L-shell Ionization Cross Sections Research Articles

Recent results from the EBIT and super EBIT at Lawrence Livermore National Laboratory

The electron beam ion trap (EBIT), and the higher-energy Super EBIT at Lawrence Livermore National Laboratory can produce any highly charged ion. These highly charged ions are used in a variety of research programs. Recent results from four different experiments are reviewed here. K-shell ionization cross sections have been measured for the hydrogenlike ions of several elements, and L-shell ionization cross sections have been measured for uranium ions. A measurement of the ground-state hyperfine transition in hydrogenlike 165Ho66+ is notable because of the complete absence of Doppler shifts. A cryogenic Penning trap, injected with EBIT ions, has been used to observe a single highly charged ion as it recombines by sequential electron capture from H2 gas. A large sputtered ion yield, suggesting a surface Coulomb explosion, has been observed from insulators bombarded with very highly charged EBIT ions.

L-shell x-ray production for Rh, Ag, Cd, Sb and I with protons in the energy range from 1.6 to 5.2 MeV

Individual , , , and x-ray production cross sections for the elements Rh, Ag, Cd, Sb and I have been measured by protons in the energy range 1.6 to 5.2 MeV. The experimental results for x-ray production cross sections and cross section ratios are compared with the RPWBA - BC theoretical predictions and with the other available experimental data. The influence of using different databases for atomic parameters in the conversion of theoretical L-shell ionization cross sections to the L-shell x-ray production cross sections is discussed.

Influence of energy straggling on quantitative PIXE analysis

Examples of numerical calculations aimed to estimate the effect of proton energy straggling on quantitative PIXE are presented. Special attention has been taken to low energy PIXE and external beam PIXE. X-ray yield calculations including beam energy straggling for beam energies ranging from 0.3 to 3 MeV were carried out and the X-ray yield variations due to the energy straggling were determined. Proton energy straggling was calculated using the uncorrected Bohr's formula. K-and L-shell ionization cross sections variations were considered for the X-ray yields calculations. Results indicate that in some cases the error induced on the produced X-ray yield may be up to a few percent on low energy PIXE. For external beam PIXE and high energy PIXE the X-ray yield variations are very small (less than 0.5%).

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.

Experimental L-Shell X-Ray Production and Ionization Cross Sections for Proton Impact

Experimental values of proton-induced L-shell x-ray production and ionization cross sections are tabulated. Data for individual L x-ray transitions and L-subshell ionization are presented separately according to incident proton energy and target atomic number in two tables. Experimental details and techniques employed by different authors are summarized as well. This work covers experimental data published in the period between 1982 and December 1992. It comprises more than 6000 newly compiled L-shell cross-section data and it is a continuation of a similar tabulation produced by Sokhi and Crumpton (Atomic Data and Nuclear Data Tables 30, 49, 1984 (Ref. 1)).

Model calculations of multielectron ionization of O2 molecules by fast-heavy-ion impact.

A simple model employing the independent-electron approximation was developed to investigate orientation effects on the cross sections for multielectron ionization of diatomic molecules in fast-heavy-ion collisions. The model was used to compare K- and L-shell ionization cross sections for ${\mathrm{O}}_{2}$ molecules with internuclear axes oriented parallel and perpendicular to the beam.

Energy-Loss of Positrons in Foils

The Giessen slow positron source TEPOS delivers intense beams in the energy range between some eV and 6 keV; with postacceleration up to 80 keV. Results for remoderation are described shortly. Further the energy-losses of positrons with incident energy of 6-20 keV through thin aluminium foils were measured. Cross sections for Kand L-shell ionization of thin silver and gold targets by positron and electron impact have been determined at projectile energies of 30-70 keV. The experimental results are presented in detail; they are confirmed by calculations in plane-wave-Born approximation which include an electron exchange term and account for the deceleration or acceleration of the incident projectile in the nuclear field of the target atom. PACS numbers: 07.77.+p, 29.25.-t, 34.80.Dp

Electron and positron impact experiments on metallic foils at the giessen positron source TEPOS

The sources TEPOS for slow positrons were installed at the Giessen 65 MeV electron linear accelerator. Ratios of K- and L-shell ionization cross-sections by electron and positron impact were measured on Au-/Ag-foils as function of the incident projectile energy. With energies near threshold a departure is observed of the ratio σ(e−)/σ(e+) from unity (increasing with decreasing energy).

Light-ion-induced L i ionization of 46 ≤ Z ≤ 60 elements: First-and second-order corrections to PWBA

Recently measured proton- and 4He-ion-induced L i -subshell ionization cross sections for the elements with 46 ≤ Z ≤ 60 have been compared with the ECPSSR predictions. A recalculation of the binding correction, which leads, when the incident energy decreases, to a limit of the binding energy of the electrons concerned corresponding to a United-Atom approximation is suggested. A good agreement between the experimental and predicted L-shell ionization cross sections and a satisfactory one for the single L i -subshell ones is obtained. The introduction of vacancy rearrangement effects gives further better agreement, but not enough to reproduce completely the experimental findings.

L-shell ionization of Si, P, S, Cl, and Ar by 0.4-2.1-MeV He+ bombardment.

The L-shell ionization cross sections of Si, P, S, Cl, and Ar have been obtained for incident ${\mathrm{He}}^{+}$ ion energies 0.4--2.1 MeV in steps of 0.2 MeV. The cross sections were obtained from the Auger electron yields measured for ${\mathrm{SiH}}_{4}$, ${\mathrm{PH}}_{3}$, ${\mathrm{H}}_{2}$S, ${\mathrm{CH}}_{3}$Cl, and Ar gaseous targets. The present experimental L-shell ionization cross sections are compared to existing experimental L-shell ionization cross sections as well as to previous theoretical predictions.

The effect of the relativistic Hartree-Fock-Roothaan momentum distribution in the binary encounter approximation for inner shell ionization

K- and L-shell ionization cross sections of gold due to impact of proton and alpha particles have been calculated in the binary encounter approximation incorporating the effects of Coulomb deflection of the projectile, of increase in binding of the target electron, and of polarization and relativity. Relativistic Hartree-Fock-Roothaan (RHFR) momentum distributions for the target electron along with an approximate relativistic correction to the collision dynamics have been used in the present calculations. A comparison with the corresponding calculations using non-relativistic Hartree-Fock-Roothaan wave functions, experimental results and other available calculations is presented.

Investigation of the impact-parameter dependence of electrons emitted in 30-, 100-, 350-keV H+ and 100-keV 3He

Energy distributions of electrons ejected in collisions of light ions with Ar atoms were measured as a function of projectile scattering angle. For this purpose the ion-electron coincidence technique was combined with the time-of-flight method for electron momentum analysis. Thus, the simultaneous detection of electrons with energies from a few hundred meV up to some keV and ions at different projectile scattering angles could be performed. The electron analyzer could observe electrons at ejection angles of 40/sup 0/--140/sup 0/ with respect to the beam axis. A time resolution of less than 400 ps was achieved with a recently developed position-sensitive ion detector. For the first time, absolute double differential yields for electron emission were measured as a function of impact parameter and electron energy for the system H/sup +/+Ar at collision energies of 30, 100, and 350 keV. Also L-shell ionization yields were determined for 30-, 100-, and 350-keV H/sup +/ and 100-keV He/sup 2+/ impact on Ar. The results are compared with predictions of the plane-wave Born approximation, an atomic orbital expansion, and the semiclassical approximation. Total L-shell ionization cross sections were deduced and compared to other measurements.

Threshold behaviour of L-shell ionisation in slow symmetric ion-atom collisions with 32⩽Z⩽54

Total L-shell ionisation cross sections have been investigated for the collision systems Ge-Ge, Kr-Kr, In-In, Mo-Mo, and Xe-Xe by measuring the X-ray yield in the impact energy range 50 keV<Elab<3 MeV. Energy steps between 2.5 and 40 keV were selected in order to get detailed information about the ionisation threshold behaviour. The influence of multiple collisions on the ionisation process was studied by comparing the X-ray yield obtained from gaseous and solid targets. The experimental data was compared with theoretical results deduced from coupled-state, as well as simplified two-state, calculations. The authors found that in addition to the well known 4f phi -4f delta -4f pi -4f sigma rotational coupling with the initially unoccupied 4f phi MO, further couplings with orbitals originating from the M shell have to be included in order to explain satisfactorily the observed structure in the threshold cross sections.

Investigation of L-subshell ionization of gold by low velocity proton bombardment

Proton induced X-ray emission was used to measure absolute L-subshell and total L-shell ionization cross sections of gold in the energy range 0.2–0.6 MeV. The experimental results were compared with the predictions of two versions (RPWBA-BC, CPSSR) of the plane wave Born approximation.

Proton impact L-shell ionisation of atoms

L-shell ionisation cross sections of Ar, Cu, Kr, Ag and Xe have been computed using Vriens' expressions for ionisation of atoms by proton impact. The effects of Coulomb deflection of the incident proton and the increase in binding of the target electron have been investigated separately. Hartree-Fock velocity distributions for the target electrons have been used in the present calculations. The results have been compared with the available experimental results and calculations of other workers. The simple modified BEA is found to be as effective in predicting proton impact L-shell ionisation cross sections of atoms as the complicated CPSSR theory.

Helium-induced L-shell ionization cross sections

The helium-induced L-subshell ionization cross sections have been calculated and compared graphically with recent experimental results. The calculations have been made by the relativistic plane-wave Born-approximation theory, using the Dirac wave functions for the target electron. The corrections for the increased binding energy and Coulomb-deflection effects have been taken into account. The experimental values for various target elements are normalized to the corresponding theoretical values and plotted against an effective reduced velocity parameter. This universal plot shows satisfactory agreement between theory and experiment for L 1 and L 3-shell ionization cross sections. However, for the L 2 shell the calculated values are systematically smaller than the experimental data. Possible reasons for the discrepancy are discussed.

Approximate relativistic correction factors in L-shell ionisation

Simple approximate correction factors for electronic relativistic effects in L-shell ionisation cross sections by low-energy charged-particle impact have been derived. The factors obtained are compared with the relativistic plane-wave Born approximation calculations using the Dirac wavefunctions and with other approximate correction methods. In the energy region of interest, the present method is found to represent well the relativistic effects.

SIGMAL: A Program for Calculating L-Shell lonization Cross-Sections

SIGMAL is a short (∼ 100-line) Fortran program designed to rapidly compute cross-sections for L-shell ionization, particularly the partial crosssections required in quantitative electron energy-loss microanalysis. The program is based on a hydrogenic model, the L1 and L23 subshells being represented by scaled Coulombic wave functions, which allows the generalized oscillator strength (GOS) to be expressed analytically. In this basic form, the model predicts too large a cross-section at energies near to the ionization edge (see Fig. 1), due mainly to the fact that the screening effect of the atomic electrons is assumed constant over the L-shell region. This can be remedied by applying an energy-dependent correction to the GOS or to the effective nuclear charge, resulting in much closer agreement with experimental X-ray absorption data and with more sophisticated calculations (see Fig. 1 ).

L-Shell Ionization Cross Sections of BA, CE, ND, SM and DY for Proton Energies from 0.5 to 2.0 MeV

Protons from the University of Texas at Arlington's 2 MV Van de Graaff accelerator were used to produce characteristic L x-rays from thin targets of Ba, Ce, Nd, Sm and Dy. Incident proton energies were varied from 0.5 to 2.0 MeV. The x-rays were detected by an ORTEC High Purity Germanium Low Energy Photon Spectrometer (LEPS) mounted at 90° to the beam direction. The incident proton beam was monitored by a particle detector also mounted at 90° to the beam direction. The measured x-ray production cross sections were converted to ionization cross sections by the use of subshell fluorescence yields and Coster-Kronig transition probabilities taken from Bambynek. Comparison of these ionization cross sections as well as the ratios of selected subshell x-ray yields with the predictions of the Plane Wave Born Approximation (PWBA) and the Binary Encounter Approximation (BEA) show that the PWBA predictions fit the experimental results much better than the BEA predictions.

Thick and thin Au and Pb L-subshell ionisation cross sections for protons

L1,L2,L3 and total L-shell ionisation cross sections were measured for thick and thin Au and Pb targets over the proton energy range 0.6 to 3.0 MeV. The results have been compared with the plane-wave Born approximation (PWBA), the relativistic RWBA (PWBAR) and the Coulomb deflection corrected and perturbed stationary states relativistic PWBA (CPSSR) of Brandt and Lapicki (1979). The experimental subshell cross sections have also been used to produce the L alpha , L beta , L gamma ratios for the ten strongest X-ray peaks observed for Au and Pb with modern high resolutions Si(Li) detectors.