Electrons from accelerators

Absorption of high energy electron beams occurs through interaction with the orbital electrons and the electromagnetic field at the atom. The processes are summarized in Table 6.1 and Figure 6.8. In order to distinguish between electrons from accelerators and those from /3-decay we refer to the latter as -particles. 

Inertial confinement is a pulsed operation system. Small pellets of solid D2 and T2 ( 1 mm) are placed into the middle of a chamber where the pellets are irradiated by intense beams of photons (from lasers) or electrons (from accelerators). The surface of the pellet rapidly vaporizes, resulting in a jet-stream of particles away firom the pellet and an inq>ulse (temperature-pressure wave) which travels into the pellet, increasing the central temperature to > 10 K. This causes a small fusion explosion, producing energetic He and n (17.17). Because the particle density is high, the pulse time can be very short and still meet the... 

If the electrodes are moved closer together, the positive column begins to shorten as it moves through the Faraday dark space because the ions and electrons within it have a shorter distance through which to diffuse. Near the cathode, however, the electric-field gradient becomes steeper and electrons from the cathode are accelerated more quickly. Thus atom excitation through collision with electrons occurs nearer and nearer to the cathode, and the cathode glow moves down toward the electrode. 

Ions are directed onto the first plate (dynode) of an electron multiplier. The ejected electrons are accelerated through an electric potential so they strike a second dynode. Suppose each ion collision causes ten electrons to be ejected, and, at the second dynode each of these electrons causes ten more to be ejected toward a third dynode. In such a situation, arrival of just one ion causes 10 X 10 = 10 = 100 electrons to be ejected from the second dynode, an amplification of 100. Commercial electron multipliers routinely have 10, 11, or 12 dynodes, so amplifications of 10 are... 

Each ion from an incident ion beam causes two electrons to be emitted from the first dynode. These electrons are accelerated to the second dynode, where each causes two more electrons (now four in all) to be ejected. These in turn are accelerated to a third dynode and so on, eventually reaching, say, a tenth dynode, by which time the initial two electrons have become a shower of 2 electrons. 

If an electric potential is applied between two electrodes in a gas, electrons are released from the cathode (negative electrode). The electrons are accelerated by the electric field and collide with atoms or molecules of gas. 

The electrons are accelerated from the filament by an anode, which has a positive potential with respect to the filament. 

Electrons from a spark are accelerated backward and forward rapidly in the oscillating electromagnetic field and collide with neutral atoms. At atmospheric pressure, the high collision frequency of electrons with atoms induces chaotic electron motion. The electrons gain rapidly in kinetic energy until they have sufficient energy to cause ionization of some gas atoms. 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W) or lanthanum boride (LaB ). W sources are typically heated to ca 2500—3000 K in order to achieve an adequate electron brightness. LaB sources require lower temperatures to achieve the same brightness, although they need a better vacuum than W sources. Once created, these primary electrons are accelerated to some desired energy with an energy spread (which ultimately determines lateral resolution) on the order of ca 1.5 eV. 

In sea water with a pH of 8, crevice pH may fall helow 1 and chloride concentration can be many times greater than in the water. The crevice environment becomes more and more corrosive with time as acidic anions concentrate within. Areas immediately adjacent to the crevice receive ever-increasing numbers of electrons from the crevice. Hydroxyl ion formation increases just outside the crevice—locally increasing pH and decreasing attack there (Reaction 2.2). Corrosion inside the crevice becomes more severe with time due to the spontaneous concentration of acidic anion. Accelerating corrosion is referred... 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

In a synchrotron, electrons are accelerated to near relativistic velocities and constrained magnetically into circular paths. When a charged particle is accelerated, it emits radiation, and when the near-relativistic electrons are forced into curved paths they emit photons over a continuous spectrum. The general shape of the spectrum is shown in Fig. 2.4. For a synchrotron with an energy of several gigaelectronvolts and a radius of some tens of meters, the energy of the emitted photons near the maximum is of the order of 1 keV (i.e., ideal for XPS). As can be seen from the universal curve, plenty of usable intensity exists down into the UV region. With suitable mono-... 

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

For detection of secondary ions a laterally resolving detector is necessary. In the first step a channel plate for amplification is used secondary electrons from the output of this device are accelerated either to a fluorescent screen or to a resistive anode. If a fluorescent screen is used the image is picked up by a CCD camera and summed frame by frame by use of a computer. The principal advantage of this system is unlimited secondary ion intensities, but compared with the digital detection of the resistive anode encoder the lateral and intensity linearity is not as well-defined. 

Field emission applicable to flat-panel display is one of the most advanced and energetically studied applications of CNTs. The apparatus is illustrated in Fig. 12 along with pictures of closed-tips of MWCNTs. In spite of their high workfunction (4..3 eV for MWCNTs [39]), CNTs emit electrons from their tips when high voltages (100-1000 V) are applied between the metal grid of the accelerator and the CNT film which are separated by 20 im [38]. 

Alternatively, the same coatings can be cured by electrons from an electron accelerator without the use of photoinitiators. Electrons from a 150-600 kV accelerator are energetic enough to create free radicals on impact with the polymer molecules and curing ensues. Clear and pigmented coatings can be cured. Electron accelerators are extremely expensive, but are cheap to run. 

Based on the photoelectric effect, electrons in evacuated tubes (photoelectrons) are released from a metal surface if it is irradiated with photons of sufficient quantum energy. These are simple photocells. Photomultipliers are more sophisticated and used in modem spectrophotometers where, via high voltage, the photoelectrons are accelerated to another electrode (dynode) where one electron releases several electrons more, and by repetition up to more than ten times a signal amplification on the order of 10 can be obtained. This means that one photon finally achieves the release of 10 electrons from the anode, which easily can be measured as an electric current. The sensitivity of such a photomultiplier resembles the sensitivity of the human eye adapted to darkness. The devices described are mainly used in laboratory-bound spectrophotometers. 

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