Solid scintillator proportional counter

Fig. 9.4. The quantum efficiencies q(%) of the more important radiation detectors. (a) silicon photodiode (b) pholomultiplyer tube with glass (/) and with quart . (2) (c) solid scintillator proportional counter with gas-filled detector...

A different detection unit is the solid-scintillator proportional counter [6]. It consists of a scintillator with ultraviolet emission in a multi wire chamber filled with an organic vapor. The organic molecules are ionized by the ultraviolet photons and the resulting photoelectrons are detected in the multi wire chamber. A popular molecule in this aspect is TMAE (tetrakis (dimethylamino) ethylene). Figure 9.4 presents the spectral sensitivities of the detectors mentioned. 

The basic function of the spectrometer is to separate the polychromatic beam of radiation coming from the specimen in order that the intensities of each individual characteristic line can be measured. In principle, the wide variety of instruments (WDXRF and EDXRF types) differ only in the type of source used for excitation, the number of elements which they are able to measure at one time and the speed of data collection. Detectors commonly employed in X-ray spectrometers are usually either a gas-flow proportional counter for heavier elements/soft X-rays (useful range E < 6keV 1.5-50 A), a scintillation counter for lighter elements/hard X-rays (E > 6keV 0.2-2 A) or a solid-state detector (0.5-8 A). 

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 three most commonly used detectors are the gas proportional counter, the scintillation detector and solid-state detectors (Pecharsky and... 

The content of tritium is usually determined by measuring its activity. Solid samples may be converted into liqnid or gaseous compounds, and the tritium content measured by a liquid scintillation counter or a gas proportional counter, respectively. 

The recommended procedure RP515 from U.S. D.O.E. proposes to count the membrane by gas flow proportional counter or by liquid scintillation. This approach was not followed. It was preferred to eluate the fixed strontium fi-om the disk using a solution of disodic EDTA. The reason of this approach was a concern for an eventual interference of the solid matrix (filter) in the scintillating cocktail. 

The standard sources have been designed in order to allow the calibration of all the classical detectors of a, p, e, y, n, X radiation (ionisation chambers, Geiger-Miiller or proportional counters, scintillation or solid-state counters, etc.). They are classified as alpha sources, electron sources, beta sources, gamma sources, neutron sources. X-ray sources, heat flux sources, and sources for radiation protection dose meters. 

Gas-filled detectors are used for X-rays or low energy gamma rays. These include ionization chambers, proportional counters and Geiger-Miiller counters. Scintillation detectors are used in conjunction with a photomultiplier tube to convert the scintillation light pulse into an electric pulse. Solid crystal scintillators such as Csl or Nal are commonly used, as well as plastics and various liquids. 

Four types of counters are currently in use proportional, Geiger, scintillation, and semiconductor. All depend on the power of x-rays to ionize atoms, whether they are atoms of a gas (proportional and Geiger counters) or atoms of a solid (scintillation and semiconductor counters). A general treatment of the first three types has been given by Parrish [7.8]. 

Besides EDXRF spectrometers that are intended for use in the laboratory, a number of portable EDXRF instruments are also available. These devices are used in various fields for on-site analysis of works of art, environmental samples, forensic medicine, industrial products and waste materials etc. In their simplest form, the instraments consist of one or more radioisotope sources combined with a scintillation or gas proportional counter. However, combinations of radio-sources with ther-moelectronically cooled soHd-state detectors are also available in compact and lightweight packages (below 1 kg). In Fig. 11.21, schematics of various types of radiosource based EDXRF spectrometers are shown. In Fig. 11.21a, the X-ray source is present in the form of a ring radiation from the ring irradiates the sample from below while the fluorescent radiation is efficiently detected by a solid-state detector positioned at the central axis. Shielding prevents radiation from the source from entering the detector. In Fig. 11.21b and c, the X-ray source has another... 

Most of the radioisotopes used as isotopic labels in activation analysis decay with beta (positron and negatron) radiations and/or gamma rays. By convention, beta-emitting radionuclides are usually measured by gas-filled or gas-flow proportional counters or Geiger counters. Sometimes, liquid scintillation counters are used to complete a beta-ray measurement. The more conventional method for gamma-ray measurements involves the use of a gamma-ray spectrometer equipped with either a scintillation or solid-state detector. Stevenson (918) discusses the characteristics of radioactive decay and gives details on the methods and instruments used to detect emitted radiations. 

All chromatographic methods (liquid, liquid-liquid, liquid-solid, gas, gas-liquid, and gas-solid chromatography) are used in radiochemical analysis of food and environmental samples. Equipment for the evaluation of paper and thin-layer radiochromatograms is commercially available. The radiation detectors used vary according to the type and energy of the radiation emitted by the measured radionuclides. Use is made of ionization chambers, proportional counters, and Geiger-Miiller, scintillation, and semiconductor detectors. 

The usual detectors for low-energy y-radiation are thin Nal(Tl) scintillation crystals coupled to a photomultiplier, and gas-filled proportional counters. Considerable improvements in energy resolution can be achieved with solid-state detectors such as the Li drifted Ge counter. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Scintillation counters usually consist of a sodium iodide crystal doped with 1% thallium. The incident X-ray photons cause the crystal to fluoresce producing a flash of light for every photon absorbed. The size of the light pulse is proportional to the energy of the photon and is measured by a photomultiplier. A deficiency associated with scintillation counters is that they do not provide as good energy resolution as proportional or solid state detectors. 

Hence, the photographic film vaguely resembles a ratemeter because the intensity is extracted from the degree of darkening of the spots found on the film - the darker the spot, the higher the corresponding intensity because the larger number of photons have been absorbed by the spot on the film surface. The three most commonly utilized types of x-ray detectors today are gas proportional, scintillation, and solid-state detectors, all of which are true counters. 

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