Electronic Stopping Power Maximum Research Articles

Backward and forward electron emission induced by helium projectiles incident on thin carbon foils: Influence of charge changing processes

The backward and forward electron emission yields γB and γF have been calculated by Monte Carlo simulations for helium (He++, He+ or He0) ions incident on thin amorphous carbon foils with energies around the electronic stopping power maximum (0.2–2MeV). Besides the direct excitation of target electrons by the incident projectile, we have taken into account the different charge changing processes (He++↔He+↔He0) undergone by the helium ion in the target. We discuss in particular the connection between the electron emission yield γ and the electronic stopping power (dE/dx)e. We compare our results with previously published experimental results.

Ion tracks in amorphous SiO2 irradiated with low and high energy heavy ions

Latent ion tracks induced by heavy ions in amorphous silicon dioxide (a-SiO2) were studied by scanning electron microscopy after chemical etching in an aqueous HF solution. The evolution of etched pore diameter and etching efficiency was studied as a function of the electronic energy loss for each ion species. Etching was possible only above a threshold of (dE/dx)e≈1.5keV/nm when using energies below 1MeV/u, as demonstrated both from pore size and etching yield results. In a transition regime 1.5keV/nm<(dE/dx)e<4.0keV/nm only a fraction of all pores were revealed by etching. For (dE/dx)e>4.0keV/nm all tracks were revealed due to the formation of continuous tracks. Above the etching threshold the pore diameter increased with energy and electronic stopping power. A comparison with ions having energies above 1MeV/u shows that the etching threshold depends on the energy regime. Furthermore, the pore diameter reached a maximum value before the electronic stopping power maximum of the respective ion was attained.

Ion track formation below 1 MeV/u in thin films of amorphous SiO2

Only a few studies have so far been reported on ion track formation using low energy ion beams. Therefore we have performed a systematic study using different heavy ions at energies below 1MeV/u for irradiation of thin films of amorphous silicon dioxide (a-SiO2). The induced latent ion tracks were studied by selective chemical etching in an aqueous HF solution and the resulting pores were investigated by scanning electron microscopy.The evolution of pore diameter and etch yield were studied as a function of energy for each ion species. The track etching threshold in the electronic stopping power (dE/dx)e and specific energy was found to be approximately 1.5keV/nm and 0.04MeV/u, respectively, which is lower than previously observed. In a transition regime, 1.5keV/nm<(dE/dx)e<4.0keV/nm, etching yields were less than 100%. For (dE/dx)e>4.0keV/nm all tracks were revealed, i.e. etching efficiency was 100%, corresponding to continuous tracks. Above the etching threshold the pore diameter increased with energy and (dE/dx)e for each ion species, and attained a maximum value before the electronic stopping power maximum of the respective ion was reached.

Theoretical study on the role of bound electron in backward secondary electron emission induced by He + incident on solid surfaces

A theoretical study on the role of bound electron in the process of backward secondary electron emission for He + incident on Al, Cu and C has been made. A Monte-Carlo computer code based on the theoretical model for the simulation of this phenomenon has been developed. The code can be used to calculate the yield, distribution statistics and other physical parameters of emitted electrons. The calculated results are compared with experimental data of other authors, and a good agreement is found. We also calculated the yield for He + incident on Al and Cu at the energy near the electronic stopping power maximum.

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

H+ projectiles from a 2.5 MV van de Graaff accelerator were used to bombard a carbon target (500 Å) under high vacuum conditions. By varying the H+ projectile energy (50–160 keV/u) it was possible to scan the low energetic Lindhard–Scharff–Schiøtt region (LSS), the maximum, and the high energetic Bethe–Bloch region (BB) of the electronic stopping power (dE/dx)e. The secondary ion yields Y(H+,C2Hx+,C3Hx+,C4Hx+,C5Hx+) 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 (dE/dx)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.

Secondary electron emission near the electronic stopping power maximum

We report on measurements of light ion $(Z=2)$ and heavy ion $(Z=14)$ induced electron emission yields from the entrance and exit surfaces of thin carbon foils near the electronic energy loss per unit path length $(dE/dx)$ maximum. At constant $dE/dx,$ secondary electron yields are lower for high projectile velocities than for lower projectile velocities. This velocity effect, which is well known for secondary ion emission, is observed for backward electron emission for both He and Si. In the case of Si projectiles such a velocity effect is also observed for forward electron emission. The results can be qualitatively understood in the framework of recent electron transport and nuclear track models.

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.

Direct measurements of the stopping power for antiprotons of light and heavy targets

Measurements of antiproton stopping powers around the stopping-power maximum are presented for targets of Al, Si, Ti, Cu, Ag, Ta, Pt, and Au. The Low Energy Antiproton Ring antiproton beam of 5.9 MeV is degraded to 50--700 keV, and the energy loss is found by measuring the antiproton velocity before and after the target. Target thicknesses have been determined accurately by weighing and Rutherford backscattering techniques. The antiproton stopping powers are found to be reduced by around 35% for both light and heavy elements near the electronic stopping-power maximum as compared to the equivalent proton stopping power. The antiproton stopping powers and the Barkas effect; that is, the difference in stopping power between protons and antiprotons is compared to theoretical estimates, based on a harmonic-oscillator model and an electron-gas model, and good agreement is obtained.

Measurement of the Barkas effect around the stopping-power maximum for light and heavy targets

The first direct measurements of antiproton stopping powers around the stopping power maximum are presented. The LEAR antiproton-beam of 5.9 MeV is degraded to 50–700 keV, and the energy-loss is found by measuring the antiproton velocity before and after the target. The antiproton stopping powers of Si and Au are found to be reduced by 30 and 40% near the electronic stopping power maximum as compared to the equivalent proton stopping power. The Barkas effect, that is the stopping power difference between protons and antiprotons, is extracted and compared to theoretical estimates.

Generalized Thomas-Fermi-Weizsäcker approach to screening and slowing down of swift charged particles in jellium.

A generalized Thomas-Fermi-Weizs{umlt a}cker theory is proposed to investigate the screening and slowing down of charged particles moving through a homogeneous jellium with arbitrary nonrelativistic velocity. This theory involves a velocity-dependent modification of the Thomas-Fermi kinetic-energy density near the projectile. Nonlinear inhomogeneous integrodifferential equations are derived for the dynamically screened potentials of protons and antiprotons. The stopping power of a noninteracting electron gas for the rotational-symmetric limit of the screened potential is approximated in a simple form. At typical metallic densities, this approximation of the antiproton stopping power finds about half of the equivalent proton stopping power near the electronic stopping power maximum. At lower velocities the results are in fair agreement with previous density functional calculations. In the large-velocity region the two stopping powers merge. Higher approximations including the breakdown of the rotational symmetry are discussed in detail. {copyright} {ital 1996 The American Physical Society.}

Measurement of the antiproton stopping power of gold - the Barkas effect

The stopping power of gold has been measured for antiprotons in the energy range 0.2–3 MeV using a novel time-of-flight technique. The antiproton stopping power is found to be less than half the equivalent proton stopping power near the electronic stopping power maximum. In the high-energy limit the two stopping powers merge.

An Approximation for Solving the Transport Equation of the Angular Flux of Ions in Matter

A method is presented for the calculation of ion flux and range distribution in a low incident energy region (below electronic stopping power maximum). The approximate method developed for ions a t high incident energy is applied to the low energy region. [Russian Text Ignored].

Damage calculations at high ion energy-part II

Abstract A method is presented for damage energy distribution calculations for incident energies above the electronic stopping power maximum. The method is based on the approximate solution of the transport equation for stationary angular flux developed in a previous paper. In order to compare the results with experimental data and other theoretical rates, calculations were made at incident energies under the electronic stopping power maximum (for Al ions at 4.6, 5 and 5.4 MeV incident energies in Al target and for 46.7 MeV Ni in Au target). Combining the large energy transfer approximation and the path length approximation, we obtain a simple approximation for the calculation of the “tail” of the damage rates at high incident energies.