Thermionic emission source

The most popular thermionic detector (TID) is the nitrogen-phosphorus detector (NPD). The NPD is specific for compounds containing nitrogen or phosphorus. The detector uses a thermionic emission source in the form of a bead or cylinder composed of a ceramic material impregnated with an alkyl-metal. The sample impinges on the electrically heated and now molten potassium and rubidium metal salts of the active element. Samples which contain N or P are ionized and the resulting current measured. In this mode, the detector is usually operated at 600-800°C with hydrogen flows about 10 times less than those used for flame-ionization detection (FID). 

A second thermionic emission source uses lanthanum hexaboride (LaBe). This has a much lower work function than tungsten and so will emit electrons when heated to only 1,800 K (tungsten operates at -2,500 K). LaBe is reactive at its operating temperature, but the emitter is a direct replacement for a tungsten filament, requiring only very minor alteration of the instrument. Cerium hexaboride is an alternative source material of the same type. 

One important sem source that is not based on thermionic emission is the field emission (fe) source. Fe-sem systems typically give images of much higher resolution than conventional sems due to the much narrower energy distribution (on the order of 0.25 eV) of the primary electron beam. A fe source is a pointed W tip from which electrons tunnel under the influence of a large electric field. This different mechanism of electron generation also results in a brightness comparable to a conventional thermionic source with much less current. 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

The most common conventional gas source is an electron impact (El) source. This consists of a metal chamber with a volume of a few cm3, through which the sample flows in the form of a gas. Electrons produced by thermionic emission from a heated tungsten filament are passed through this gas, and accelerated by a relatively low voltage ( 100eV), causing ionization within the sample gas. A plate inside the chamber carries a low positive potential (the repeller ) which ejects the positive ions into a region which contains a series of plates (called lenses) and slits, which serve to focus, collimate, and accelerate the ion beam into the next part of the system... 

The electron probe is the reduced image of an electron source supplied by a gun. The tungsten filament thermionic emission gun is currently the most suitable source for X-ray microanalysis. It is not very expensive to run, can be easily aligned and offers satisfactory stability. It delivers high current intensities (1 to 1000 nA) for probe diameters of the order of a micrometre. 

Electron Production Processes. The important electron production processes occur in the gas phase and, in the case of discharges with electrodes, at electrode surfaces. The major surface processes are (a) secondary electron emission on ion impact at the cathode, (b) field emission at sharp points on electrodes, and (c) thermionic emission in the case of arc-type discharges where electrodes become strongly heated. These are the sources of the primary electrons in d.c. and low frequency discharges. 

Second, channel potential measurements by Kelvin probe force microscopy and the four-probe method, which are described later in this chapter, indicate that the contact resistances and temperature dependences associated with the individual source and drain electrodes are nearly identical. From a thermionic emission... 

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