Secondary electron coefficients

However, a much coarser approach can yield significant results. Several authors, for example, have attempted to relate the kinetic-secondary-electron coefficient to the stopping-power relationships used for the analysis of atomic ion ranges in solids. A very lucid paper on this approach has been presented by Beuhler and Friedman and their perspective will be summarized in the following paragraphs. 

Secondary-electron coefficients are strongly dependent upon the condition of the surface. The presence of adsorbed gas or surface roughness can significantly alter the number of secondary electrons. Moreover, much of the work in this field predates ultra-high-vacuum technology and the associated surface-characterization tools (for reviews see Refs. 144-146). In addition, surfaces exposed to a plasma are not well characterized. Therefore, crude, estimates of the magnitude of the secondary-electron coefficients seem to be the most useful type of data in the present context. 

Analytical technique. Secondary electrons are produced by inelastic collisions between high-energy beam electrons and atoms within the specimen (Goldstein et al., 1992). The number of secondary electrons emitted, and the resulting secondary electron coefficient, is... 

Channel Electron Multiplier (CEM) or channeltron is an electron detector that is used to multiply each electron (up to 10 times) to provide a pulse output suitable for further amplification by conventional electronic circuits. This is a bent tube that is coated with a photoelectric material (of a specific work function) with a high secondary electron coefficient. The tube is kept at a potential of about 2.5 kV. When the electrons pass through the inlet aperture of the CEM and strike the surface of the CEM, a collision of sufficient energy between the ultraviolet radiation and the CEM wall will eject at least one electron. When an electron strikes the mouth of the tube, a number of secondaries is produced that is accelerated in the channeltron. A local electric field created by the bias voltage of the power source accelerates these... 

The primary-ion bombardment leads not only to the desorption of sputtered elements and molecules but also to the emission of further. secondary species, among them electrons. The number of emitted electrons per primary ion (the so called ion-induced secondary-electron coefficient) can be as large as 10 according to the particular material. Ion bombardment of insulating surfaces therefore leads to the buildup of positive charge at the surface which can se-... 

Once electrons have been emitted by the photocathode, they are accelerated by an applied voltage induced between the photocathode and the first dynode (Uq in Figure 3.17). The dynodes are made of CsSb, which has a high coefficient for secondary electron emission. Thus, when an electron emitted by the photocathode reaches the first dynode, several electrons are emitted from it. The amplification factor is given by the coefficient of secondary emission, S. This coefficient is defined as the number of electrons emitted by the dynode per incident electron. Consequently, after passing the first dynode, the number of electrons is multiplied by a factor of 5 with respect to the number of electrons emitted by the photocathode. The electrons emitted by this first dynode are then accelerated to a second dynode, where a new multiplication process takes place, and so on. The gain of the photomultiplier, G, will depend on the number of dynodes, n, and on the secondary emission coefficient, 5, so that... 

Fig. 9. Relative secondary-electron yield as a function of ion energy for Ne "—Na ", Ar —K ", and Kr —Rb. A constant quantity equal to the estimated potential-secondary-emission coefficient has been substracted from the raw noble gas data. (From Ref. )...

A MCP is used in the PIMMS as a secondary electron multiplier (see Sect. 3.7). The electron current measured after MCP compared to the initial ion current is amplified by a factor of 10-1,000. The secondary electron emission coefficient is an averaged number of secondary electrons emitted after each impact. This number depends on the initial energies of the electrons or ions and so on the voltage applied to the MCP. The amplification factor of a MCP configuration is expressed as ... 

After ionising the gas molecules, the positive ions generated migrate to the cathode and can liberate secondary electrons. The efficiency of this secondary electron production is given by the second Townsend coefficient 7 which is the fraction of secondary electrons liberated. This coefficient depends on the material of the cathode and is typically in the range around 0.1 [14]. 

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