Tungsten filament electron source

The usual source of electrons is a tungsten filament electron gun held at a negative potential (typically 10-30 kV), and magnetic lenses focus the beam into a fine probe incident on the surface of the specimen. A probe diameter of 0.2-1 pm is typical, with a current of 1-100 nA. 

Source Optical tungsten filament Electron tungsten filament Field emission source ... 

The particle size distribution of South Afriea Coneentrate was measured using the laser diffractometer MS 2000 which is produced by Malvern Instruments. The moisture eapacity of raw material was measured in the self-made instrument in the laboratory. [5-6]The morphology of the particle surface were performed using scanning electron microscope (SEM) equipped with a tungsten filament emission source. 

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. 

There are three major types of electron sources thermionic tungsten, LaBg, and hot and cold field emission. In the first case, a tungsten filament is heated to allow... 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

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... 

An electron source. or electron gun traditionally the most frequent is the triode gun comprising a tungsten wire cathode heated to around 2 700 K. The electrons emitted by the cathode are accelerated by the electric field set up between this negatively polarised filament and the earthed anode. This gun can provide an electron beam of 30 keV with a current density per unit solid angle (also called bright-... 

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. 

The electron sources used in most sems are thermionic sources in which electrons are emitted from very hot filaments made of either tungsten (W)... 

Figure 25-1 9 The Spectronic 20 spectrophotometer. A photograph of the instrument is shown in (a), while the optical diagram is seen in (b). Radiation from the tungsten filament source passes through an entrance slit into the monochromator. A reflection grating diffracts the radiation, and the selected wavelength band passes through the exit slit into the sample chamber. A solid-state detector converts the light intensity into a related electrical signal that is amplified and displayed on a digital readout. (Courtesy of Thermo Electron Corp., Madison, WI.)...

In 1913, Coolidge [COO 13] imagined another kind of X-ray source. The cathode is comprised of a tungsten filament heated by the Joule effect. According to the Edison effect, this filament emits electrons that are accelerated by an electrical field and bombard the anticathode which then emits X-rays. The entire device is placed in a sealed tube inside which the pressure must be as low as possible. A schematic view of such a tube is shown in Figure 2.3. 

Most commercially available X-ray spectrometers utilize a sealed X-ray tube as an excitation source, and these tubes typically employ a heated tungsten filament to induce the emission of thermionic electrons in a vacuum chamber. After acceleration by means of a high voltage X the electrons are directed towards a layer of high purity metal (e.g., Cr, Rh, W, Mo, Rh, Pd,. ..) that serves as anode. In the metal layer, a bremsstmhlung continuum is produced, onto which the characteristic lines of the anode material are superimposed. The broad band radiation is well suited for the excitation of the characteristic lines of a wide range of atomic numbers. The higher the atomic number of the anode material, the more intense the beam of radiation produced in the tube. Fig. 11.11 shows a schematic cross-section of a sealed X-ray tube. 

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