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Secondary Electron Emission and Atomic Shell Structure
Abstract
DOI:https://doi.org/10.1103/PhysRev.80.925
... In fact, there is no mentioning of a target-collector biasing scheme that is necessary to create the retarding field for the separation of the SEs from the backscattered electrons. This is explicitly stated by Sternglass [75] and implies that the total (secondary plus backscattering) yield is measured. Taking into account the fact that the backscattering yield can be roughly regarded as an increasing function of the atomic number above 500 eV, this partly explains why the deviations of the Kollath maxima from the Walker and Bronstein maxima increase for medium and high-Z materials. ...
The description of secondary electron emission, as presented by plasma-material interaction fusion compendia, is demonstrated to be outdated both in its theoretical and experimental aspects. As a consequence, the recommended treatment leads to a strong overestimation of the secondary electron emission yields for tokamak relevant materials. Reliable experimental data-sets, in fusion energy ranges, are identified after a detailed review of a recently updated electron-solid interaction database and previously published experimental results. A novel semi-empirical approach is proposed for the description of the secondary electron emission yield. Application of the approach for a large number of solids reveals an unprecedented agreement with experimental data. The present results can serve as a reliable input for future quantitative investigations of the effect of secondary electron emission on various aspects of scrape-off-layer physics.
... (.. .) The simple correlation of [the maximum value of the true secondary electron yield of an element] with [its] position in the periodic system would seem to indicate that a satisfactory theory of secondary emission must take into account atomic electrons more firmly bound than the valence or conduction electrons in a metal" [32]. James Joseph Lander was to solve the difficulty two years later at the Bell Telephone Laboratories, in an article the title of which explicitly invoked Auger electrons: Auger Peaks in the Energy Spectra of Secondary Electrons from Various Materials [33]. ...
It has been claimed by R. Sietmann that the attribution of the discovery of the so-called 'Auger' effect to Pierre Auger was a false attribution and that Lise Meitner should have got the credit for that discovery. However Sietmann himself recognised that Meitner's description of this effect was 'buried in' two larger papers whose primary concern was nuclear physics. Sietmann only mentioned Auger's 1925 article and did not mention his 1923 article, an omission now found in many places. We examine again L. Meitner's and P. Auger's contributions to the description of the 'Auger' effect. Meitner's concern was the exact nature of the (nuclear) beta radiations about which she opposed Ch. D. Ellis, and this had been the subject of an intense Berlin-Cambridge controversy where Ellis' description eventually prevailed. Auger's observations were the central theme of his PhD thesis at the J. Perrin's laboratory on the composed photoelectric effect. We thus believe that while L. Meitner should have shared the Nobel Prize with O. Hahn, the Auger effect has rightly been attributed to Auger.
This classified bibliography, sponsored by Subcommittee XI on Electron Microstructure of Metals of ASTM Committee E-4 on Electron Metallography, was compiled through the efforts of many scientists from several countries and includes contributions from individuals who, although not members of the ASTM subcommittee, contributed substantially to this effort.
In the microprobe analyzer, a portion of the high energy electrons impinging on the surface are backscattered from the sample and re-emitted at high energy levels. Low energy (less than 50 eV) or secondary electrons also ate emitted. Both the electron backscatter yield and the secondary electron yield are related to the mean atomic number of the target material and, hence, may be used to provide information about the target composition. Unfortunately, however, the secondary electron yield is very sensitive to the surface condition of the specimen and various instrument parameters. This complicates the otherwise simple linear relationship between sample composition and electron backscatter yield.
It is shown that the effects due to secondary electrons can be minimized by biasing the sample, and that good results can be obtained in the analysis of binary systems. The limitations and utility of the method are discussed, and backscatter yields are determined.
Parallel zur Durchstrahlungs-Elektronenmikroskopie, bei der eine Abbildung von Oberflächen nur mittelbar durch Anwendung der Abdruck- bzw. Dekorationstechnik möglich ist, werden in zunehmendem Maße elektronenoptische Verfahren zur Direktabbildung von Oberflächen eingesetzt. Als wichtigste Verfahren sind in diesem Zusammenhang die Oberflächen-Rasterelektronenmikroskopie, die Emissionsmikroskopie und die Spiegel-Elektronenmikroskopie zu nenen. Die Oberflächen-Rasterelektronenmikroskopie — im folgenden kurz als Raster-Elektronenmikroskopie (REM) bezeichnet— bietet dabei mit Abstand die universellsten Einsatzmöglichkeiten.
This chapter reviews that the promise of the electron beam as a means of addressing large quantities of digital information with high speed and low systems cost has intrigued many workers, since the early days of modern computing machines. The major advantages of any beam memory, whether photons, electrons, or ions, lies in its ability to address a location on the storage surface randomly from the third dimension. The beam is a flexible interconnection that eliminates the need for extensive hard wiring and, as such, introduces considerable flexibility into the memory design itself. Relatively large amounts of data are stored, using few components, with the consequent advantage of low cost. This review is limited to the electron beam addressed memories (EBAMs) with emphasis focused on write/read systems rather than on data recording. The objective is to review the current status of research and, in so doing, stimulate activity in an area that appears fundamentally attractive. The references are extensive but not exhaustive.
Treffen Elektronen auf feste Körper auf, so treten aus den festen Körpern wiederum Elektronen aus (Fig. 1). Diese Erscheinung der Sekundärelektronen-Emission (SE) wurde schon um 1900 entdeckt und ist seither Gegenstand vieler Untersuchungen gewesen, insbesondere seit sich die Technik für diesen Gegenstand zu interessieren begann. Man kann sich auf den Standpunkt stellen, daß es sich grundsätzlich einer Feststellung entziehe, wie weit solche „sekundären“ Elektronen ursprünglich dem Metallverband angehörten und wie weit es sich um irgendwie gestreute Elektronen des auftreffenden Strahls, um Primärelektronen (P), handelt. Die genauere Untersuchung der Erscheinung hat aber gute Gründe dafür geliefert, mit dem Namen „Sekundärelektronen“ (S) nur einen gewissen Teil der von dem Festkörper ausgehenden Elektronen mit ganz bestimmten Eigenschaften zu bezeichnen. Daß zum mindesten ein Teil der „sekundären“ Elektronen nicht primäre Elektronen sein können, geht schon daraus hervor, daß in vielen Fällen mehr „sekundäre“ Elektronen den Festkörper verlassen, als primäre aufgetroffen sind.
The interrelation of the secondary-electron emission (SEE) coefficient δm
with the target-atom size, the electron configuration of the valence shells thereof, and the principal quantum number of 44 elements is established. It is revealed that the traditional concept of the δm
dependence on the atomic number Z of elements is incorrect because parameter Z “hides” various characteristics of the electronic structure of an atom. The coefficient δm
is demonstrated to depend linearly on the size of the atom r and the quantum number N of valence shells in such a way that the value of δm
decreases with increasing r and decreasing N. The “dipole” mechanism of SEE based on the obtained results and G.V. Samsonov’s electron-localization model is proposed.
This chapter presents a study of secondary electron emission from solids. The phenomenon of secondary emission (SE) from solids was discovered in 1902 and has been the subject of numerous experimental and theoretical investigations. It consists in the following process: If primary electrons (P) impinge on a solid, secondary electrons (S) are observed leaving the surface in free space. The maximal experimental information about these S can be obtained (neglecting spin) by measuring the number of S emitted per second from 1 cm2 of the surface with energy E in the direction Ω(ν,φ). This function is the detailed current density of observed S, denoted by js(E,Ω). The js(E,Ω) can depend only on the states of the interacting systems, that is to say, on the properties of the primary beam arid on the physical and chemical properties of the emitter, such as chemical composition, crystal structure, surface conditions, temperature, and so on. In this chapter, a qualitative description is presented of how the different elementary processes of SE are connected with one another. It discusses the distribution function. Experimental investigations, particularly those published in recent years, are also discussed.
Electron emission from solids may be induced by various means: high temperatures, strong electric fields, and bombardment with photons or particles, particularly charged particles. The chapter discusses the emission induced by electron bombardment called as “secondary electron emission,” discovered during a study of the reflection of electrons from metals. The theory of secondary emission is far from complete. The main reason for this state of affairs is the complexity of the problem, which becomes evident when one considers qualitatively what happens in the secondary emission process. This first step in the process presumably involves the interaction of a beam of incident electrons of energies in the range between 100 ev–10 kev with the solid. This leads to a cascade process of ionization and excitation, combined with elastic and inelastic scattering of the cascade electrons. The second step is concerned with the number of cascade electrons that escape from the surface and with their energy distribution.
In the preceding chapter, we have described a single EHD or EMHD fluid model as a continum for dusty and dirty plasmas. For tenuous dusty plasmas, however, dust particles can highly be charged due to a variety of processes such as electron and ion collection from the ambient plasma, photoelectron emission, secondary electron emission, electric field emission, thermionic emission, triboelectric emission and so on as discussed in this chapter. For such cases, environments need to be considered a multi-component plasma or fluid, including a dust component. In this chapter, we therefore describe a multi-component fluid model for such a plasma, comprising electrons, ions (positive or negative), uncharged and charged (negatively or positively) dust.
Based on the main physical processes of secondary electron emission, experimental results and the characteristics of backscattered electrons (BE), the formula was derived for describing the ratio (βangle) of the number of secondary electrons excited by the larger average angle of emission BE to the number of secondary electrons excited by the primary electrons of normal incidence. This ratio was compared to the similar ratio β obtained in the case of high energy primary electrons. According to the derived formula for βangle and the two reasons why β > 1, the formula describing the ratio βenergy of β to βangle, reflecting the effect that the mean energy of the BE WAVp0 is smaller than the energy of the primary electrons at the surface, was derived. βangle and βenergy computed using the experimental results and the deduced formulae for βangle and βenergy were analyzed. It is concluded that βangle is not dependent on atomic number z, and that βenergy decreases slowly with z. On the basis of the two reasons why β > 1, the definitions of β and βenergy and the number of secondary electrons released per primary electron, the formula for βE-energy (the estimated βenergy) was deduced. The βE-energy computed using WAVp0, energy exponent and the formula for βE-energy is in a good agreement with βenergy computed using the experimental results and the deduced formula for βenergy. Finally, it is concluded that the deduced formulae for βangle and βenergy can be used to estimate βangle and βenergy, and that the factor that WAVp0 increases slowly with atomic number z leads to the results that βenergy decreases slowly with z and β decreases slowly with z.
The feasibility of electric field measurements by the double sphere technique both inside and outside the magnetosphere is critically reviewed. In particular, influences resulting from photo emission, secondary electron emission and the plasma environment are analyzed. It is concluded that a double sphere aerial, 50-100 m long, can measure dc electric fields as low as 0.1 mV m-l, in the solar wind and during conditions with ambient electron current density below 10-6 A m-2 in the magneto sphere, provided certain precautions are taken regarding probe symmetry. In the magnetosphere, for current densities larger than 10-6 A m-2, the space charge of photoelectrons escaping from the satellite will give rise to potential asymmetries near the probes, and an electron gun is required for control of the satellite potential and reduction of space charge asymmetries.
The physical mechanism of secondary electron emission under the impact of high-speed heavy particles is analyzed. The treatment is based on the formation of secondaries according to the Bohr-Bethe theory of ionization, the diffusion of the slow secondaries to the surface, and their subsequent escape in the vacuum. The yield is found to be proportional to the rate of energy loss of the incident particles, and it is shown to be essentially the same for all metals, independent of their work function, conductivity, and other bulk properties. The observed energy distribution of the secondaries, the effect of adsorbed layers and the dependence of the yield on temperature, particle charge, and velocity are found to be explained in terms of this mechanism. The application to the general problem of the escape of electrons from metals and to the study of electron capture and loss by ions passing through solids is discussed.
For the secondary electron emission studies a series of poly- and nano-crystalline diamond films with different characteristics (crystallinity, morphology and dopants) were produced using an innovative CVD apparatus that allows the doping of diamond layers. Moreover additional hydrogenation processes were carried out after the first emission experiments in order to obtain H termination at three diamond surface and enhance the electron yield. At 1 KeV a gain of 5.5 has been obtained for Ti-doped diamond layers and for diamond layers containing C-sp2 clusters.
The stopping cross sections of manganese, copper, germanium, selenium, silver, tin, antimony, gold, lead, and bismuth are reported for protons in the energy range 400 to 1000 kev. The cross sections are roughly proportional to the square root of the atomic number of the stopping element and inversely proportional to the velocity of the incident protons. There is some evidence that the stopping cross section increases more rapidly than Z12 as the various p shells are filled.
A survey of data from the first year of the P78-2 (SCATHA) satellite operations showed that severe spacecraft frame charging (πf) both in sunlight (-340 V) and in eclipse (> -8 kV) occurred on 24 April 1979. Analysis of the data indicates that if the sunlight charging environment had been present during eclipse, the vehicle would have charged in excess of - 15 kV, which is above any known charging level observed to date for the SCATHA satellite. Therefore, the environment at the peak of the sunlight charging at -0650 UT 24 April 1979 was chosen for this "worst case" study. The environment at this time is characterized by an injection of high-energy (30-335 keV) electron fluxes whose combined current correlates with πf with a correlation coefficient of 0.95. The fluxes were highly anisotropic, maximizing perpendicular to the magnetic field. The low-energy (<4 keV) electron population had a density <1 cm⁻³ and the low-energy ions were near background. The measured electron distribution functions, when fitted to double Maxwellians by a least-squares technique, show that throughout the sunlight charging period the high and low temperatures remained nearly the same, while the density of the high-energy component followed the charging levels. The injection occurred simultaneously with the rapid return of the magnetospheric magnetic field to a more dipolelike configuration. © American Institute of Aeronautics and Astronautics, Inc., 1983, All rights reserved.
The secondary emission ratio as a function of primary energy is determined for targets of bismuth, gallium, lead, and mercury for the metals in both the liquid and solid state. The secondary emission characteristics for liquid surfaces are shown to be very nearly like those for solid surfaces, and in general the shape of the secondary efficiency curves for these materials are similar to those for other pure metals. A comparison of the observed maximum secondary ratios with predicted values is made.
It is shown that a simple theory based on the constant energy loss per unit path length of primary electrons accounts quantitatively for the variation of secondary electron emission yield below its maximum value. The theory can be extended formally to include a Bethe-type energy loss at high primary energies. An attempt was made to clarify the present situation concerning the relationship between the secondary electron emission and the atomic structure of the elements, and some new relations are indicated. The mechanism of the secondary emission of insulators and semiconductors is also discussed.
Deep dielectric charging is the suspected mechanism for formation of potential barriers aboard the ISEE 1 spacecraft. Energetic electron distribution functions in the plasmasheet were examined for both surface and deep dielectric charging. Surface charging was found to be dependent on whether the satellite surfaces were in shadow. The surface potential is regulated by photoelectric emission, and is two orders of magnitude higher than other mechanisms. Deep dielectric charging deposits charge within dielectrics, and is independent of surface effects, such as photoemission and radiation-induced conductivity. Deposition of electrons into solar array cover cells begins at approximately 10 keV.
Based on a simple atomic model giving the potential between electrons and atoms as V(r)=Ze2as −1/srs, the range of electrons penetrating solid targets is derived. Starting from the generalized power law involving the energy loss, the Lenard-type absorption law, and the assumption that the distribution of the secondary electrons due to both incident and backscattered electrons within the target is isotropic, a theoretical universal reduced yield curve of secondary electrons and the resulting maximum yield, which are found to be in good accordance with results obtained experimentally, are deduced as a function of three parameters such as atomic number, resonance potential and backscattering coefficient.
The high yield of secondary electron emission from insulators due to electron bombardment may be the result of an increase of the depth of escape. The free-electron scattering theory is applied to the high energy of the primary beam, but it cannot be applied to the low energy of the secondary escaping beam because of the large energy gap of insulators. Then the plasmon loss with the valence electron is considered when the secondary electrons escape. Secondary electron emissions from insulators are calculated from the assumptions that the distribution of the secondary electrons due to both incident and back-scattered electrons within the target is isotropic and that it follows the absorption law of Lenard type. The universal yield-energy curve of secondary electron emission is derived.
Following a suggestion by Bronshtein and Fraiman that the total secondary emission coefficient for solid mercury measured by Brophy was too low, a programme of research was initiated, and a series of measurements produced evidence that there are two mechanisms for contamination of a mercury target in a measuring tube evacuated by a mercury diffusion pump. The first, which lowers the values of the total coefficient, is thought to be water vapour on the surface of the target. The second, which raises the value, may be backing pump oil contamination.
A system has been devised with which it is possible to control and eliminate these contaminants. The results obtained for the total and partial coefficients for pure, uncontaminated samples agree with those predicted by Bronshtein and Fraiman from their study of regularities in the behaviour of the coefficients with atomic number. The lower values of Brophy are shown to be due to contamination.
There is no change in the values of the coefficients as mercury solidifies. However, starting from room temperature, the values of the total and partial coefficients have been found to fall and rise respectively as the temperature is lowered.
A new and thorough examination of secondary electron (SE) yield as a function of primary energy (EPE) and atomic number Z for the 44 elements in the database1 is made. The principles of the semiempirical universal law for the SE yield are described and a template for Monte Carlo (MC) simulation is produced accordingly. Both universal curve fitting and MC simulation are made for the 44 elements. The resulted maximum SE yield δm, corresponding primary energy , SE excitation energy ε, and effective escape depth λ are tabulated and plotted as a function of atomic number Z. It is found that similarities exist in the profiles of ε and λ, δm and , and all of these parameters seem to have characteristics associated with atomic shell filling. Copyright © 2005 John Wiley & Sons, Ltd.
Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.
The order of magnitude of the equilibrium potential of a surface exposed to the particle environment outside the magnetospheric plasmasphere is calculated. This is done under the assumption that no solar irradiation reaches the surface, thus no photo-emission becomes effective. It is shown that for a given surface material the equilibrium potential depends strongly on the energy distribution of the particle environment. In some cases secondary electron emission helps to avoid high negative equilibrium potentials. It can be concluded that materials with high secondary electron yields such as BeCu and SiO2 have the most useful properties to the extent that highly negative surface charges are avoided in nearly all particle environments which are encountered inside the magnetosphere.
Presented here are electron-induced electron yield measurements from high-resistivity, high-yield materials to support a model for the yield of uncharged insulators. These measurements are made using a low-fluence, pulsed electron beam and charge neutralization to minimize charge accumulation. They show charging induced changes in the total yield, as much as 75%, even for incident electron fluences of <3 fC/mm2, when compared to an uncharged yield. The evolution of the yield as charge accumulates in the material is described in terms of electron recapture, based on the extended Chung and Everhart model of the electron emission spectrum and the dual dynamic layer model for internal charge distribution. This model is used to explain charge-induced total yield modification measured in high-yield ceramics, and to provide a method for determining electron yield of uncharged, highly insulating, high-yield materials. A sequence of materials with progressively greater charge susceptibility is presented. This series starts with low-yield Kapton derivative called CP1, then considers a moderate-yield material, Kapton HN, and ends with a high-yield ceramic, polycrystalline aluminum oxide. Applicability of conductivity (both radiation induced conductivity (RIC) and dark current conductivity) to the yield is addressed. Relevance of these results to spacecraft charging is also discussed.
A theoretical model is developed for the density and temperature of confined electrons and the plasma potential in low-density hot-filament discharges. These three parameters are found from a simultaneous solution of the equations for ion particle balance, electron particle balance, and electron energy balance. In the model, electrons are lost by diffusion in velocity over the potential barrier determined by the plasma potential. The confined electrons are heated by the unconfined electrons that are the secondaries from the wall and, to a lesser extent, by the primary electrons from the filaments. The plasma parameters calculated from the model agree with parameters measured in a double plasma device that has been modified to have a clean wall that gives a single value for the confining potential.
Experience has indicated a need for uniform criteria, or guidelines, to be used in all phases of spacecraft design. Accordingly, guidelines have been developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy (less than approximately 50 kev) space charged-particle environment. Interior charging due to higher energy particles was not considered. This document is to be regarded as a guide to good design practices for assessing and controlling charging effects. It is not a NASA or Air Force mandatory requirement unless specifically included in project specifications. It is expected, however, that this document, revised as experience may indicate, will provide uniform design practices for all space vehicles. Refs.
Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. A study of changes in electron emission yields as a result of internal charge buildup due to electron dose is presented. Evolution of total, backscattered, and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton and Al2O3. Reliable yield curves for uncharged insulators are measured, and quantifiable changes in yields are observed due to <100-fC/mm2 fluences. Excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve is found; this includes a decrease in the rate of change of the yield as incident energies approach the crossover energies and as accumulated internal charge reduces the landing energy to asymptotically approach a steady state surface charge and unity yield. It is also found that the exponential decay of yield curves with fluence exhibit an energy-dependent decay constant alpha(E). Finally, physics-based models for this energy dependence are discussed. Understanding fluence and energy dependence of these charging processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulators
An object, whether conducting or not, takes on charge in a plasma.
If the object is a spacecraft, and the plasma is provided by a planetary
magnetosphere, the act of charging can produce a variety of unwanted
effects. During the last twenty years, spacecraft have been flown
specifically to measure charging and discharging effects; ground
experiments, performed to measure the fundamental properties of
materials used in spacecraft, have provided charging parameters and
testing procedures and guidelines for entire spacecraft design. This
review summarizes the progress
The production of secondary emission by the interaction of bombarding electrons with the valence electrons of a metal target is quantum-mechanically treated. When the results are modified by considerations having to do with the relative rates of absorption of the primary and secondary particles, yield-vs.-bombarding energy curves are obtained which approximate the results of experiment. The primary voltage required for maximum yield, the effect of work function on the emission, and the energy range of the secondary particles are semi-quantitatively derived in terms of the properties of the target material.
A theory of secondary electron emission from metals is formulated on the basis of the Sommerfeld free-electron model, momentum transfer between electrons and lattice being included by introducing a finite mean free path for elastic scattering. The approach to the problem is similar to that of Kadyschewitsch, but the development is simpler and comparison with experiment is made in more detail. An understanding is reached of the influence of work function and the width of the conduction band, making it clear why, on the average, metals with large work function might be expected to be the best emitters. The observed effect of changing the work function of a given metal by surface layers of foreign atoms is interpreted, an inverse relationship between emission and work function being obtained which is in qualitative agreement with experiment. The theory also accounts for the velocity distribution of the secondaries, giving the general shape of the curve and determining the approximate position of the maximum, and is consistent with the observed angular distribution of the secondaries. The investigation is not carried far enough to give new theoretical information concerning the variation of secondary emission with primary energy. However, the relation of the present theory to some work of Bruining is indicated, and attention directed to an important empirical relationship.
- B. F. J. Schonland