Thermionic sources

Thermionic Sources. Richardson s Law relates the current density from the source, J, to the operating temperature T  

Tungsten has the necessary high melting temperature (3660 K) to be employed as a thermionic source, and lanthanum hexaboride (LaB6) is also employed because of its low work function. 

A tungsten wire filament may be bent into a V shape (a hairpin filament, figure 5.3) and its typical lifetime at 100 kV operating voltage is 100 h. LaB6 crystals are grown with a (110) orientation in order to enhance their electron emission, and their corresponding lifetime is of the order 500 h. 

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The collector contains an electrically-heated rubidium salt used as the thermionic source. During the elution of a molecule of a nitrogen compound, the nitrogen is ionized and the collection of these ions produces the signal. The detector is very sensitive but Its efficiency is variable subject to the type of nitrogen molecule, making quantification somewhat delicate. 

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. 

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. 

Two kinds of electron sources may be employed in local analysis. The first is a thermionic source which produces electrons when heated, and the second is... 

Modern spectrometers only require electron beam currents in the range 0.1 lOnA and hence probe sizes of 20-200 nm may be readily achieved with thermionic sources and 5-15 nm with a FEG. Spatially resolved compositional information on heterogeneous samples may be obtained by means of the Scanning Auger Microprobe (SAM), which provides compositional maps of a surface by forming an image from the Auger electrons emitted by a particular element. 

The important and stimulating contributions of Kebarle and co-workers 119 14 > provide most of the data on gas-phase solvation. Several kinds of high pressure mass spectrometers have been constructed, using a-particles 121>, proton- 123>, and electron beams 144> or thermionic sources 128> as primary high-pressure ion sources. Once the solute A has been produced in the reaction chamber in the presence of solvent vapor (in the torr region), it starts to react with the solvent molecules to yield clusters of different sizes. The equilibrium concentrations of the clusters are reached within a short time, depending on the kinetic data for the... 

The precise nature of broadening due to the energy analyzer depends on analyzer design and may not be easy to compute, even in the ideal case. Measurement may also be difficult. The determination of the broadening function sM(x) may be attempted with the aid of narrow spectral lines or spectral lines of known shape, or by supplying the analyzer with electrons from a thermionic source (Lee, 1973). 

Organolead compounds may be detected by turning off the heating to the thermionic source and running in the FTID mode. In this mode, the combustion of organolead compounds lead to long-lived negative-ion products which are detected at the TID collector. 

The alkali flame-ionization detector, sometimes called an NP or nitrogen-phosphorus detector, contains a thermionic source, such as an alkali-metal salt or a glass element containing rubidium or other metal, that results in the efficient ionization of organic nitrogen and phosphorus compounds. It is a selective detector that shows little response to hydrocarbons. 

Another consideration is the choice of electron sources. The field emission electron guns provide better lateral coherence, smaller probes, and higher brightness in general than the thermionic source they are more suitable for CBED and NED. 

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

Fig. 8 Transfer function. The microscope transfer function t k) as a function of the spatial frequency k calculated for Scherzer conditions at 300 kV. Dashed line, microscope equipped with a thermionic source (LaBg) and Cs = 1.3 mm full line, FEG microscope, Cs = 1.2 mm.

Thermionic source brightnesses are about 10 A sr maximum current several microamperes. 

All modem thermionic ionization detectors (TED) employ a solid surface, composed of a ceramic or glass matrix doped with an alkali metal salt in the form of a bead or cylinder, molded onto an electrical heater wire as the thermionic source [254,270,271]. 

Electron sources used in EBL exposure tools are similar to those used in conventional electron microscopes. They can be divided into two main groups— thermionic or field emission—depending on the way in which they emit electrons. The sources that rely on the emission of electrons from a material that is heated to a temperature at which electrons are emitted from the surface are referred to as thermionic sources. These sources are fabricated from materials such as tungsten, thoriated tungsten, or lanthanum hexaboride (LaBe). ... 

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