Detectors Based on Ionization

Many detectors have been developed to collect and amplify the primary ionization created by nuclear particles. In principle, the careful measurement of this ionization provides the most information about the particle and its energy. The devices with the highest resolution are these detectors based upon ionization. Broadly speaking, ionization-based detectors have the common feature that the incident radiation creates ion pahs in an active volume of the device. An electric field is applied to the active volume to separate the charge pairs and sweep the ions to the electrodes. 

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IVpical data for detectors based on ionization mechanisms are given in Table lS-7. 

Detectors based on ionization continue to be developed, and they do not always lit these simple categories, but these categories provide a foundation for selecting and using modem ionization detectors. 

More recently, chemiluminescence detectors based on redox reactions have made possible the detection of many classes of compounds not detected by flame ionization. In the redox chemiluminescence detector (RCD), the effluent from the column is mixed with nitrogen dioxide and passed across a catalyst containing elemental gold at 200-400°C. Responsive compounds reduce the nitrogen dioxide to nitric oxide. The nitric oxide is reacted with ozone to give the chemiluminescent emission. The RCD yields a response from compounds capable of undergoing dehydrogenation or oxidation and produces sensitive emissions from alcohols, aldehydes, ketones, acids, amines, olifins, aromatic compounds, sulfides, and thiols. 

These and many other variations (see below) make it possible to find a chromatographic system suitable for application to most complex mixtures. The species to be separated may be large or small, polar or nonpolar, isomeric or homologous, molecular or ionic, volatile or nonvolatile, and, of course, colored and thus visible (as with Tswett s work) or, more commonly, invisible, requiring a sensitive detector based on UV adsorption, selective ionization, and so on. 

The quantitative determination of X-ray intensities with a photon energy up to 150 keV can be realized by three types of detectors semiconductor-based detectors which call for efficient cooling, ionization chambers which have a low quantum detection efficiency (< 50%), and detectors based on luminescent materials. The latter type of detector is of interest here, because it is realized by the combination of a luminescent material with a photodiode. 

The reaction equipment is operated by means of a data acquisition and control program. The reactor is of stainless steel 316, with 9 mm internal diameter. It is provided with a fixed bed of catalyst diluted with alumina as inert and operates in isothermal regime. The reaction products are analysed by gas chromatography (Hewlett Packard 6890) by means of detectors based on thermal conductivity (TCD) and flame ionization (FID). The separation of products is carried out by means of a system made up of three eolumns 1) HP-1 semicapillary column for splitting the sample into two fi actions a) volatile hydrocarbon components (C4.) and polar components (ethanol, water and diethyl ether) b) remaining products (C5+). 2) SUPEL-Q Plot semicapillary column for individually separating out both volatile components and polar components, which will be subsequently analysed by TCD and FID. 3) PONA capillary column for separation of Cs+ hydrocarbons, which will be analysed by FID. 

Fig. 2.15. (a) X-ray counting detector based on silicon (photo courtesy SECTRA), (b) high-pressure gas ionization detector (Xcounter. From Digital Mammography, eds. ED Pisano, MJ Yaffe, CM Kuzmiac. Lippincott, Williams and Wilkins, a Walters Kluwer Company, 2004. With permission)... 

A variety of portable instruments commonly referred to as sniffers are used. They tend to be nonspecific detectors based on the principles of flame ionization and photoionization. More recently portable gas chromatographs and gas chromatographs coupled with mass spectrometers have become available for use at the fire scene. These provide more information about the sample than sniffers but require more training in their use and interpretation of data. 

There are three different main types of radiation detectors. These are detectors based on gas ionization, scintillation detectors, and semiconductor detectors. 

Detectors based on gas ionization are the ionization chamber, proportional counter, and Geiger-Miiller counter. These devices measure the electric current pulses induced by radiation between electrodes in a gas-filled chamber. The ionization chamber has a low... 

The semiconductor detector is also based on ionization. The ionization takes place in the p-n jvmc-tion of two semiconductor materials. At room temperature the signal-to-noise (S/N) ratio of semiconductor detectors is poor but increases when the detector is cooled, e.g., by liquid nitrogen. The semiconductor detector has a very good energy resolution. 

Abstract Most radiation related to nuclear properties is outside the visible part of the electromagnetic spectrum or involves submicroscopic particles, hence is invisible. Detectors -devices to sense the radiation, and perhaps measure its properties - are essential. The emphasis in research has moved from the characteri2ation of radioactivity, through simple nuclear reactions, to explorations of the extremes of nuclear matter, but the central importance of suitable radiation detectors has persisted. This chapter emphasi2es detectors associated with measurements of radioactivity, as opposed to nuclear reactions. Thus, much of the current creative work is excluded, but otherwise the scope of these volumes would at least double. Detectors are classified broadly as based on ionization of gases, conduction in semiconductors, or scintillation. The concluding section is an introduction to systems based on two or more components of one of these basic types. 

Some techniques have been described that are based on the concept of flame ionization used in gas chromatography. The results are generally unsatisfactory because it is necessary to evaporate the solvent prior to introducing the mixture into the detector. 

Fast concentration and sample injection are considered with the use of a theory of vibrational relaxation. A possibility to reduce a detection limit for trinitrotoluene to 10 g/cnf in less than 1 min is shown. Such a detection limit can by obtained using selective ionization combined with ion drift spectrometry. The time of detection in this case is 1- 3 s. A detection technique based on fluorescent reinforcing polymers, when the target molecules strongly quench fluorescence, holds much promise for developing fast detectors. 

The contractor at Site H had established area and personnel sampling consistent with HAZWOPER requirements. A photo ionization detector (PID) and a real-time aerosol monitor (RAM) were used on a daily basis to screen for potentially hazardous levels of contaminants. On a weekly basis, personal air samples were collected and submitted for laboratory analysis. PPE requirements, however, were often not based on this data because the oversight agency had established inflexible minimum PPE requirements. The audit team found many of the PPE requirements on Site H to be excessive in light of site monitoring data and hazard determinations. 

The flame ionization detector is capable of measuring only gaseous hydrocarbons, in other words, hydrocarbons that have a low boiling point. Emission gases can, however, also contain hydrocarbons in liquid form at ambient temperature and pressure. Therefore, analyzers based on flame ionization detection are generally equipped with heating elements to keep rhe sampling line and the detector at about 200 °C. 

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