Electrons secondary emission coefficient

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. )...

It should be pointed out that the gain mechanism in a PMT tube operates as a random process. The number of secondary electrons is different for each primary electron. The relative width of the distribution can be expected at least of the size of the standard deviation of a poissonian distribution, n, of the secondary emission coefficient, n, at the first dynode. Therefore the single-photon pulses obtained from a PMT have a considerable amplitude jitter, see Sect. 6.2.5, page 226. 

Table 2-14. Secondary Emission Coefficient y for the Potential Electron Emission Induced by Collisions with Metastable Atoms...

Another secondary electron emission mechanism is related to surface bombardment by excited metastable atoms with an excitation energy exceeding the surface work functiom This so-called potential electron emission indnced by metastable atoms can have a quite high secondary emission coefficient y. Some of them are presented in Table 2-14. 

Photomultipliers are now often replaced by microchannel plates. They consist of a photocathode layer on a thin semiconductive glass plate (0.5-1.5 mm) that is perforated by millions of small holes with diameters in the range 10-25 xm (Fig. 4.111). The total area of the holes covers about 60 % of the glass plate area. The inner surface of the holes (channels) has a high secondary emission coefficient for electrons that enter the channels from the photocathode and are accelerated by a voltage applied between the two sides of the glass plate. The amplification factor... 

The energy spectrum of electrons emitted by a specimen in an electron beam has two maxima. One is at high energy where most of the back-scattered electrons are, and the other is at only a few eV. These are secondary electrons, which by definition include all electrons emitted at less than 50 eV. The number emitted divided by the number of incident electrons is the secondary emission coefficient 5. For 20keV incident electrons in the SEM, the coefficient is close to... 

The secondary emission coefficient increases as the beam energy falls from 20keV, and this is to be expected. At low beam energies, more of the interactions occur near the surface, so more of the low energy electrons produced can escape. Eventually at very low beam energy the coefficient falls, as the incident electrons do not have the energy to do very much. [60,71]. 

The relevant characteristic of the active surface in any electron multiplier is the secondary emission coefficient. 

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 ... 

Bombardment of a Cu-Be alloy dynode (an electron multiplier material) by PHJ and PDJ ions caused secondary electron emission. Deuterated ions gave a smaller emission coefficient [9]. 

PH + and PD + ions were used to excite secondary electron emission from a Cu-Be target. The emission coefficients are smaller for PD + [8]. 

The Secondary-Electron Emission Coefficients of Lead Silicate Glasses... 

The composition and chemical state of surface layers of glasses determine their emission capacity, i.e., the secondary electron emission coefficient. The mechanism of secondary electron emission is studied in [51]. The authors developed theory of a plasmon mechanism of secondary emission of electrons by dielectrics, in which the main role is attributed to the process of generation and disintegration of plasmons arising as a consequence of inelastic interaction of primary electrons with the electron structirre of solids. The probability of plasmon excitation depends on the concentration of valence electrons and the minimum... 

The coefficients a, y, x, p were calculated from the deconvolution of 01s and Pb4f spectra. The modifier-lead was assumed not to contribute into value of secondary electron emission coefficient. 

Using these formulas, A.L. Shakhmin determined the coefficients of secondary electron emission for lead silicate glasses. A good qualitative correlation was obtained between experimental and estimated variation regularities of the secondary electron emission coefficient on varying the lead oxide content in glasses (Table 7). This confirms the validity... 

Table 7 lists the results of estimation of secondary electron emission coefficients for binaiy lead silicate glasses implemented by decomposition of Pb 4f and 0 1s spectra. The relative content of modifying lead is substantially overestimated, i.e. the coefficient x in the calculation formula for Nx is overestimated, which is the main reason for the elevated values of Oest. The values of secondary electron emission coefficients calculated by us agree well with experimental data. 

Table 7. The results of estimation of secondary electron emission coefficients for binary lead silicate glasses...

In Eq. (3), ad is the number of electrons generated by an electron leaving the cathode and arriving as an electron avalanche at the anode (at a distance d from the cathode), and y is the so-called secondary ionization coefficient, defined as the number of secondary electrons produced perprimary ionization. These secondary-electron processes include (1) electron emission from the cathode as it is struck and photons, positive ions, and metastable molecules and (2) gas processes such as photoionization of the gas. Physically, Eq. (3) states that when each initial... 

The secondary electron emission coefficient is dependent not only on the material coated on the dynode surfaces, but also on the momentum of the incoming electrons. When the electrons are accelerated through a potential difference of 200 V between adjacent... 

Energetic ion bombardment of a surface causes the emission of secondary electrons. Metals generally have a secondary electron emission coefficient of less than 0.1 under ion bombardment while the secondary electron emission coefficients of oxide surfaces are higher. Secondary electron emission from electron bombardment is much higher than from ion bombardment. 

When the dc discharge is first ignited at a constant pressure and voltage, there is a decrease in cathode current with time. This is due to removing the oxides, which have a high secondary electron emission coefficient, from the cathode surface, and heating of the gas, which reduces its atomic/molecular density. The plasma is not in equilibrium until the discharge current becomes constant. 

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