.5–8 MeV H +, He 2+, Li 3+, B 5+ and C 6+ impact on aluminum and gold targets was measured. The stopping power of bare ions is investigated. Stopping power values calculated according to the Bethe term with and without Barkas and Bloch corrections and to the Z 2 scaling of hydrogen values are compared. The relation of electron emission yield and the stopping power is explored. A significant deviation of a proportionality of the yield to the stopping power is found. Using a — slightly modified — model by Borovsky and Suszcynsky [Phys. Rev. A 43 (1991) 1433] the experiments could be well represented. In this model the collective electric field generated along the projectile's path was taken into account. Consequently, the positive ion channel appears to be the dominant mechanism that leads beyond a projectile independent yield/stopping power ratio.

Electron-emission yield of Al, Cu, and Au for the impact of swift bare light ions.

The electron emission yield induced by mega-electron-volt ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, ${\mathrm{B}}^{5+}$, and ${\mathrm{C}}^{6+}$ impact on aluminum, copper, and gold targets was measured. We found a significant deviation of the results from a simple proportionality to the stopping power, especially for heavier ions and low projectile velocities. Using a slightly modified model by J. E. Borovsky and D. M. Suszcynsky [Phys. Rev. A 43, 1433 (1991)] our experiments could be well represented. In this model the collective electric field generated along the projectile's path was taken into account. Consequently, the positive ion channel appears to be the dominant mechanism that leads beyond a projectile-independent yield--to--stopping power ratio.

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.

Monte Carlo study of incident-angle dependence of ion-induced kinetic electron emission from solids

The incident angle dependence of ion-induced kinetic electron emission (KEE) from solids is calculated using a Monte Carlo simulation of the transport of incident ions and recoiling target atoms, and a semi-empirical theory of KEE. The emphasis is put on the origin of the deviation from the inverse cosine law. The effect of the high-energy recoiling target atoms on the deviation is much greater than that of the trajectory distribution and backscattering of the incident ions, except for light and low-energy ion impact. The positive (negative) contribution of the recoiling target atoms to the incident angle dependence is dominant for high (low) impact energy. The deviation can be explained by the increase in the electron excitation by recoiling target atoms localized near the surface and the increase in the emission of the atoms, i.e., sputtering.

Present status of the experimental L-shell ionization cross sections for light ion impact

Compilations of all available experimental L-shell X-ray production and ionization cross sections for proton and light ion impact have recently been produced by the group from the National University of Singapore (I. Orlic, C.H. Sow and S.M. Tang, At. Data Nucl. Data Tables 57, in press; I. Orlic, Experimental L-shell x-ray production and ionization cross sections for light ion impact, internal report, National University of Singapore, Department of Physics, 1993 [1,2]). Together with the previous tabulations more than 9700 experimental data points for proton impact and approximately the same number of data points for heavier ion impact have become available. An overview of all available experimental data is given in this work. Discussed are annual growth and decline of published data, distribution of L-shell cross section data vs. target atomic number as well as distribution of number of data vs. incident ions. Experimental data are compared with the corresponding ECPSSR predictions in the usual manner: plotting the ratio S = σ exper./ σ theory vs. reduced velocity parameter for each individual subshell L1, L2 and L3 separately for proton and heavier ion impact. Mean experimental cross sections are obtained using the statistical procedure recommended by the Particle Data Group (Particle Data Group, Phys. Lett. B 204 (1988) 5 [3]). Statistical errors of mean values are significantly smaller than errors of individual experimental results allowing for some generalization. A brief review of the obtained results is presented.

Delta-electron emission in fast heavy ion ? atom collisions: observations of new phenomena and breakdown of common scaling laws

Double differential cross sections for the emission of Delta-electrons have been measured in fast uranium-rare gas collisions. The well-known Binary Encounter peak reveals unexpected structures for certain observation angles and its intensity increases towards smaller angles, which is in contradiction to results and scaling laws obtained by experiments with light ion impact. The observed dependencies are fairly well described by recent calculations in the framework of IA and CTMC. From systematic experimental as well as theoretical studies we can derive that the potential of the partially stripped projectile ion gives rise to rainbow and glory scattering of the target electron in the field of the projectile. The rainbow scattering is observed in the laboratory frame as pronounced interference structures, whereas the glory scattering is responsible for the steep increase of the cross sections for binary-encounter electrons towards small laboratory ejection angles. The observed effects have a dramatic influence on the commonq2 scaling laws derived from experiments with light ions. Furthermore, since the binary-encounter electrons ejected at forward angles have approximately twice the projectile velocity, these new phenomena have an important influence on the electronic stopping power of heavy ions and therefore have to be taken into account for the investigation of radiation damage by these ions e.g. in biological matter.

Calculation of Incident Angle Dependence of Ion-Induced Kinetic Electron Emission from Aluminum

The Monte Carlo simulation of transport of incident ions and recoiling target atoms is combined with the semiempirical theory of ion-induced kinetic electron emission (KEE) for calculating the incident angle dependence of the KEE yields. The calculation was made for light and heavy ion impact on Al in the energy range from 100 eV to 1 MeV. The present results are consistent with the experimental data and have revealed the origin of the deviation from the inverse cosine law, which corresponds to the contributions from backscattering ions and recoiling target atoms.

Li Overlayer Formation, Oxidation and Sputtering Characteristics of Al-Li Alloys and W/Al-Li Composites for Fusion Applications

The next generation of long pulse fusion devices will impose severe requirements on the properties of plasma-facing materials. In devices such as ITER, a divertor design is being considered, using a divertor plate which would be either tungsten or a low-Z material such as graphite or beryllium. Graphite and beryllium have a relatively high light ion erosion rate. Tungsten has a much lower sputtering rate for light ion impact, but it is subject to runaway self-sputtering. Because of its limited thermal conductivity, it must be used as a relatively thin plate which might be subject to damage during a disruption. Strongly segregating lithium alloys have been proposed as a means of producing a self-sustaining low-Z overlayer which lowers plasma Zeff and resists self-sputtering. Aluminum-lithium alloys are among the better-characterized lithium-bearing alloys, and it has been demonstrated that lithium segregates strongly in aluminum. However, aluminum has a relatively low melting point, and for low lithium concentrations, the lithium diffusion rate is too slow to replenish lithium at the rate at which it is eroded by the incoming plasma. It has been suggested previously that the β phase Al-Li alloy (48–54 at.% Li) should have high enough diffusivity to be able to replenish surface lithium, and that incorporation of the β-phase AlLi in a composite with tungsten would provide improved high temperature strength and melt layer stability, along with significantly better thermal conductivity than pure tungsten. Such a composite has been fabricated, as well as a variation containing titanium as a means of controlling oxidation at grain boundaries. The Li overlayer formation, erosion, and replenishment are characterized for the β-phase LiAl alloy, and W-AlLi and W-Ti-AlLi composites. It is found that Li diffusion is extremely rapid, and the composites form an oxygen-free Li overlayer which is stable under continuous ion beam sputtering.

INNER-SHELL IONIZATION BY LIGHT-ION IMPACT

The ECPSSR theory to calculate the inner-shell ionization cross sections by light-ion impact is studied. Starting from the general formulation of the plane-wave Born approximation, the corrections for the energy-loss effect (E) and the Coulomb-deflection effect (C) of the projectile, and for the binding-polarization effect in the perturbed-stationary-state approach (PSS) and the electronic relativistic effect (R) of the target electron are described. The calculated values in the ECPSSR theory are compared with the experimental data. In general, agreement is good, but there are systematic deviations for low-energy projectiles. The comparison with other theoretical treatments for the inner-shell ionization is discussed.

L3 subshell alignment of U by light-ion impact

The alignment induced in the L3 subshell of U by proton and alpha-particle impact in the specific energy range 0.35–1.00 MeV/u has been determined from the angular distribution of the L1 line relative to the beam direction. The results of the anisotropy parameter, 2 as a function of υ1/υl3 (where υ1 and υL3are the projectile and the L3-electron velocities, respectively) show the minimum predicted by the theory, due to the projectile deflection in the target Coulomb field. For protons, these alignment results are in good agreement with the values previously obtained for gold, showing the validity of υ1/υL3as a universal parameter, in spite of the Coulomb deflection effect. For alpha-particles, our results seem to question this universality. There is a fair agreement with theory based on the second-order Born approximation.