R. Wünsch

Fluence dependent electron emission as a measure of surface modification induced by swift heavy ions

Abstract From the fluence dependence of low energy electron yields induced by swift heavy ions, one can deduce a cross section for surface transformation by electronic processes. For a clean metal surface exposed to a contaminant gas in ultrahigh vacuum, this cross section is related to the electronic desorption. In general, this cross section is also related to structural modification of the surface. The dependence of electron yields, surface transformation cross section and secondary ion yields for carbon foils show a different dependence on the electronic stopping power which is a hint for different ejection mechanisms involved.

Atomic collisions in secondary ion production by hydrogen near the electronic stopping power maximum

Abstract H + projectiles from a 2.5 MV van de Graaff accelerator were used to bombard a carbon target (500 A) under high vacuum conditions. By varying the H + projectile energy (50–160 keV/u) it was possible to scan the low energetic L indhard– S charff– S chiott region (LSS), the maximum, and the high energetic B ethe– B loch region (BB) of the electronic stopping power (d E /d x ) e . The secondary ion yields Y (H + ,C 2 H x + ,C 3 H x + ,C 4 H x + ,C 5 H x + ) from the beam entrance surface were measured with a time of flight spectrometer (TOF). The measurements reveal a non-linear behaviour between the electronic stopping power (d E /d x ) e calculated with TRIM, in the maximum and the Bethe–Bloch region, and the secondary ion yields. However, the experimental results show good agreement with a new Pereira et al. [Int. J. Mass Spectrom. Ion Proc. 174 (1998) 179] effective energy loss model.

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.

VELOCITY EFFECT IN SECONDARY ELECTRON EMISSION BELOW AND ABOVE THE ELECTRONIC STOPPING POWER MAXIMUM

We studied secondary electron emission yields γ from a carbon surface with helium projectiles near the electronic stopping power maximum (0.2–2 MeV). We ask the question: “Does γ depend on the electronic stopping power (dE/dx)e only or is there an additional dependence on the projectile velocity VP?” A velocity effect is indeed found for backward emission. The results are discussed in the framework of recent electron transport and track models and compared with the yield of secondary ions from the same collision system.

Pre-equilibrium charge state distributions of Ni26+ (74 MeV/u) in carbon foils

Abstract We measured pre-equilibrium charge state distributions of Ni26+ ions at 74 MeV/u traversing carbon foils as a function of target thickness (28–49000 μg/cm2). We also present the evolution of the mean final charge for this collision system and for Cu25+ at 9.6 MeV/u. The experimental data agree well with numerical simulations performed with the new computer code ETACHA written by Rozet et al. [Nucl. Instr and Meth. B 107 (1996) 67].

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.

Temperature and dimension of nuclear tracks

Abstract From secondary ion mass and energy spectroscopy we can obtain information on the target properties and on track formation mechanisms. We present measurements of initial velocity and mass distributions of organic fragment ions desorbed from C, Al, and Au substrates by fast heavy atomic and molecular ion (e.g. 0.9 MeV N + /1.8 MeV N 2 + ) bombardment. We show that molecular ions can be used to control the local and temporal distance of nuclear track formation processes. Information on the dimensions of infra- and ultratrack is obtained from a comparison between secondary ion emission from binary molecular and single incident ions.

Transport of swift heavy ion induced electrons in solids

Abstract We investigated the transport of heavy ion induced electrons in solids by both numerical simulations and measurement: Electron yields and bouble differential spectra of electrons have been studied as a function of target thickness d . We measured electron yields from the beam entrance and exit surfaces of thin carbon foils bombarded with swift highly charged heavy ions at 13.6 MeV/u (Ar, foil thickness d ≈ 4 μ g/cm 2 –360 μ g/cm 2 ) and 74 MeV/u (Ni, d ≈ 3 μ g/cm 2 –50 mg/cm 2 ) and compared the results to numerical simulations. Also, double differential (in velocity and angle) spectra of electrons emitted into the forward direction from C foils induced by Ar 17+ at 13.6MeV/u reveal the dependence of the shape of the “binary encounter” electron peak on thickness d .

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.

The temperature of the track

Abstract The energy deposition of swift charged particles penetrating solids is accompanied by such processes as particle (electron, atom, ion, molecular ion, photon …) emission and/or a change of the solid along the particle track. The energy, velocity and mass distribution of such secondary particles obtained from thin solids (such as carbon, polyhydrocarbon, isolators and conductors) penetrated by projectiles (e.g. Ar 1.8 MeV) was measured quantitatively. A number of direct and indirect production mechanisms contribute to the internal source of electrons and secondary ions. The analysis of the ejected radicals give information on the emission processes, the temperature, the charge and the time scale of energy deposition near the surface of the solid which, eventually, become responsible for the track formation.