Semi-conductor detectors

All methods of radiometric analysis involve, of course, the use. of various radiation detection devices, The devices available for measuring radioactivity will vary with the types of radiations emitted by the radioisotope and the kinds of radioactive material. Ionization chambers are used for gases Geiger-Miiller and proportional counters for solids liquid scintillation counters for liquids and solutions and solid crystal or semi-conductor detector scintillation counters for liquids and solids emitting high-energy radiations. Each device can be adopted to detect and measure radioactive material in another state, e.g., solids can be assayed in an ionization chamber. The radiations interact with the detector to produce a signal,... 

The energy dispersive. spectrometer (Fig. 7.12) comprises a semi-conductor detector (diode) that collects the entire X-ray spectrum and transmits it to a multi-channci analyser which classes the various X-ray spectrum lines as a function of their energy. 

The CCD is a semi-conductor detector. It can be used as an X-ray detector (for reviews see Allinson (1982), Milch et al (1982) and Allin-son (1989)) either by direct illumination of the X-rays onto the silicon or by conversion of the X-rays in a phosphor to visible light which is then incident onto the silicon. The previous section dealt with phosphor coupled systems based on television cameras. The CCD can replace the television camera. This is the basis of the system being developed by Strauss et al (1987) and Westbrook (1988) see figure 5.25. An alternative is to use direct illumination onto a so-called deep depletion CCD. CCD... 

A silicon (Lithium drifted) semi-conductor detector, obtained from Kevex Corporation, California, USA, is used. It is a 3 mm thick disc with a sensitive surface area 80 mm mounted at the anterior end of a horizontal steel hollow cylinder (Fig. 3) which is cooled by liquid nitrogen contained in a Dervar drum. 

Instrumentation. Traditional methods of alpha and beta spectrometry instrumentation have changed little over the past decade. Alpha spectrometric methods typically rely on semi-conductor or lithium-drifted silicon detectors (Si(Li)), or more historically gridded ion chambers, and these detection systems are still widely used in various types of uranium-series nuclide measurement for health, environmental, and... 

In INAA, a rock or mineral sample is irradiated in the reactor. The irradiated sample is removed from the reactor, and the dangerous radioactivities are allowed to decay. Then the sample is placed into a counter and the y-rays emitted by each element in the sample are counted. A variety of counters are used, including scintillation counters, gas ionization counters, or semi-conductor counters. For the most precise results, background counts in the detectors produced by electronic noise, cosmic rays, and other radioactive decays must be eliminated. The technique is very sensitive, and samples as small as a few tens of milligrams can be measured. 

The properties of the defect solid state are fundamental to our understanding of all reacting systems involving a solid in fact, there is little of the metallurgical and chemical industries which is not based on the chemical properties of defect solids. A little reflection will make this so obvious that there is no necessity to enumerate specific examples even the electrical industry depends on the ability to produce materials having controllable defect properties, e.g. luminescent materials for fluorescent lamps and cathode-ray tube screens, oxide materials for cathode coatings, a variety of semi-conductors for resistors, rectifiers, detectors and photoelectric devices. 

Figure 12.8 The two mtegories of detectors used for energy dispersive X-ray fluorescence spectrometry. (a) Proportional counter used in pulse mode (b) Cooled Si/Li diode detector using Peltier effect (XR detector by Amptek Inc.) (c) Functioning principle of a scintillation detector containing a large size reverse polarized semi-conductor crystal. Each incident photon generates a variable number of electron-hole pairs. The very high quantum yield enables the use of low power primary sources of X-rays (a few watts or radio-isotopic sources).

The semi-conductor transducer (scintillation counter). Each X-ray photon increases the conductivity of the active zone (the junction) of a lithium-doped silicon diode (one electron for around 3.6 eV). The background noise is reduced if the sensor is maintained at low temperature (cooled by liquid nitrogen or a Peltier device). The entry surface is protected by a beryllium film of a few pm (transparent for Z > 11) (Figure 12.8). In one or other cases the impulse furnished by the detector allows to go back to the energy of the incident photon. 

Finally, different types of VOC detectors based on semi-conductor techniques are available. Sensors of this type are commonly called air quality sensors and give an output signal scaled as 0-100% air quality. Only limited experience of the use of such devices has been presented in the literature. However, a series of tests carried out by Fahlen et al. (1992) showed that a common feature of such sensors is that they generally are highly sensitive both to temperature and humidity. 

In practice, the laboratory air was monitored by eight semi-conductor gas detectors placed at high, low and bench levels around the laboratory, all connected to a switchable indicator and common audio alarm. As a result, the fans could be throttled back or turned off, as necessary, to make the laboratory a more comfortable working environment over long periods. 

Potential barriers can be commonly seen in semi-conductor ceramics. The applications include the use of SiC, thus insulated, as substrate (see section 11.6.1), grain boundary layer capacitors (section 11.6.2), PTC thermistors (section 11.6.6), varistors (section 11.6.7), ferrites (section 11.6.8) and gas detectors [WOL 91]. 

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