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... 

Now calculate the minimum light power that can be measured with a photomultiplier using the photocathode of Exercise 3.5 and with 10 dynodes, each of which has a secondary emission coefficient of 5 = 6. Estimate these minimum powers if the photocathode is cooled down to 5 °C. Assume a bandpass width of 1 Hz. 

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. 

In No at pressure 10 Torr, interelectrode distance of 10 cm, electrode area of 100 cm, and secondary emission coefficient y = 10 , the maximum dark discharge current is... 

The gain factor G is not constant, but shows fluctuations due to random variations of the secondary-emission coefficient q, which is a small integer. This contributes to the total noise and multiplies the rms shot noise voltage by a factor a > 1, which depends on the mean value of [4.131]. The Johnson noise of the load resistor R at the temperature T gives an rms-noise current... 

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. 

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