Secondary Electron Emission Characteristics Research Articles

The “ Q factor” method: quantitative microPIXE analysis using RBS normalisation

The main uncertainties in the absolute quantitation of PIXE elemental concentrations are the total deposited beam charge and the local matrix composition. This is particularly true in the case of microbeam PIXE analysis, where local inhomogeneities in the sample may affect both the secondary electron emission characteristics (and hence the recorded charge) and the PIXE yield. The use of simultaneous RBS analysis can solve this problem in many cases. Provided the RBS spectrum can be well modelled, its shape gives information on the local matrix composition, while the total area depends on the total beam charge. The ratio between the true charge and the measured charge (the “ q factor”) can then be used to normalise the PIXE data. Once the system is calibrated, standarless accuracy of 5–10% can be achieved in most cases. The method is described together with its limitations. Examples of the use of the method are presented.

Recent developments in data acquisition and processing on the Oxford scanning proton microprobe

A new data acquisition system has been developed for the Oxford SPM facility. This is based on a single PC-compatible computer with a custom designed interface unit and permits the recording of several channels of pulsed detector data together with x and y scanning voltages, beam charge and deadtime. In addition, the system will also control the beam position allowing the generation of irregular scan areas, line scans at any angle and the selection of point analyses using the mouse. The hardware and software of the system are described and some examples of data collected using the system are presented. One problem of quantitative microanalysis using focused beam PIXE is the fact that the local matrix composition and secondary electron emission characteristics may change on a scale comparable with the beam size across the sample. This leads to uncertainties in the measured charge (especially at the low currents found in microbeams) and local self-absorption correction which can introduce significant inaccuracy into the analysis. This problem has been largely solved at Oxford by the simultaneous use of RBS analysis. The shape of the RBS spectrum yields the local matrix composition and its area gives a measure of the total charge, so that the dependence on precise charge measurement is much reduced. This “ Q factor” technique is described in detail and it is shown that once the system has been calibrated routine standardless accuracy of the order of 5–10% can be obtained.

Onset characteristics of secondary electron emission from Si(100) coadsorbed by Cs and Cl

Second derivative energy spectra of secondary electron emission (SE) near the onset have been measured in the coadsorption of Cl and Cs on Si(100). Two SE peaks are observed when Cl atoms are deposited on Cs-covered Si(100), while only one onset peak for the reverse order of deposition of first Cl and second Cs. Both two peaks are ascribed to the SE onsets due to cascade electrons rather than electrons due to the electron density of state above the vacuum level. The appearance of the two SE onsets is quite sensitive to primary electron beam energies, E p. For E p < 150 eV two SE onsets appear, while for E p > 200 eV a single onset appears. This fact suggests that the two SE onsets occur when the secondary electrons are preferentially generated in the selvedge. The surface structure with two SE onsets is still stable even after the Cl atoms are removed by electron stimulated desorption. From a deconvolution analysis of the double peak, we propose a surface structure with Cs rich patches.

Surface damage effects on secondary electron emission From the negative electron affinity diamond surface

ABSTRACTThe effects of surface damage on the secondary electron emission characteristics of a natural diamond (100) surface have been investigated using ultraviolet photoelectron spectroscopy and scanning electron microscopy. Surface damage was intentionally induced by abrading the (100) diamond face with diamond paste. Removal of the damage was achieved by a sequence of ion implantation, graphitization, electrochemical etching and oxygen/argon plasma etching. Prior to characterization performed between steps in the sequence, the surface was hydrogenated by exposure to a hydrogen plasma in attempts to create a negative electron affinity surface condition. Upon removal of the surface damage, the secondary electron yield from the negative electron affinity surface was enhanced by a factor of ˜20 over that from the damaged negative electron affinity surface.

Secondary electron emission gaseous detectors for fast X-ray imaging

A new class of X-ray imaging gaseous detectors was developed, based on photon conversion in a thin film and the detection of the emitted secondary electrons with a low-pressure avalanche wire chamber. Their main features are parallax-free imaging with a submillimeter spatial resolution over a broad range of X-ray energies, a nanosecond response and a counting rate capability above 1 MHz/mm 2. The imaging characteristics of a large area secondary electron emission (SEE) detector equipped with CsI, Ag and Ta photoconvertors, in the photon energy range of 6–60 keV are summarized. In localization capability this detector can successfully compete with commercial image intensifier systems and medical film-screen combinations.

Characteristics of secondary electron emission from CsI induced by x rays with energies up to 100 keV

A microscopic model for low energy electron interaction in alkali halides was used to simulate secondary electron emission from CsI induced by x rays with energies up to 100 keV. The integral ‘‘current’’ and ‘‘pulse’’ yields were calculated as function of the x-ray energy, CsI convertor thickness, and angle of incidence. We observe a decrease in true low energy (&amp;lt;50 eV) secondary electron yields at increasing x-ray energies and discuss the effectiveness of CsI convertors coupled to gaseous electron multipliers developed for fast, high resolution x-ray imaging.

Ion beam treatment of potential space materials at the NASA Lewis Research Center

Ion source systems, in different configurations, have been used to generate unique surface morphologies for several NASA space applications. The discharge chamber of a 30 cm ion source was successfully used to texture potential space radiator materials for the purpose of obtaining values of thermal emittance greater than 0.85 at 700 and 900 K. High absorptance surfaces were obtained using ion beam seed-texturing, for space radiator materials that were flown on the long duration exposure facility (LDEF) for 5.8 years in space. An ion source discharge chamber was also used to develop electrode surfaces with suppressed secondary electron emission characteristics for use in collectors in microwave amplifier traveling wave tubes. This was accomplished by sputtering textured carbon onto copper as well as texturing copper using tantalum and molybdenum as sacrificial texture-inducing seeding materials. In a third configuration, a dual-ion-beam system was used to generate high transmittance diamond-like carbon (DLC) films.

Multi-electron secondary emission from solid surface bombarded by fast atomic particles

This article reviews the most recent experimental approaches to the study of, and the results of investigations of multi-electron secondary emission resulting from the bombardment of the surface of a solid by atomic particles with energies of 1–103 keV. The results of measurements of the differential characteristics of secondary electron emission (emission statistics, energy and angular distributions) are discussed. Theoretical approaches to a quantitative description of the principal characteristics of multi-electron secondary emission are considered, and the results of a Monte Carlo modeling, reproducing the principal observed MUSE effects are discussed.

Ion implantation in China

Ion implantation in silicon has been widely used in the production of semiconductor devices and integrated circuits in China. Research of low energy ion implantation, high energy ion implantation and the SOI technology is in progress. Fully Si-implanted planar GaAs dual-gate MESFETs, high linear GaAs Hall effect sensors, light emitting diodes, laser devices, and infrared detectors made of HgCdTe have been fabricated by ion implantation and have been applied in telecommunication, transportation, medicine, and in a ground satellite station. The research of ion implantation in metals began in 1978 in China. Ion implantation for improving wear, corrosion, oxidation, fatigue-resistance and secondary electron emission characteristics has progressed. The artificial joints treated by ion implantation have been used in trial medical treatment. Thin films of high T c superconductors of Y 1Ba 2Cu 3O 7 zero resistance temperature of 86 K have been worked out with reactive ion beam coating. Recently, ion beam enhanced deposition has been used to synthesize Si 3N 4 and other compound films. The process of ion beam enhanced deposition has been studied by a dynamic ion implantation model and Monte Carlo computer simulation. It was at the end of the 1960s that China began to work on ion implantation. China now has more than 80 units involved in the field of research and application of ion implantation, including scientific research institutes, universities and factories.

Secondary Electron Emission Characteristics of Heavy-Metal Oxide Coatings for CRT Shadow Mask

In order to reduce the misalignment caused by the thermal expansion of the shadow mask originated from the deposition of the primary electron energy, the secondary electron emission (SEE) characteristics of Bi2O3 and Fe3O4 coatings for the CRT shadow mask are studied. The total SEE yield of the Bi2O3 coating varies from 0.96 at a primary electron energy of 15 keV to 0.75 at 30 keV. These values are about twice as high as those of the Fe3O4 coating. The backscattered electron yield of the Bi2O3 coating increases almost linearly from 0.36 at a primary electron energy of 15 keV to 0.40 at 30 keV. That of the Fe3O4 coating also increases in the same manner, although its magnitude is much smaller. These measured values agree with the estimated values from the reported data which give the SEE yield as a function of the atomic number. The average energy of the backscattered electrons can be also estimated to be about 24% of the incident primary energy for the Bi2O3 coating, and 12% for the Fe3O4 coating. These results shows that the Bi2O3 coating effectively reduces the deposition of primary electron energy onto the shadow mask by the backscattering of incident primary electrons. Therefore, the Bi2O3 coating will result in the reduction of the temperature rise and the thermal expansion of the shadow mask to a greater extent than the Fe3O4 coating.

Secondary electron emission characteristics of oxidized beryllium cathodes

AbstractOxidized beryllium surfaces are used in a number of microwave crossed‐field amplifier (CFA) tubes as secondary electron emission cold cathodes. During operation the cathode surface is subjected to electron and ion bombardment in a complex process which produces both erosion and replenishment of the oxide surface, in addition to a significant build‐up of amorphous carbon. The result is a general degradation of tube performance. Special techniques were developed to opoen an evacuated tub and transfer the cathode to a UHV surface analysis system with minimal contamination from the atmosphere. Three CFA tubes with various histories of operation were examined and measurements from made of the secondary electron emission (delta), first crossover, voltage at delta maximum, and the surface chemical composition by AES at several points on each cathode. Significant differences were found which reflected their number of hour of operation, as well as variations in manufacturing procedures. A generalized linear cofrrelation between secondary electron emission and the surface carbon atomic fraction was obtained when the data for all the tubes was combined and analyzed. The deleterious effects of carbon in operated CFA tubes were confirmed and suggest that reduction of potential sources of carbon contamination in the tube during manufacture, processing, and operation should improve overall performance.

Improvement of secondary electron emission characteristics by ion implantation

Improvements of the secondary electron emission coefficient and oxidation resistance of copper grid by Cr ion implantation have been investigated. In comparison with the copper grid coated with carbon, ion implantation is a prospective and practical method for improving the secondary electron emission characteristics.

Carbon and carbon-coated electrodes for multistage depressed collectors for electron-beam devices—A technology review

The efficiency of multistage depressed collectors (MDC's) for electron-beam amplifier tubes is strongly affected by the secondary electron emission characteristics of the collector electrode surfaces. Specifically, to recover the maximum kinetic energy from the tube's spent electron beam and therefore to achieve high collector efficiency, the electrode surfaces must have low secondary electron emission properties. Copper, the most commonly used MDC electrode material, has relatively high emission properties. While several methods for reducing these emission properties have been applied with some success, graphite or carbon electrodes are currently being considered as much more promising electrode materials. Furthermore, experimental studies have demonstrated that the already low emission properties of these carbon forms can be reduced to the lowest levels observed thus far by ion texturing their surfaces under the proper conditions. Recently, a method of applying a highly textured carbon film on a copper substrate was developed that has a particularly significant potential for the MDC electrode application. This paper reviews important aspects of the carbon and carbon-coated MDC electrode technology developed at the NASA Lewis Research Center as well as related technology developed under NASA sponsorship. The following topics are included: electrode material properties, surface treatment procedures, surface physical characteristics, secondary electron emission measurements, and MDC fabrication methods. The performance of MDC's with untreated graphite electrodes, ion-textured graphite electrodes, and textured, carbon-coated copper electrodes is also discussed.

The use of three-component polymeric composites in preparing a channel electron multiplier

The characteristics of secondary electron emission (SEE) of three-component composites including organic compounds have been investigated. The maximum SEE yield for conductive composites which consist only of polyvinylchloride and electroconductive particles such as NATCNQ and carbon black is low (less than 1). The addition of a third component, such as titanium, magnesium or beryllium oxides, sodium chloride, or the complex of TCNQ with anthracene, has enabled the authors to produce composites characterised by high maximum SEE yields for low energy of primary electrons. The study of the SEE yield from such electron-conductive polymeric composites shows that the SEE yield depends above all on the species and quantity of the third compound. On the basis of the data obtained a channel electron multiplier CEM has been developed. The CEM made of the three-component system is characterised by a gain of 107 to 108 (3 kV) at the maximum output current of 10-6 A.

Surface flashover of solid dielectric in vacuum

The surface of a solid dielectric insulator becomes electrically charged when subjected to a high-voltage stress in vacuum. A method for calculating the surface flashover voltage based on the assumption that the discharge occurs in a layer of desorbed gases from the insulator surface is proposed. The electric field strength required to cause surface flashover is calculated by taking into account the secondary electron emission characteristics of the dielectric material. The dependence of the surface flashover field on the insulating material is deduced. A dependence of the flashover voltage on the insulator length to a power law of 0.5 is theoretically predicted. The calculated surface flashover voltage is compared with the previously reported measurements and good agreement is obtained.

Characteristics of Secondary Electrons Ejected from Solid Targets by Multiply Charged Ions with Energy up to 20 MeV/Nucleon

AbstractDetailed characteristics of secondary electron emission from silicon after bombardment by multiply charged ions Al, C, O, Ca, Si, Fe with energies up to 20 MeV/nucleon are deduced from Monte‐Carlo simulations. The calculated integral yields η in forward direction to approach the η—Zeff law. The discrepancies between calculated and experimental data for fission fragments are discussed. Limits for ion charge resolution by secondary particle counting are obtained.

Determination of secondary electron emission characteristics of lunar soil samples

A procedure is described for the determination of the “apparent crossover voltage”, i.e. the value of the primary (bombarding) electron energy at which an insulating sample surface changes the average sign of its charge. This apparent crossover point is characteristic of the secondary emission properties of insulating powders such as the lunar soil samples. We found that lunar core samples from well-defined, distinct soil layers differ significantly in their secondary emission properties. This observation supports the suggestion that soil layers were deposited by an electrostatic transport process.

Secondary electron emission from polymers and its application to the flexible channel electron multiplier

The basic characteristics of secondary electron emission (SEE) from various organic compounds have been investigated, and a channel electron multiplier with high gain and flexibility has been developed. The maximum SEE yield is higher for the aliphatic compound than for the aromatic, and is higher for the organic solid with high ionization potential. By studying the SEE yields from the electron-conductive polymeric compositions which consist of plasticized poly(vinyl chloride) and electroconductive particles (NaTCNQ or carbon black), it is shown that the SEE yield depends mainly on the characteristics of the matrix polymer and is almost independent of the addition of electroconductive particles which inherently have low SEE yields. Adding less than 5% stabilizers to these polymeric compositions has little effect on the SEE yields. A flexible channel electron multiplier (FCEM) made of the electron-conductive polymeric composition shows the following characteristics: gain ≃ 108 (applied voltage of 3 kV); rise time ≃ a few nanoseconds; background count rate < 0.1 cps; and maximum output current ≃ 10−6 A. As a photon detector in the vacuum UV region, the FCEM shows a threshold value of 8.4 eV for photoelectric emission.

Secondary electron emission characteristics of dielectric materials in AC-operated plasma display panels

From the measurement of the ion-induced secondary electron emission yield γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> it was shown that with increasing γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> of the dielectric panel material the firing voltage of an ac gas discharge display panel decreased. The measurement of the energy distributflon of the secondary electrons showed that the dielectric material with higher γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> has lower most-probable energy (MPE), and half-width (HW) values. The energy distribution introduced by Stolz in the ease of secondary electrons emitted from a metal was applied to the dielectric panel material and solved by using a computer. From this result, it was shown that the lower the electron affinity of the dielectric panel material, the higher is the γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> . Under the condition when γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> became greater than unity, unstable self-sustained emission was observed even though the primary ion beam was cut off.

Secondary Electron Emission from Insulators by Ion Bombardment

The characteristics of secondary electron emission from a glass surface by Ar/sup +/ ions were investigated experimertally. The yields from glass and quartz microsheets were larger than those from stainless steel. It was first observed that field-enhanced emission and seif-sustained emission were caused by the surface charge. Because increasing of the yield has technological demand, HF etching and sandblast were attempted as one of the means of increasing yield. Then the yields of etched and sandblasted samples were about two times larger than that of nonetched or nonsandblasted sample. Field-enhanced emission and seif-sustained emission cannot be irterpreted only with kinetic ejection or potertial emission theory. A simple model is proposed in which the work function decreases by the band bending with the accumulation of positive charge near the surface. (auth)