Proportionality between the secondary electron yield and the electronic stopping power for proton impact on aluminium

doi:10.1016/0168-583X(93)95807-H

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 78, Issues 1–4, 3 May 1993, Pages 255-259

https://doi.org/10.1016/0168-583X(93)95807-H Get rights and content

Abstract

Many experiments have shown that the backward yield of secondary electrons induced by protons in solid targets is proportional to the electronic stopping power S_e: γ_B = Λ_BS_e. The parameter λ_b can be considered as a “material parameter” because it is constant over a wide proton energy range. The forward yield, which can be measured with very thin targets, seems to exhibit the same property. Using a Monte Carlo model for the electron transport in aluminium, we have shown that the anisotropy of the angular distribution of the electrons excited by the proton plays a dominant role together with the number of elastic collisions suffered by an electron before it escapes. When this number is low (forward emission at low proton energy), the yield is approximately proportional to the total excitation cross section and, when it is large (backward emission), the yield is proportional to the stopping power.

References (13)

C. Smidts et al.
Nucl. Instr. and Meth.
(1992)
E.J. Sternglass
Phys. Rev.
(1957)
P. Sigmund et al.
Electron emission from solids during ion bombardment
J. Schou
Phys. Rev.
(1980)
J. Schou
Scanning Microsc.
(1988)
D. Hasselkamp
Die Ioneninduzierte kinetische Elektronenemission von Metallen bei mittleren und grossen Projektilenergien, Habilitationsschrift
(1985)

There are more references available in the full text version of this article.

Cited by (17)

Monte Carlo study of ion-induced backward and forward secondary electron emission from thin Al foil
2008, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
Like several experiments [5,7,9] on thin metallic and carbon files our simulation also gives larger forward than backward electron yield for high-energy ions, despite the fact that energy of the projectile ion is lower at the exit. It may be due to the reason that the electrons are preferentially excited in forward direction and deposit their energy downstream from their point of creation [9,24]. Fig. 5 shows two-dimensional distributions of the excitation points of the electrons emitted from Al target in backward and forward direction upon impact of 5, 15 and 35 keV Cu+ ions.
A direct Monte Carlo program has been developed to calculate the backward (γ_b) and forward (γ_f) electron emission yields from 20 nm thick Al foil for impact of C⁺, Al⁺, Ar⁺, Cu⁺ and Kr⁺ ions having energies in the range of 0.1–10 keV/amu. The program incorporates the excitation of target electrons by projectile ions, recoiling target atoms and fast primary electrons. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. The calculated backward electron emission yield and the Meckbach factor R = γ_f/γ_b are compared with the available experimental data, and a good agreement is found. In addition, the effect of projectile energy and mass on the longitudinal and lateral distribution of the excitation points of the electrons emitted from front and back of Al target has been investigated.
Backward and forward electron emission induced by helium projectiles incident on thin carbon foils: Influence of charge changing processes
2007, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
Recent experimental studies [6–10] have shown variations of these specific yields as a function of the incident ion energy (proton or helium), particularly for the forward specific yield. For protons incident on aluminium or carbon targets, “microscopic” theoretical calculations (see [9–12] for instance) have shown similar variations, i.e. ΛB varies slightly with the incident projectile energy while ΛF is strongly depending on the incident proton energy. This result is explained by electron transport effects and by the anisotropy of the electron source.
The backward and forward electron emission yields γ_B and γ_F 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 (0.2–2 MeV). 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.
Influence of the target thickness on the backward and forward electron emission characteristics induced by protons incident on thin carbon foils
2005, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
For thin metallic or carbon foils, experiments [5] have shown that Rγ is always larger than one for all projectile and target combinations. This is due to the anisotropy of the “source” of excited electrons inside the material, i.e. to the fact that the electrons are preferentially excited in the forward direction [6,7]. In particular, close collisions lead to high energy electrons excited in the forward direction, that can be called δ-electrons [8].
Saturation effects in highly charged ion interaction with thin carbon foils
2003, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
Citation Excerpt :
In our case, the ratio QP/vP varies from a value of 2 × 10−3 for a proton (weak perturbation) up to a value of 2 for Mo39+ (strong perturbation) at 9.2 MeV/u. We have shown that the CDW-EIS theory can be used to go beyond the linear response theory [4] and that it gives a better description of the variation of the backward yield γB versus QP. A similar trend is observed for the stopping power dependence on QP.
We present a set of experimental values for relative cross sections of Auger electron production in carbon foils for projectile charges Q_P ranging from 6 to 39 and fixed projectile velocity 19.2 atomic units. We deduce the 1s single and double ionisation cross sections and their relative variation with respect to the projectile charge. The cross section saturates for the largest projectile charges. However, the comparison of the experimental data with continuum distorted wave-eikonal initial state (CDW-EIS) and classical trajectory Monte Carlo (CTMC) calculations shows that this saturation effect is not well reproduced by the calculations. We suggest that the observed saturation effect in 1s-shell ionisation could be due to multiple electron effects not included in these models based on independent electron approximation. We have then simulated the backward electron emission yield γ_B taking into account this strong saturation effect for the 1s shell. We obtained a better agreement for both the emission yield γ_B and the stopping power. The electron emission simulation gives some insight on the relative contribution of valence and 1s-shell to saturation effects.
Experimental and theoretical study of the ratio between the electron emission yield and the electronic stopping power for protons incident on thin carbon foils
2002, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
It is often assumed that the kinetic electron emission yield γ is proportional to the electronic stopping power dE/dx for ions incident on solid targets. Though these two phenomena are based on the same physical process in which the incident ions lose their energy in the target in electron excitations, there is no reason why the ratio Λ=γ/(dE/dx) should be constant. We present in this paper a comparison between experimental and theoretical results for protons incident on thin carbon foils in a wide energy range (200 keV–9.2 MeV). The ratios Λ_B=γ_B/(dE/dx) and Λ_F=γ_F/(dE/dx) for both the backward (B) and forward (F) emission yields are studied as a function of the incident proton energy as well as the target thickness.
Simulation of particle-induced electron emission in aluminum and copper
2000, Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms
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.

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^∗: Chercheur qualifié du Ponds National Belge de la Recherche Scientifique.

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Section V. Electron emissionProportionality between the secondary electron yield and the electronic stopping power for proton impact on aluminium

Abstract

Nucl. Instr. and Meth.

Phys. Rev.

Electron emission from solids during ion bombardment

Phys. Rev.

Scanning Microsc.

Die Ioneninduzierte kinetische Elektronenemission von Metallen bei mittleren und grossen Projektilenergien, Habilitationsschrift

Section V. Electron emission
Proportionality between the secondary electron yield and the electronic stopping power for proton impact on aluminium