M. Kuzel

Elemental composition of different types of wood

Abstract For the use of wood as an environmental sensor the average or normal elemental composition of wood has to be known (here with Z > 12). Since we find with PIXE that the elemental composition depends on the species, environmental influences can only be deduced indirectly from deviations from average elemental concentrations, which have to be known for each examined species. The average elemental composition was calculated from the concentrations of more than 10 samples from almost as many sites for beech and oak. Other species were also analysed but not from more than three different sites. Significant correlations between certain elemental concentrations were found (in particular between Ca and Mn), also studied was the influence of the growing site on the elemental concentrations.

Contribution of transfer ionization to cusp electron production in proton on argon collisions

Cusp-electron emission in proton on argon collisions has been investigated at impact energies 33, 50 and 75 keV. By detecting the electrons in coincidence with the charge-state analysed outgoing H+,0 projectiles, the process of electron capture to the continuum (ECC), as well as the process of simultaneous ECC and bound-state capture (transfer ionization, TI) were identified. The ratios of the yields for cusp-electron production by TI and ECC are found to decrease with increasing projectile energies. At the lowest proton energy (33 keV) the measured TI/ECC ratio is about four times smaller than the corresponding ratio for incident He+ measured in earlier work. Significant is the finding that there exists a cusp associated with TI events, and, moreover, that this cusp is present for neutral outgoing projectiles.

Secondary-ions from atomic collision processes in solid surfaces

Abstract Using time-of-flight technique (TOF) we investigated the secondary ion emission from an uncleaned carbon surface induced by fast helium bombardment near the maximum of electronic stopping power ( d E d x ) e . We examined the production of secondary ions dependend on projectile properties only, as specific projectile energy ( 50 keV/u ≤ E P m P ≤ 500 keV/u ) and incident charge state (0 ≤ qi ≤ 2), but not on target properties, as surface conditions. The “pre-equilibrium stopping power” plays a major role in secondary ion production in the examinated projectile velocity regime.

A systematic investigation of ionization occurring in few electron collision systems: H0, He0 impact on He

Absolute doubly differential cross sections for electron emission occurring in fast (0.5 MeV/amu) H0 — and He0 — He collisions have been measured using standard non-coincidence techniques as well as emitted electron — charge state analysed projectile coincidence techniques. The comparison of these data with results obtained for H+ and He+ impact provides insight into the influence of one or two loosely bound projectile electrons on the probabilities for projectile, target, and simultaneous projectile — target ionization. PWBA calculations for these systems demonstrate good agreement with the experimental data for target and projectile ionization and indicate the importance of including simultaneous ionization processes in the theoretical treatment.

HOW CAN A NEUTRAL ATOM CAPTURE AN ELECTRON TO CONTINUUM STATES

Abstract We report on a systematic experimental study of the cusp peak associated with the ionization of the target (known as electron capture to the continuum, ECC) by impact of neutral atoms. In a measurement carried out for He 0 on Ar collisions, the dependence of ECC on the impact energy was investigated. The energy was varied between 18.9 and 250 keV/amu. The relative contributions (“fractions”) of the different processes (electron capture and loss to the continuum, double electron loss) to the cusp have been determined. For the ECC channel absolute doubly differential cross sections (DDCS) were also measured and compared to the corresponding DDCS values obtained with He + projectiles at the same experimental conditions. In another experiment an accurate measurement was performed for the shape of the ECC cusp by impact of goound-state and excited (metastable) He projectiles. The aim of this latter experiment was to check the validity of a model based on the formation of a low-lying virtual resonance state of the intermediate e − + He(2 1 S) system. The case of the neutral hydrogen as projectile was also considered: the ECC cusp observed from 100 keV H 0 on He collisions is compared with the corresponding ECC cusp obtained with protons.

Cusp-electron production due to transfer ionization in 25-250 keV/amu He2+ + Ar collisions

Abstract Cusp-electron emission has been investigated in 25–250 keV/amu He2+ on Ar collisions. Two processes which contribute to cusp electrons for this collision system, Electron Capture to the Continuum (ECC) and Transfer Ionization (TI), were distinguished by detecting the electrons in coincidence with the charge-state analyzed outgoing He2+ and He+ projectiles, respectively. The ratios of the electron yields for the TI and ECC channels were determined over the energy range investigated. These ratios can provide information on the role of the electron-electron interaction in the two-electron TI process. The measured TI ECC ratios were compared to the measured total double- to single-bound state capture cross-section ratios. These ratios are found to be consistent with a simple relationship based on the independent particle model, indicating a lack of electron-correlation effects for cusp-electron production by TI in these collisions.

Charge state analysis of heavy ions after penetration of uncleaned and sputter cleaned conducting surfaces

Abstract The evolution of the charge state distribution of fast ions inside a solid is of basic interest in various research fields as stopping power measurements etc. The existing models for the charge state evolution differ in the treatment of the projectile-exit-surface interaction, which has a strong influence on the final charge state distributions. We measured the charge state distributions for C+, N+, and O+ (30 ≤ E/M ≤ 130 keV u ) impact on thin C, Cu, and Au foils, where the surface properties were modified by sputter cleaning. The measurements show a pronounced change of the mean projectile charge state to lower values in the case of sputter cleaned surfaces. This result underlines the importance of the projectile-surface interaction for the generation of the outcoming charge state distribution.

Electron emission from binary collisions with ZProjectile ≤ ZTarget

Abstract We have measured doubly differential cross sections for electron emission in ion-atom collisions. In one set of experiments, 0.5 MeV/u He 2+ projectiles impinged on the target atoms He, Ne, Ar, Kr and Xe and molecules N 2 and O 2 ; in another set, H + (energy ranging from 2.0 to 6.0 MeV) and 0.75 MeV/u N q + ( q = 2–7) collided with Ar targets. In both cases, emitted electrons were observed under 0°. Major interest was directed upon the position of the Binary Encounter Peak, which is commonly assumed to follow a simple kinematic model. Surprisingly the peak is significantly shifted towards lower energies. For comparison we calculated spectra with the Plane Wave Born Approximation and the Plane Wave Approximation. These do not show in general good agreement with our experimental data, presenting large discrepancies not only in absolute values but also in the behaviour with the target atomic number.

QUASIFREE ELECTRON SCATTERING ON ATOMS IN THE INVERSE KINEMATIC SYSTEM

Abstract The measurement of elastic electron scattering is a sensitive tool to test ionic and atomic potentials. Because a dense ionic target is difficult to produce, we use inverse kinematics, i.e. we exchange the roles of projectile and target and transform the cross sections. The absolute singly and doubly differential cross sections for the projectile ionization of He 0 (0.1 MeV/u) scattered by Ne 0 , measured for the direct and the inverse system in the whole angular range from 0° to 180°, show the validity of the transformation. We find a pronounced generalized Ramsauer-Townsend minimum in the angular-dependent singly differential cross section. Changing to the Ne 3+ target the loss peak is absent in our measurements for larger emission angles. This may be explained by a large capture cross section into bound states of the neon ion, which depends on the impact parameter.

Beam - foil convoy electrons emerging from uncleaned and cleaned surfaces in coincidence with charge-analysed projectiles including neutrals

We discuss measurements of convoy electrons performed with incident (50, 150 keV) and (100, 200, 400 keV) ions, in coincidence with charge-analysed projectiles emerging from thin (150 - ) C and Cu foils. A strong dependence of convoy electron yields on the projectile charge, as well as cusp-electron spectra in coincidence with neutrals are observed and compared with the characteristics of presently available models for beam - foil convoy electron emission. Measurements performed with sputter-cleaned target surfaces confirm that the transmission through the surface potential is important in determining both the coincident convoy electron yields and the final charge distribution of the emerging projectiles.