Shell Ionization Cross Sections Research Articles

Experimental verification of theoretical cross sections for FIB–PIXE

X-ray production cross sections were found for films of Cr, Cu, Ge, Ag, W and Au, using incident H+ and Be+ ions at energies from 300keV to 3.5MeV. These experimental cross section results were compared with the cross section results obtained using software which calculates inner shell ionization and X-ray production cross sections. The software uses the ECPSSR-UA approach to finding X-ray production cross sections. This program was found to be useful for predicting cross sections for H+ and Be+ ions at the energies in this study. The software was then used to predict results for Li+, Be+ and B+ ions at 280keV, energies available in the Arizona State University focused ion beam laboratory.

M-shell ionization of heavy elements by0.1–1.0MeV∕amuH1,2andHe3,4ions

The $M$-shell ionization in high-$Z$ atoms by low-energy light $_{1}^{1}\mathrm{H}$, $_{1}^{2}\mathrm{H}$, $_{2}^{3}\mathrm{He}$, and $_{2}^{4}\mathrm{He}$ ions have been studied systematically in the energy range $0.1--1.0\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\mathrm{amu}$ in order to verify the available theoretical approaches describing the $M$-shell ionization by charged particles in asymmetric collisions. The present low-energy data, combined with our earlier results reported for $M$-shell ionization by hydrogen and helium ions for higher energies, form a systematic experimental basis to test the theoretical predictions of $M$-shell ionization based on the plane-wave Born approximation (PWBA), the semiclassical approximation (SCA), and the binary-encounter approximation (BEA). In the PWBA based approaches the energy loss (E), Coulomb deflection (C), perturbed stationary state (PSS), and relativistic (R) effects were considered within the ECPSSR theory and its recent modification, called the ECUSAR theory, in which a description of the PSS effect was corrected to account for the united- and separated-atom (USA) electron binding energy limits. In the SCA calculations with relativistic wave functions the binding effect was included only in the limiting cases of separated-atom and united-atom limits. Possible contribution of the electron capture, multiple ionization, and recoil ionization to the $M$-shell vacancy production, which is dominated for light ions impact by direct single ionization process, are also discussed. The universal scaling of measured $M$-shell x-ray production and ionization cross sections was investigated in detail. Using the present data the isotopic effect has been studied by comparing the measured $M$-shell ionization cross-section ratios for equal-velocity hydrogen $_{1}^{1}\mathrm{H}$ and $_{1}^{2}\mathrm{H}$ as well as helium $_{2}^{3}\mathrm{He}$ and $_{2}^{4}\mathrm{He}$ isotopes. In addition, the ratios of measured ionization cross sections for $_{1}^{2}\mathrm{H}$ and $_{2}^{4}\mathrm{He}$ were used to investigate the role of the binding effect. The present results are of practical importance for the application of particle-induced x-ray emission technique in trace element studies.

Empirical model for the electron-impactK-shell-ionization cross sections

The total cross sections of electron-impact single-$K$-shell ionization of 14 atomic targets ranging from H to U $(1\ensuremath{\leqslant}Z\ensuremath{\leqslant}92)$ are calculated using a modified version of the BELI formula [Bell et al., J. Phys. Chem. Ref. Data 12, 891 (1983)] by incorporating both ionic and relativistic corrections in it. The proposed modified Bell model with a single set of parameters is found to provide an excellent description of the experimental data in the reduced energy range $1\ensuremath{\leqslant}E∕{I}_{K}\ensuremath{\leqslant}{10}^{6}$ ($E$ and ${I}_{K}$ are, respectively, the incident energy and ionization potential) with a performance level at least as good as any of the existing methods and models.

Double 1s shell ionization of Si induced in collisions with protons and heavy ions

The double 1s ionization of Si induced in collisions with protons and heavier ions (C, Ne) was studied by measuring the K X-ray emission of a solid Si target. In order to resolve the hypersatellite contributions in the spectra, high-resolution crystal diffractometry was employed yielding subelectronvolt energy resolution. Experimentally obtained hypersatellite yields were used to determine the double to single K shell ionization cross-section ratios σ KK/ σ K corresponding to the investigated collisions. The experimental ratios for collisions with heavy ions, where direct Coulomb ionization and electron capture need both to be considered, were compared to the theoretical values calculated within the independent electron approximation employing single electron ionization probabilities calculated by the three body classical trajectory Monte-Carlo (CTMC) method. For proton collisions where direct ionization solely contributes to 1s ionization the semiclassical approximation (SCA) was employed.

Measurement of Cu K-shell and Ag L-shell ionization cross sections by low-energy positron impact.

Inner shell ionization cross sections by low-energy positron impact have been measured. Development of an x-ray detector with thin Si(Li) crystals has enabled the first measurements of the absolute cross sections for the positron impacts in the energy range below 30 keV. Threshold behavior of the measured cross sections for the Cu K shell and Ag L shell are compared with the theoretical results of Gryzinski and Kowalski [Phys. Lett. A 183, 196 (1993)]] and Khare and Wadehra [Can. J. Phys. 74, 376 (1996)]]. Good agreement has been found for the Cu K shell, while the experimental values for the Ag L shell were found to be smaller than the corresponding theoretical results.

Measurements of absoluteK-shell ionization cross sections andL-shell x-ray production cross sections of Ge by electron impact

Results from measurements of absolute $K$-shell ionization cross sections and $L\ensuremath{\alpha}$ x-ray production cross sections of Ge by impact of electrons with kinetic energies ranging from the ionization threshold up to $40\phantom{\rule{0.3em}{0ex}}\text{keV}$ are presented. The cross sections were obtained by measuring $K\ensuremath{\alpha}$ and $L\ensuremath{\alpha}$ x-ray intensities emitted from ultrathin Ge films deposited onto self-supporting carbon backing films. Recorded x-ray intensities were converted to absolute cross sections by using estimated values of the sample thicknesses, the number of incident electrons, and the detector efficiency. Experimental data are compared with the results of widely used simple analytical formulas, with calculated cross sections obtained from the plane-wave and distorted-wave Born approximations and with experimental data from the literature.

Differential partial ionization cross sections for 10–24 keV electrons colliding withhelium andneon atoms

The relative differential partial ionization cross sections of helium and neon atomsfor 10–24 keV electron impact have been measured by using an ejectedelectron–ion coincidence technique. Measurements were accomplished byemploying a time-of-flight mass spectrometer for charge analysis of the ionsand a channel electron multiplier working in a pulse-counting mode fordetecting the slow electrons of non-discriminated energies ejected at 90°to the incident electron beam direction. The charge state fractionsFn of ions withcharge states nare found to be nearly invariant with the impact energy. The present resultsshow that the multiply charged ions of the target atoms are produced bydifferent mechanisms, namely by a direct multiple ionization or by inner shellionization followed by non-radiative transitions, for instance, by an Augerprocess combined with shake-off transitions. A good agreement is obtainedbetween our experimental results and the calculations based on inner shellionization cross sections folded with charge state abundances, which resultfrom the decay of the corresponding initial inner shell vacancies.

Accurate cross sections for microanalysis

To calculate the intensity of x-ray emission in electron beam microanalysis requires a knowledge of the energy distribution of the electrons in the solid, the energy variation of the ionization cross section of the relevant subshell, the fraction of ionizations events producing x rays of interest and the absorption coefficient of the x rays on the path to the detector. The theoretical predictions and experimental data available for ionization cross sections are limited mainly to K shells of a few elements. Results of systematic plane wave Born approximation calculations with exchange for K, L, and M shell ionization cross sections over the range of electron energies used in microanalysis are presented. Comparisons are made with experimental measurement for selected K shells and it is shown that the plane wave theory is not appropriate for overvoltages less than 2.5 V.

Ejectedelectron-ion coincidence measurements of multiple ionization ofargon by 10-24 keV electron impact

The charge state spectra of argon ions produced in singlecollisions by electrons having energies from 10 to 24 keVwith argon atoms have been measured by using the ejectedelectron-ion coincidence technique. Charge analysis of theions is accomplished by a time-of-flight mass spectrometer equipped with a channeltron while the ejected electrons aredetected by another channeltron, both being used in thepulse-counting mode. The ions and the electrons are detected at90° to the incident electron beam direction. Therelative abundance of argon ions in charge state n up to n = 4 + was deduced from the experiment. The charge state fractionsFn of ions with charge states n are found to be constantin the investigated impact energy range. However, their valuesare seen to decrease rapidly with n. A good agreement isobtained between our experimental results and the calculationsbased on inner shell ionization cross sections folded with charge state abundances, which result from the decay of thecorresponding initial inner shell vacancies. This agreementsupports the idea that the multiple ionization of argon atomsby energetic electrons is caused mainly due to inner shellvacancy production followed by relaxation processes resultingin an increase of the charge state of the generated ion. Further,our results show that the Ar+ ions are produced mainlythrough dipole transitions while ions having n>1 + aredominantly produced by the non-dipole transitions in the entireenergy range of investigation. The partial ionizationcross sections for Arn + ions (n = 1-4) are found tobe in close agreement with the results of other workers in the overlapping region of impact energies.

Measurements of germanium K-shell ionization cross sections and tin L-shell x-ray production cross sections by electron impact

This article reports experimental measurements of germanium K-shell ionization cross sections and tin L-shell x-ray production cross sections by electron impact. In order to avoid the difficulty of preparing thin and uniform self-supporting targets, thin targets with thick substrates are used in the experiments. Since electrons reflected by the substrate can induce additional ionization of target atoms, we developed a method to correct the measured x-ray intensities and eliminated the influence of the substrate on measured data. Our measured cross sections are in good agreement with previous measurements. The present results are also compared with the predictions of theories and semiempirical and empirical formulas. It is found that the K-shell ionization cross sections of Ge at energies from near threshold to 40 keV can be reasonably described by the semiempirical formula of Green and Cosslett. Experimental absolute L-shell x-ray production cross sections of Sn are presented. It seems that, so far, none of the available theories can adequately interpret our L-shell results at low energies.

Transmission Electron Diffraction at 200 eV and Damage Thresholds below the Carbon K Edge.

Transmission electron diffraction patterns from ultra-thin aromatic and aliphatic organic films at beam energies of 200 eV-1 keV have been recorded in a custom low energy electron transmission (LEET) chamber. A significant reduction of the molecular damage cross-section, measured by fading of diffraction spots, was found for thin films of the aromatic perylene when the beam energy was reduced from 400 to 200 eV. The corresponding measurements for the aliphatic tetracontane showed a smaller "threshold energy" and the differences are discussed. Electron beam damage from other aromatic materials has also been studied at low energy. Comparison of the carbon K shell ionization cross-section and the measured damage cross-sections show that carbon K-shell ionization is strongly correlated with the damage observed in aromatics at beam energies higher than 284 eV. Calculation of the minimum number of unit cells needed for imaging a single molecule, and comparison of calculated elastic with measured damage cross-sections both indicate new possibilities for imaging biomolecules with low energy electrons.

Transmission Electron Diffraction at 200 eV and Damage Thresholds below the Carbon K Edge

Abstract Transmission electron diffraction patterns from ultra-thin aromatic and aliphatic organic films at beam energies of 200 eV–1 keV have been recorded in a custom low energy electron transmission (LEET) chamber. A significant reduction of the molecular damage cross-section, measured by fading of diffraction spots, was found for thin films of the aromatic perylene when the beam energy was reduced from 400 to 200 eV. The corresponding measurements for the aliphatic tetracontane showed a smaller “threshold energy” and the differences are discussed. Electron beam damage from other aromatic materials has also been studied at low energy. Comparison of the carbon K shell ionization cross-section and the measured damage cross-sections show that carbon K-shell ionization is strongly correlated with the damage observed in aromatics at beam energies higher than 284 eV. Calculation of the minimum number of unit cells needed for imaging a single molecule, and comparison of calculated elastic with measured damage cross-sections both indicate new possibilities for imaging biomolecules with low energy electrons.

Virtual PIXE and RBS laboratory

An interactive software package for a complete simulation of Particle Induced X-ray Emission (PIXE) and Backscattering Spectra (BS) is being developed. The user is in a position to define all experimental parameters such as incident ions (protons, deuterons or He ions), their energy, excitation and detection geometry, detector parameters and filters as well as sample composition and thicknesses of a number of layers. This is all done through an easy to operate interactive user interface. Simulated PIXE and RBS spectra are immediately displayed on the screen and can be saved either as bitmaps and/or files suitable for further processing. Each output comes with a complete set of experimental parameters, absolute and relative line intensities (including all major K and L lines), peak to background ratio and detection limits for all elements of interest.The program has also a number of utility routines to calculate various fundamental parameters such as photon cross sections, K and L shell ionization and production cross sections, energy loss, and detector efficiency. All these routines use the state-of-the art data base sources.The program operates on a personal computer under the MS Windows operating system. The simulation is fast and the program is easy to operate. The package will be useful in many ways. Firstly, it will be an excellent teaching tool for researchers/students without experience with PIXE/RBS. Secondly, it will be of immense help in planning and optimizing PIXE and/or RBS experiments. The user can ‘run’ a simulated experiment for any sample/experimental parameters and test various settings/scenarios to achieve optimal results without actually carrying out the experiment, thereby saving a lot of expensive machine time that would otherwise have been spent on trial and error experiments.

Quantitative AES VII. The ionization cross-section in AES

Inner shell ionization cross-sections are analyzed for use with Auger electron spectroscopy (AES). Correlations of the cross-sections of Gryzinski, Casnati et al., Jakoby et al. and Drawin are made with experimental data from the digital Auger database. Jakoby et al.'s cross-section is realistic for K shell ionizations but can become negative for other shells unless modified, so is of limited use for AES. Results for the ratios of experimental and calculated intensities for electron beam energies of 5 keV and 10 keV, for overpotentials in the range 2–60, are used to rank the effectiveness of the remaining three cross-sections. The experimental ratios show means and scatter standard deviations of 1.01±0.12, 0.99±0.05 and 1.04±0.09 when compared with the predictions for the cross-sections of Gryzinski, Casnati et al. and Drawin, respectively. Casnati et al.'s cross-section provides the best correlation, with the goodness of fit being similar for ionizations in the K, L, M and N shells. Casnati et al.'s cross-section is therefore recommended for future use in AES. The use of Gryzinski's or of Drawin's cross-sections, by comparison, can lead to errors as high as 50% at the low value of 1.5 for the overpotential (i.e. for high-energy Auger electrons). © Crown copyright 1998. Reproduced with the permission of the Controller of Her Majesty's Stationery Office.

Inner shell ionization of carbon atom by electron impact

A prescription is proposed for the calculation of inner shell ionization cross sections of neutral atoms (e.g. carbon, argon etc.) by electron impact in coplanar geometry. As a first step, triple differential cross sections (TDCS) are calculated for the K-shell ionization of carbon atom by fast electron impact for highly asymmetric kinematics. The distinguishing feature noted in the TDCS structure is the presence of intense recoil peak which for certain parameters is even larger than the binary peak, an unusual feature for the outer shell ionization process. Results are compared with the relative measurement performed on the C2H2 molecule as target. The binary to recoil peak intensity ratio of the present TDCS is found to be in good accord both with the experiment as well as with another theory.

Correction of substrate effect in the measurement of 8-25-keV electron-impact K-shell ionization cross sections of Cu and Co elements.

The experimental measurement of electron-impact K-shell ionization cross sections of Cu and Co elements within the energy region 8--25 keV was reported. The influence of a substrate of thin targets on ionization cross sections has been corrected by a method based upon a bipartition model of electron transport [Z. M. Luo, Phys. Rev. B 32, 812 (1985); 32, 824 (1985)]. The measured cross sections are in good agreement with existing data and are well reproduced by the empirical formula of Green and Cosslett [Proc. Phys. Soc. London 78, 1206 (1961)]. \textcopyright{} 1996 The American Physical Society.

K-shell ionization of B, O, and F by 0.4-2.0-MeV He+ ions.

Absolute {ital K}-shell ionization cross sections of B, O, and F have been obtained for incident He{sup +} ion energies of 0.4{endash}2.0 MeV in steps of 0.2 MeV. These ionization cross sections were obtained from Auger-electron yields measured for atomic boron in BF{sub 3}; for atomic oxygen in CO, CO{sub 2}, and O{sub 2}; and for atomic fluorine in C{sub 2}H{sub 2}F{sub 2}, C{sub 2}F{sub 6}, and C{sub 4}F{sub 8}. For F-containing molecules, the {ital K}-shell ionization cross sections per atom are found, both in this laboratory and at other laboratories, to decrease as the number of F atoms in the molecule increases. The experimental {ital K}-shell ionization cross sections are compared to existing experimental {ital K}-shell ionization cross sections as well as to various theoretical predictions. {copyright} {ital 1996 The American Physical Society.}

K shell ionization cross sections of Au, Pb and Bi induced by low energy proton impact

K X-ray production cross sections induced by proton impact were measured on thick targets of Au, Pb and Bi in the energy range 0.5 < E 0 < 1.7 MeV. The yields of characteristic X-rays were normalised to the yields of backscattered protons. For proton energies E 0 > 0.8 MeV, the deduced K shell ionization cross sections are uncertain by 12%, these uncertainties being smaller than those of the data published before. Our experimental results are compared with the semiclassical cross sections calculated within the united atom binding procedure. Experimental values agree well with theoretical ones, their difference being generally smaller than our experimental errors. The ionization cross sections measured for E 0 < 0.8 MeV are too large to be explained by proton induced direct ionization. We showed that for thick targets and proton energies smaller than E 0 < 0.8 MeV, the K-shell ionization is produced notably by fluorescence on account of bremsstrahlung photons emitted in the collisions between protons and target nuclei.

Use of low-energy accelerators in Slovenia

From activities connected with nuclear physics research in the mid 70s, the step towards studies in atomic physics and the application of nuclear spectroscopic techniques in interdisciplinary fields has been made. In the field of atomic physics progress from the measurement of single K and L shell ionization cross sections to studies of multi-electron ionization and decay processes was achieved. In parallel, various accelerator-based methods were installed such as PIXE (energy and wave-length dispersive), PIGE, RBS and CAM. These methods have been successfully used for case studies in archaeology, in studies of hard coatings, occupational health and for some biomedical research. The important role of the accelerator has also been demonstrated in education.

PIXE and RBS analysis of sediments from El-Kansera barrage on Oued Beht river in Morocco

Elemental analyses of sediments from the barrage El-Kansera on the river Oued Beht in Morocco, are reported. These analyses were performed by means of proton induced X-ray emission (PIXE), and Rutherford backscattering spectroscopy (RBS). The RBS analysis allowed the determination of the sediments matrix composition, which was used for the PIXE analysis. The quantitative analyses of the PIXE spectra were performed by using the binary encounter approximation (BEA) for the inner shell ionization cross section. Preliminary results are presented, and a comparison with literature data is discussed.