Comparison of inner-shell ionization by relativistic electron and positron impact

Ratios of K/L shell X-ray yields by 10–20 MeV electrin and positron impact on Au have been measured with an accuracy of better than ±1.6%. The results show that the K-shell X-ray production cross section by electron impact is about 2% larger than by positron impact.     

Search for the density effect in inner-shell lonization by ultra relativistic electron impact

We have extended the measurements ofK-andL-shell ionization cross sections by electron impact into the ultra relativistic energy region, 0.9≦E≦2.0 GeV, in order to search for a saturation of the cross section. This phenomenon, which is due to the polarization of the target medium, is called density effect. It is predicted to occur at several hundred MeV impact energy and preferentially for lowZ target elements. Theoretical calculations are presented, based on the one-photon-exchange approximation. The absolute measurements of theK-andL-shell cross sections for Ni(K), Cu(K), Ag(K, L) and Au(L) performed at the 2.5 GeV electron synchrotron of the Bonn University, however, exhibit that the cross sections show no saturation but are still increasing. Furthermore, from theK X-ray yields, obtained at 0.9 and 2.0 GeV by bombarding the lowZ elements S, Ca, Mn, Ni and Ge, we obtain for the corresponding cross section ratio σ _K (2GeV)/σ _K (0.9GeV)=1.08±0.01 on the average. TheK X-ray yield of a composite Ca — Mn target amounts to $$[\sigma _K (Ca;2.0GeV)/\sigma _K (Mn;2.0GeV)]/[\sigma _K (Ca;0.9GeV/\sigma _K (Mn;0.9GeV)] = 0.99 \pm 0.02.$$ . All three results are in disagreement with theory. This severe discrepancy is discussed but the origin for it is not yet understood.     

快电子碰撞碳原子K-壳层电离的三重微分截面

利用3C、DS3C和S3C模型分别计算了共面非对称几何条件下快电子碰撞碳原子K-壳层电离的三重微分截面(TDCS),将S3C模型计算结果与其它理论结果和实验数据进行了比较.表明:内壳层电离的TDCS呈现出一强的recoil峰,对于某些参量,recoil峰甚至高于binary峰.这一点与外壳层电离过程不相同.S3C模型能够较好地描述这样的电离过程.?     

Total ionisation cross sections for chlorofluoromethanes and CClx radicals by electron impact

We report here the total ionisation cross section for chlorofluoromethanes, namely CCl₃F (Freon 11), CCl₂F₂ (Freon 12), CClF₃ (Freon 13), CHCl₂F (Freon 21), CHClF₂ (Freon 22), CH₂ClF (Freon 31), CCl₄ and CCl_x (x = 1–3), radicals by electron impact from ionisation threshold to 2 keV. The total inelastic cross section is obtained employing a complex optical potential formalism and solving the Schrödinger equation through partial wave analysis. Using the complex scattering potential-ionisation contribution method, the total ionisation cross section is derived from the inelastic cross section for these targets. The results obtained are then compared with the existing experimental and theoretical data, wherever available. The present result shows reasonable agreement with previous data. For the CCl_x radicals, the ionisation cross section is predicted for the first time. The data reported here have immense interest to atmospheric and technological plasma modelling.     

Triple differential cross section of electron impact ionisation of Na(3s, 3p, 3d)

The fivefold differential cross section (5DCS) of the ionisation by electron impact of atomic sodium is determined theoretically for its fundamental 3s(² S) state and the excited 3p(² P) and 3d(² D) states by a procedure which employs in the transition matrix element of the first order Born approximation, the correlated double continuum (3C) wave function. This permits us to determine the statistical M-state population and the orientation and alignment tensors in (e,2e) detection. It is also shown that, the use of Gamow correlation term, in the independent particle (2C) model, reproduces, only in some situations, the shape of the angular distribution of the 5DCS obtained by the (3C) wave function. Received: 17 November 1997 / Received in final form: 16 March 1998 / Accepted: 21 March 1998     

Triple differential cross sections for low energy electron impact ionisation of helium

Measurements are reported of the triple differential cross sections for the ionisation of helium by electron impact. Data are presented for incident energies of 100 and 80.5 eV at several scattering angles and compared with previous measurements and theoretical calculations.     

K-shell ionisation of atoms by positron and electron impacts

Recent developments in the calculation of the K-shell ionisation cross sections of atoms by positron and electron impacts are reviewed. The application of the first Born and other approximations derived by the modification of the first Born are discussed. It is pointed out that the first Born approximation modified to include exchange and the effect of the atomic field on the projectile yields satisfactory results for the light atoms.     

Cross-section for impact ionisation in semiconductors

The dependence of the impact ionisation cross-section upon the carrier energy in a semiconductor is obtained by a numerical method. The result is used to calculate the field dependent concentration of the nonequilibrium carriers. The agreement of the results with the experimental data is found to improve in comparison to other simplified calculations.     

Positron impact ionisation of He ion

Triple differential cross-sections (TDCS) of a hydrogenic (He⁺) ion has been studied by positron impact using coplaner geometry for both symmetric and asymmetric kinematics in the intermediate and medium high incident energy region. TDCS has also been studied of He⁺ ion by electron impact for symmetric kinematics taking account of the electron exchange effect. The final state wavefunction is chosen as the correlated 3-body Coulomb wavefunction satisfying the exact asymptotic boundary condition. The long range Coulomb interaction in the initial channel between the ionic target and the projectile has also been taken into account properly. For positron impact, the collision is found to be almost recoilless at lower incident energies, in contrast to the strong recoil peak noted in the case of electron impact ionisation. For electron impact, the exchange effect is found to be significantly high for equal energy sharing in the final channel. Received 10 July 1999 and Received in final form 7 December 1999     

Velocity dependence of double inner-shell ionization in vanadium by proton and α-particle impact

Ratios of double (KL) to single (K) ionization cross sections for proton and α-particle impact on vanadium are measured with velocities V = 0.53 to 1.7 (in units relative to the L-shell electron velocity) and are found to be reproduced by the recipe of McGuire and Richard.     

Post-collision interaction effects on the triple-differential cross-section for the ionization of helium by fast positrons

Triple differential cross-sections for the ionization of helium by fast positrons are calculated in a ‘correlated’ first Born approximation supplemented by the inclusion of post-collision interaction effects. The results are analysed with respect to electron-helium experimental data of Jung and coworkers in coplanar asymmetric geometry.     

Study of the background in the experiments of inner-shell ionization of atoms by positron impact

One of main difficulties in the experiments of inner-shell ionization of atoms by positron impact near threshold energy region is the relatively high low-energy background, which is caused by the deposited part of energy in semi-conductor X-ray detectors of 0.511 MeV γ rays that are produced by positron annihilations in targets and target chamber. In this paper, by using the Monte Carlo method, we simulated the backgrounds for the X-ray detectors with the sensitive layer thickness of 0.3 mm and 3 mm in the case of 0.511 MeV γ rays impacting vertically on a Ti plate of 0.2 mm in thickness, and compared the simulation results with the experimental observations of the other research group and our own. Moreover, we also simulated the backgrounds for a simplified experimental setup in the case of 20 keV positrons impacting vertically on a thick Ti target and observed that the backgrounds for the X-ray detectors with the sensitive layer thickness of 0.3 mm and 3 mm, are very similar.     

Polarization ofn=3-terms after electron capture by beam-foil interaction

The electron capture by protons into excitedn=3 states as function of the distance from the surface is calculated in OBK-approximation. The target electrons are treated as free electron gas bounded by a step potential. Electron loss processes are considered by weighting the density matrix with a survival probability for different capture depths. The result is used to determine the Stokes parameters of the emitted light. Comparison is made with polarization measurements of HeI (n=3) states after beam foil excitation.     

Site-specific ionisation edge fine-structure of Rutile in the electron microscope

Combined Bloch-wave and density functional theory simulations are performed to investigate the effects of different channelling conditions on the fine-structure of electron energy-loss spectra. The simulated spectra compare well with experiments. Furthermore, we demonstrate that using this technique, the site-specific investigation of atomic orbitals is possible. This opens new possibilities for chemical analyses.     

On the role of electron capture in the inner-shell alignment of atoms in ion-atom collisions

The alignment of the $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ vacancy in Mg resulting from collisions with protons is calculated. Two possible mechanisms of ionisation are considered: direct Coulomb ionisation and electron capture by protons. The inclusion of electron capture leads to a marked improvement of the agreement between theory and experiment.     

Thresholds for impact ionisation in InSb at 77K

The thresholds in energy for 5 different impact ionisation processes in InSb at 77K were calculated on the basis of a critical review of the available bandstructure data for largerk values. An accurate threshold value of 243 meV ± a few meV is given for the main process. It is shown that production of light holes by impact ionisation is highly improbable. It is suggested that double ionisation and light hole initiated ionisation may be equally important in interpreting quantum efficiency data. Impact ionisation by L-band electrons may contribute significantly to the avalanche in Gunn domains, explaining the rapid quenching of the latter.     

Excitation of inner-shell states of CO2, COS,CS2, N2O and C2H2 by low energy electron impact

The design of an electron impact spectrometer for use in the study of inner-shell electric-dipole-forbidden transitions is described. The use of the spectrometer to investigate spin-forbidden transitions to inner-shell excited states of CO₂, COS, CS₂, N₂O and C₂H₂ is reported. For all these molecules, excitation of inner-shell triplet states has been observed.     

Inner-shell excitation-ionization in potassium by electron impact

A new autoionization peak in potassium at 20 eV is attributed to excitation of the 3p⁵4s²-levels at 18.72 and 18.98 eV. Metastable ions contribute to the total ionization cross section for energies above 24.4 eV.