O. Benka

Helium elastic scattering from carbon for 30° to 150° in the energy region from 2 to 4.8 MeV

Abstract The differential cross-sections for elastic scattering of 4 He ions by carbon atoms were measured at scattering angles of 30°, 45°, 60°, 135° and 150° in the energy range from 2 to 4.8 MeV. Up to now mostly data for angles larger than 150° were published in the literature. A thick carbon target with a thin evaporated Cu layer on the surface was used for the measurement. The number of impinging projectiles was obtained from the He ions scattered by the Cu layer assuming Rutherford cross-sections. The carbon scattering cross-sections were then obtained from comparison of measured He energy spectra with the simulated ones. Above 2 MeV all evaluated cross-sections become non-Rutherford. Deviations from Rutherford cross-sections are about 50% for 30° scattering angle and amount up to a factor 30 for 150° scattering angle. The measured experimental cross-sections were compared with the calculated theoretical cross-sections and already published data. Satisfactory agreement was obtained for all measured scattering angles and energies.

A new setup for elastic recoil analysis using ion induced electron emission for particle identification

Abstract We describe a new setup for elastic recoil detection analysis (ERDA) using our recently developed particle identification method. Before the ions and elastic recoil atoms from the target reach a silicon surface barrier detector for energy analysis, they penetrate a set of thin foils (e.g. carbon). The ion induced electron emission yield from the foils depends on the nuclear charge of the penetrating ion and it is roughly proportional to the energy loss in the foil. The emitted electrons are accelerated towards a microchannel plate (MCP), which gives a signal amplitude proportional to the number of emitted electrons. This signal is measured in coincidence with the energy signal from the surface barrier detector using our dual-parameter multichannel analyzer system M2D. Since the energy resolution is not measurably deteriorated by the particle identification our setup offers optimum depth resolution for light elements. Due to the compact design large solid angles for high sensitivity can be achieved. A new measuring chamber has been built which offers considerable improvements. The ERDA scattering angle (30° or 45°) and the target orientation can be selected for optimum depth resolution or sensitivity. Element separation for light elements has been enhanced by several improvements: A new geometry of the foil setup improves the collection efficiency for ion induced electrons onto the MCP, coating of the carbon foils with insulators enhances the electron emission yield. Finally, a new data evaluation procedure has been developed in which the pulse height spectrum of the MCP is considered to be a linear combination of individual spectra from the incident ion and of the recoil atoms. The normalized shapes of these spectra are taken from calibration measurements, the intensities are then calculated using a linear fitting algorithm and finally give the depth profiles of the elements in the target. For hydrogen in near surface layers even isotopic separation is possible. Examples for 1H and 2H in a Be matrix will be given.

Electron emission yield of Al, Cu and Au targets induced by fast hydrogen and helium ions

Abstract The electron emission yield γ is measured for impact of H + , He + , He 2+ ions and of electrons on Al, Cu, and Au targets. The ion energy is between 0.5 MeV (H + ) and 4.8 MeV (He 2+ ), the electron energies are around 1 keV. The bare ion yield is compared with predictions of a model, which assumes the yield to be proportional to the stopping power and takes collective electric fields generated by the projectiles into account. The He + yield is explained by investigating the additional electron production of stripped electrons. It is found that the mean depth He + ions penetrate the surface layer without losing their bound electron and is about 60% of the mean escape length of the emitted electrons.

Statistics of electron emission induced by MeV H+ and He2+ ions☆

Abstract In this work we describe an experimental setup for measuring the statistical distribution of the number of electrons which are emitted from a target surface at ion impact. Emission statistics is measured for MeV He 2+ and H + projectiles impinging on Au and Cu targets. The total electron yield obtained from these measurements is compared to that obtained from current integration. The results agree within the estimated uncertainties of 3–5%. The distribution of the number of emitted electrons can be very well approximated by Polya distributions. For He 2+ impact this distribution is close to a Poisson distribution, whereas for H + impact the measured distributions are completely different. It is shown that the deviations from Poisson statistics can be explained by the statistical distribution of the energy deposited by the projectile in a surface layer with a thickness which corresponds to the mean escape depth of ion induced electrons.

Determination of H recoil cross-sections for He ions incident at 2.5–4.5 MeV and recoil angles from 30° to 60°

Proton diAerential recoil cross-sections were measured for impact of He ions in the energy range from 2.5 to 4.5 MeV and for recoil angles from 30∞ to 60∞. For the first time, H recoil cross-sections at angles from 45∞ to 60∞ are reported. All the measured cross-sections, except those for 60∞, deviate from the Rutherford cross-sections. Thin melamine foils (50 lg=cm 2 ) evaporated on50 lg=cm 2 thick Cu were prepared as targets. Two surface barrier detectors positioned at forward angles were used to detect the recoiled and scattered atoms. One detector was kept all the time at 40∞ and was used to measure the loss of H due to irradiation. The experimental results were compared with the other available experimental data for lower recoil angles and a good agreement is found. A comparison is also made with the theoretical calculations obtained by fitting the phase shifts of the kinematic inverse reaction using the principle of detailed balance. Again an agreement is found for low recoil angles, but for 55∞ and 60∞ the experimental cross-sections are significantly smaller than the theoretical values. ” 2001 Elsevier Science B.V. All rights reserved.

Electron emission yield from thin Al and insulating layers induced by 3 MeV He2+ and 3 keV electron impact

Abstract The kinetic electron yield was measured for the impact of 3 MeV He 2+ ions and 3 keV electrons on thin layers of Al on Cu, of Al 2 O 3 on Al, and of CeO 2 and CaF 2 on Si backings. The dependence of the yield on the layer thickness was determined. For Al on Cu a decreasing yield was observed for increasing Al layer thickness, since the yield of Cu is higher than that of Al. For insulating layers increasing yields were measured for increasing layer thickness. The observed yield dependencies were fitted by sums of exponential functions. The characteristic lengths of the exponential terms were interpreted as emission lengths. For Al and the metal oxides a small emission length in the range of 2–3 nm was found, for CeO 2 and electron impact on Al a second exponential term with a large emission length, 20–100 nm, was necessary to describe the measured yield dependence, which is probably due to backscattered projectile electrons and δ-electrons. The Al results are compared with computer simulations. The increasing yield of CaF 2 layers could be fitted by a function with only one exponential term and an emission length of 10.8 nm for He impact and 20.6 nm for electron impact.

Elastic recoil detection analysis for large recoil angles (LA-ERDA)

Abstract In this paper, elastic recoil detection (ERD) measurements at recoil angle of 60° using ion-induced electron emission (IEE) for particle identification are presented. In our IEE system for particle identification, recoiled target atoms and scattered projectiles penetrate a set of thin carbon foils before their energy is analyzed in a solid state detector. Particle identification is based on the fact that the total number of electrons emitted from the foils depends on the particle nuclear charge. This method is characterized by its low minimum detectable energy, which stimulated us to study ERDA at 60°. Due to collision kinematics and due to the angular dependence of the scattering cross-sections, it is expected that the sensitivity can be significantly improved. In this work, the detection efficiency of the IEE particle identification system for H recoils at energies below 1 MeV was determined. LA-ERDA measurements were performed with 4He and 12C projectiles using two different types of samples with a well-known amount and depth distribution of H atoms near the surface. Sample 1 consisted of a 50 μg/cm2 melamine layer evaporated on a flat Si substrate, sample 2 was a Si wafer with implanted H. Sensitivity and depth resolution were measured using LA-ERDA with a recoil angle of 60° and ERDA with recoil angles of 30° and 45°. The results for different recoil geometries and projectiles are discussed and compared with theoretical predictions.

The influence of multiple ionization upon fluorescence yield

Abstract A short summary is given of calculations and experimental measurements of the K-fluorescence yield for multiple ionization in the K- and L-shell. It is shown that electron transfer processes into the L-shell can change the K-fluorescence yield and for Ne values of this effect are given. Further, nonstatistical population of multiplet states and the influence of outer shell electrons are discussed. Finally an upper limit is given of the effect of multiple ionization on the fluorescence yield for light ion bombardment.

Simulation of particle-induced electron emission in aluminum and copper

Abstract A Monte-Carlo computer code for the simulation of particle transport in metallic solids has been developed. Electrons or bare ion projectiles can be used. The code is able to calculate a wide variety of phenomena such as electronic energy loss, electronic energy loss straggling, particle-induced yield of emitted electrons or the statistical distribution of the number of emitted electrons per incident projectile. The theoretical models used in the simulation partially follow the basic work of Ganachaud and Cailler. However, for the loosely bound outer electrons of copper, the classical model of core ionization as it has been used by previous authors breaks down. Therefore, a fully quantum- mechanical description has been used in this work. For aluminum and copper the simulation results are compared with experimental and theoretical data. Excellent agreement is found.

Temperature dependence of the electron and proton induced electron emission yield of Al

Abstract The electron emission yield was measured for the impact of 3 keV electrons and 2 MeV H+ ions on Al as a function of target temperature. A strong influence of the surface morphology was found. For annealed samples a linear decrease of the yield was observed for increasing temperature. In the temperature range 50–440°C the yield for electron impact decreased by 4%, for proton impact by 6%. In addition, the work function of Al was measured for 25°C and for 440°C. The dependence of the yield on the work function was investigated by Monte Carlo simulation of the electron emission process for electron and proton impact. The temperature dependence of the yield is discussed in terms of work function changes.