Gas ionisation

Detectors. Two general types of detectors are used in x-ray medical imaging scintillation and gas ionisation. Scintillation detectors are used for both conventional projection and computerized tomographic imaging. Ionization detectors have been used only in CT appHcations. All detectors used in detection of x-ray radiation must be linear and have a maximum efficiency at the wavelength of the x-ray photon to be detected. 

Gas ionisation counters or scintillation counters are used as detectors. These two counters arc often fitted one behind the other, in tandem, so as to detect both the high energy X-iays (scintillator) and the low energy X-rays (gas ionisation counter). The electrical impulses from the counter are pre-amplified and subsequently processed by an appropriate electronic system. 

In Townsend discharges, free electrons ionise the gas molecules. Several processes for gas ionisation are known. Table 2.1 lists the most important ones. The ionisation of a neutral gas molecule A takes place if the kinetic energy of the electron e is greater than the ionisation potential Vt of the gas molecule (see Table 2.2) ... 

The MWPC usually consists of three planes of wires as shown in figure 5.22 (a). One set of cathode wires is parallel to the anode wires, the wires in the other cathode are orthogonal to it. At the position where an X-ray photon is absorbed and gas ionisation occurs, the resulting electrons drift in the direction of the electric field towards the anode and gas multiplication takes place. A negative pulse is formed on the anode. Simultaneously, positive pulses are induced on a few of the neighbouring cathode wires in both the cathode planes. 

These gas ionisation sources have been described in Section 2.1.5 on mass spectrometry. They are particularly suitable for the analysis of mixtures of volatile and low molecular weight compounds (<800) such as hydrocarbons, essential oils and relatively nonpolar drugs. Chemical derivatisation, e.g. trimethylsilylation, can often be employed to increase the volatility of compounds containing polar functional groups (-OH, -COOH, -NH2 etc) so that GC-MS can be used. 

At the anode of an acid electrolyte fuel cell, the hydrogen gas ionises, releasing electrons and creating H+ ions (or protons). 

Certain interactions with matter of the radiation accompanying the decay of unstable nuclides (a- and /9-particles, y rays) are the basis for the detection and measurement of radioactivity These include photochemical processes, by which a radioactive sample placed in close contact with photographic emulsion causes blackening of the latter upon development (autoradiography) gas ionisation and the deriving production of current pulses that can be analysed and measured by suitable devices excitation of orbital electrons of special molecules, either in a crystalline form or in solution, with subsequent emission of light pulses to be converted into electric current by a photoelectric detector (scintillation)... 

A sensitive photographic strip is worn by regular users of isotopes to monitor the radiation dose to which they have been exposed, when the strip is developed. Generally counting a- or p-particles or y rays depends on their energy and the physical state of the source. Either gas ionisation, scintillation or semiconductor counters are used. 

The ideal gas ionisation constant can be approximated by the following temperature-dependent function (lAPWS, 2007) ... 

Loss of one electron gives the noble gas configuration the very large difference between the first and second ionisation energies implies that an outer electronic configuration of a noble gas is indeed very stable. 

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Liquid ammonia. This can be prepared by compressing ammonia gas. It has a boiling point of 240 K and is an excellent solvent for many inorganic and organic substances as well as for the alkali metals. Liquid ammonia is slightly ionised. ... 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

Reviews of gas-phase kinetics (59) and ionisation energies (60) have also Hsted some of the advantages SF enjoys ia service as a gaseous dielectric. 

Some of the gas atoms or molecules must be stripped of one or more of their electrons. The energy required to accomplish this, called the ionisation potential, is measured in electron volts. In MHD flows of interest, the required energy is suppHed by heating the gas. Thus the ionisation process is referred to as thermal ionisation. 

Vacuum gauges may be broadly classified as either direct or indirect (10). Direct gauges measure pressure as force pet unit area. Indirect gauges measure a physical property, such as thermal conductivity or ionisation potential, known to change in a predictable manner with the molecular density of the gas. 

Conversion to acetates, trifluoroacetates (178), butyl boronates (179) trimethylsilyl derivatives, or cycHc acetals offers a means both for identifying individual compounds and for separating mixtures of polyols, chiefly by gas—Hquid chromatography (glc). Thus, sorbitol in bakery products is converted to the hexaacetate, separated, and determined by glc using a flame ionisation detector (180) aqueous solutions of sorbitol and mannitol are similarly separated and determined (181). Sorbitol may be identified by formation of its monobensylidene derivative (182) and mannitol by conversion to its hexaacetate (183). 

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

Such vessels can also be baked at a temperature of several hundred degrees, to drive off any gas adsorbed on metal surfaces. The pumping function of an ion gauge was developed into efficient ionic pumps and turbomolecular pumps , supplemented by low-temperature traps and cryopumps. Finally, sputter-ion pumps, which rely on sorption processes initiated by ionised gas, were introduced. A vacuum of 10 "-10 Torr, true UHV, became routinely accessible in the late 1950s, and surface science could be launched. 

ZnO contauns excess metal which is accommodated interstitially, i.e. at positions in the lattice which are unoccupied in the perfect crystal. The process by which ZnO in oxygen gas acquires excess metal may be pictured as follows. The outer layers of the crystal are removed, oxygen is evolved, and zinc atoms go into interstitial positions in the oxide. We represent interstitial zinc by (ZnO). However, the interstitial zinc atoms may ionise to give (Zn O) or even (Zn O). The extra electrons produced in this way must occupy electron levels which would be vacant in the perfect crystal. We represent them by the symbol (eo), and refer to them as free electrons. They can be pictured as Zn ions at normal cation sites. We see therefore that three reactions can be written, each giving non-stoichiometric ZnO ... 

Kubaschewski and Hopkins consider the conditions of the gaseous phase which influence the rate of corrosion of metals apart from major variations of composition, they refer also to the effects of minor impurities, gas pressure, flow rate and ionisation. 

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