Solid scintillation counters

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.

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). 

Scattering on the Triple-Axis-Diffractometer [1,2] at the HASYLAB high-energy beamline BW5 is performed in the horizontal plane using an Eulerian cradle as sample stage and a germanium solid-state detector. The beam is monochromatized by a singlecrystal monochromator (e.g. Si 111, FWHM 5.8 ), focused by various slit systems (Huber, Riso) and iron collimators and monitorized by a scintillation counter. The instrument is controlled by a p-VAX computer via CAMAC. 

The scintillation counter is a solid state radiation detector. 

The scintillation counter is a solid state radiation detector which uses a scintillation crystal (phosphor) to detect radiation and produce light pulses. Figure 24 is important in the explanation of scintillation counter operation. 

Scintillation counters, which constitute an extremely important group, depend upon the absorption of radiation by a scintillator to produce UV light scintillations, which are detected and converted into amplified voltage pulses by a photomultiplier (Figure 10.10). Solid scintillators are used extensively for the detection and analysis ofy-rays and X-rays, while liquid scintillators find widespread employment in the measurement of pure negatron emitters, especially where the particle energy is low (< 1 MeV). 

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,... 

Low-level counting of y-ray emitters using solid scintillation counters is an extensively used technique. The most important aspect of low-level solid scintillation counting is to decrease the counter background. Typical contributions to a solid scintillation counter s background rate from various sources are shown in Table 19.3. 

On-line detection can be classified as either homogeneous or heterogeneous. In the homogeneous system, the effluent is mixed with a liquid scintillation cocktail before passing through a flow cell that is positioned in a scintillation counter. In the heterogeneous system, the effluent passes through a flow cell packed with a solid scintillator. 

Speed of Analysis. The speed with which many immunochemical analyses can be completed illustrates a major advantage of immunochemical procedures. Immunochemical assays are most time and cost effective when the sample load is large. Parker (4) estimated that a single technician could perform 100-5000 radioimmunoassays per day with little or no assay automation in comparison to 20-40 GLC assays (3). Numerous inexpensive systems are available to decrease analysis time. These systems may include solid phase separation techniques, automatic dispensers, test tube racks which will fit directly into a centrifuge and/or scintillation counter, and data handling systems. Alternatively, there are fully automated systems based on RIA or ELISA which require very little operator attention and which handle 25-240 samples/hr. Gochman and Bowie (81) have outlined the basis of operation and summarized the features of automated RIA systems and extensive literature is available from the manufacturers. 

The nature of the tracer dictates the detection system. For the liquid phase, quite often the tracers (e.g., NaCl, H2S04, etc.) are such that the detection probe is directly inserted into the reactor and continuous monitoring of the concentration at any fixed position is obtained by means of an electrical conductivity cell and a recorder. In this case, no external sampling of liquid is necessary. If the tracer concentration measurement requires an analytical procedure such as titration, etc., sampling of the liquid is required. For the solid phase, a magnetic tracer is sometimes used. The concentration of a solid-phase tracer can also be measured by a capacitance probe if the tracer material has a different dielectric constant than the solid phase. In general, however, for solid and sometimes gas phases, some suitable radioactive tracer is convenient to use. The detection systems for a radioactive tracer (which include scintillation counters, a recorder, etc.) can be expensive. Some of the tracers for the gas, liquid, and solid phases reported in the literature are summarized in Table 3-1. 

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. 

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. 

The main parts of a scintillation counter are sketched schematically in Fig. 7.13. In the transparent crystal or liquid the radiation is absorbed and photons are emitted. At the photocathode of the photomultiplier tube the photons release electrons which are multiplied by the dynodes of the multiplier to give pulses of several mV. Some examples of solid and liquid scintillators are listed in Table 7.1. 

This operates by detecting the scintillations (fluorescence flashes ) produced when radiation interacts with certain chemicals called fluors. In solid (or external) scintillation counters (often referred to as gamma counters ) the radioactivity causes scintillations in a crystal of fluorescent material held close to the sample. This method is only suitable for radioisotopes producing penetrating radiation. 

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. 

Enzyme immunoassays (EIA) play an important role in clinical diagnostics, veterinary medicine, environmental control, and bioprocess analysis. Antibodies are coupled to enzymes like peroxidase or phosphatase, whose products can be measured after the degradation of a substrate. Because of its high selectivity and sensitivity, EIA enables the detection of a broad spectrum of analytes in complex samples. A solid phase EIA performed in a plastic microtitre plate is called an enzyme-linked immunosorbent assay (ELISA). The coloured products produced in the ELISA can be measured spectro-photometrically rather than in a scintillation counter as for the RIA. 

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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