Secondary emission of electrons

High sensitivity is featured by the electrical methods used to detect EEPs. These are based on measuring small currents that occur in the course of selective ionization of EEPs, or currents of secondary emission of electrons or ions knocked by EEPs out of the surface of solid targets (if such emission is taking place). 

Typical connections for a photomultiplier tube. Secondary emission of electrons from 5 to 15 dynodes provides gains of up to 10. The dynode resistors are usually all the same and are about 50 kfl. The capacitors on the last few dynodes store charge for improved operation with high current pulses. 

Electron multiplier (1936) n. A device that utilizes secondary emission of electrons for amphfying a current of electrons. 

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... 

One can compensate for charging by using a so-called flood gun, which sprays low-energy electrons onto the sample. Charging can also be minimized by using a beam of atoms instead of ions as primary particles. In this case, kinetic emission of electrons is the only source of charging, if we ignore the low yields of secondary ions. 

The treatment of secondary-electron emission presented in this section essentially follows the ideas and organization of Sickafus which in turn was strongly influenced by Wolff . Upon entering a solid, an energetic electron is subjected to a sequence of elastic and inelastic scattering events. The collisions produce a cascade of moving electrons. The intersection of the cascade with the surface results in the emission of electrons into the gas phase. Therefore, the externally-observed, energy-... 

Emission of electrons from the particle surface has also been used in laboratory studies to probe surface composition. Electron emission has been induced by UV irradiation, for example, by Burtscher and Schmidt-Ott (1986) to probe perylene on the surface of carbon particles. In a series of laboratory studies, Zie-mann et al. (1995, 1997, 1998) have demonstrated the potential utility of secondary electron yield measurements as a technique for probing particle surface composition. In this method, particles are bombarded with... 

Experimentally, work-function measurements which rely on the cold emission of electrons are carried out in the field-emission microscope (F.E.M.) (21). The apparatus, as shown in Fig. 11, consists of a W tip P sharpened by electrolytic polishing so that the radius of curvature is 10 cm., and an anode in the form of a film of Aquadag. A variable potential of 3 to 15 kv. is applied to the anode, and the electrons, pulled out from the point, travel in approximately straight lines to the fluorescent screen. The linear magnification obtained is of the order of 10 to 10. The secondary electrons from the screen are collected by the anode, and the field-emission current is measured by a sensitive microammeter. 

In the SIMS a primary noble gas atom or ion (e.g. Ar°, Ar+, Xe°, Xe+) beam is bombarded on the sample in ultra-high vacuum, penetrating to a depth of 30-100 A. The kinetic energy of the particle is assumed to dissipate via a collision cascade process, which causes the emission of electrons, neutral species and secondary ions, the yields of which vary with polymer surface composition and obviates the possibility of quantitative SIMS informa-... 

Samples of microspheres were mounted on aluminum specimen mounts by means of double-faced tapes. The microspheres were fractured with razor blades to expose the internal matrix. The samples were then coated with approximately 125 of gold by pulsing the sputter coater to avoid the possibility of artifact caused by heat generation. Secondary emissive scanning electron microscopy was performed with an Amray 1600 Turbo scanning electron microscope. 

Primary effects comprise recoil of the nucleus and excitation of the electron shell of the atom. The excitation may be due to recoil of the nucleus, change of atomic number Z or emission of electrons from the electron shell. Secondary effects and subsequent reactions depend on the chemical bonds and the state of matter. Chemical bonds may be broken by recoil or excitation. In gases and liquids mainly the bonds in the molecules are affected. The range of recoil atoms is relatively large in gases and relatively small in condensed phases (liquids and solids). Fragments of molecules are mobile in gases and liquids, whereas they may be immobilized in solids on interstitial sites or lattice defects and become mobile if the temperature is increased. 

These effects overlap and lead to ionization, emission of electrons from the electron shell and fluorescence. They may cause secondary reactions in molecules and subsequent reactions of the ions or excited atoms or molecules produced by these effects. 

An energetic ion incident on a solid resist-coated substrate can interact with the latter in a variety of phenomena that include sputtering of neutral atoms, emission of electrons, lattice damage, heat generation, and ion implantation, as shown in Pig. 15.7. In addition, the beam can produce secondary electrons that participate in the chemical transformations of the resist, such as bond breaking, of the kind that can expose the resist. It is this flexibility and effectiveness of ions in... 

Note that like the case of the negatron decay, it is not necessary to add or subtract electron masses in the calculation of the Q-value in EC. An example of EC is the decay of Be to Li for which it is possible to calculate that the )2-value is 0.861 MeV. This reaction is somewhat exceptional since for neutron deficient nuclei with values of Z below 30, positron emission is the normal mode of decay. Electron capture is the predominant mode of decay for neutron deficient nuclei whose atomic number is greater than 80. The two processes compete to differing degrees for the nuclei between atomic numbers 30 and 80. Electron capture is observed through the emission of electrons from secondary reactions occurring in the electron shell because of the elemental change (see 4.9). 

For the emission of electrons in APCI, a corona discharge is used instead of the filament in GC-MS (Cl) because of the rapid fusion of the filament at AP. In APCI, with nitrogen as sheath and nebulizer gas and atmospheric water vapor (also in 5.0 nitrogen sufficient quantity of water is available), N2 -and ions are primarily formed by electron ionization. These ions collide with the vaporized solvent molecules and form secondary reactant gas ions, such as HgO" " and (H20) H (Figure 1.4). 

Secondary emission (1931) n. The emission of electrons from a surface that is bombarded by particles (as electrons or ions) from a primary source. Giambattista A, Richardson R, Richardson RC, Richardson B (2003) College physics. McGraw-Hill Science, New York. 

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