Scintillators Anthracene

Scintillation detectors are substances which fluoresce when stmck by x-radiation. Scintillation can, therefore, serve to convert x-ray photons into visible or ultraviolet light. Scintillation materials include thaUium-activatedcrystals of sodium iodide, NaI(Tl), potassium iodide, KI(T1), or cesium iodide, CsI(Tl) crystals of stilbene (a, P-diphenylethylene) [588-59-0] and anthracene [120-12-7] bismuth germanium oxide [12233-56-6] ... 

Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. 

OPTICAL CRYSTAL. A comparatively large crystal, either natural or synthetic, used for infrared and ultraviolet optics, piezoelectric effects, and shortwave radiation detection. Examples are sodium chloride, potassium iodide, silver chloride, calcium tluonde, and (for scintillation counters) such organic materials as anthracene, naphthalene, shlbene, and lerphenyl. 

There are three common types of organic scintillator. The first type is a pure crystalline material, such as anthracene. The second type, the liquid scintillator, is the solution of an organic scintillator in an organic liquid, such as a solution of p-terphenyl in toluene ( 3 g solute/L solution). The third type is the solution of an organic scintillator, such as p-terphenyl, in a solid plastic, such as polystyrene. 

As shown in Figure 3, our detectors came equipped with a cylindrical plastic cell about the size and shape of a scintillation counting vial into which a U-shaped tube had been drilled. When this cell has been packed with scintillation grade anthracene a flow cell quite satisfactory for aqueous systems is obtained. 

Solid scintillators include materials such as sodium iodide, lithium iodide, anthracene, naphthalene and loaded polymers. Sodium iodide detectors are by far the most important, and subsequent discussions will be restricted to... 

Transparent organic crystals, such as anthracene, may also be used as scintillators (Table 7.1). They can be used to measme f radiation of medium or high energy, but they exhibit no special advantages. 

Beta (fl) Single crystals of anthracene, rranj-stillbene are commonly used. Sometimes naphthalene doped with anthracene can also be used. For P-Particle counting, especially low-energy particles from and tritium, liquid scintillators are commonly used (see discussion below on liquid scintillators). 

Anthracene and phenanthrene are stereoisomers that are crystals in pure form. Anthracene is a pale yellow crystal, while phenanthrene exhibits a yellow to brown hue. Besides its common name, anthracene is referred to as anthracin, green oil, or paranaphthalene. The compound is commercially produced by recovery from the coal tar distillation fraction known as anthracene oil or green oil. Anthracene is the key ingredient in the production of anthraquinone. However, it and phenanthrene are also used for the manufacture of dyes, fibers, plastics, and wood preservatives. ° Phenanthrene, also known as phenanthrin, can be produced by high-temperature fractional distillation of coal tar oil. It is additionally used for the oxidation of diphenic acid for use in polymers, as well as the production of chemical softeners, explosives, and some pharma-ceuticals. Recent research has extended the application of both isomers to scintillation counters, semiconductors, and photoconductors. ... 

Anthracene is used as an intermediate in dye production, in the manufacture of synthetic fibers, and as a diluent for wood preservatives. It is also used in smoke screens, as scintillation counter crystals, and in organic semiconductor research (Hawley 1987). Anthracene is used to synthesize the chemotherapeutic agent, Amsacrine (Wadler et al. 1986). Acenaphthene is used as a dye intermediate, in the manufacture of pharmaceuticals and plastics, and as an insecticide and fungicide (HSDB 1994 Windholz 1983). 

The materials that are efficient organic scintillators belong to the class of aromatic compounds. They consist of planar molecules made up of benzenoid rings. Two examples are toluene and anthracene, having the structures shown in Fig. 6.6. 

No activator is needed to enhance the luminescence of organic crystals. In fact, any impurities are undesirable because their presence reduces the light output, and for this reason, the material used to make the crystal is purified. Two of the most common organic crystal scintillators are anthracene and trans-stilbene. 

Anthracene has a density of 1.25 X 10 kg/m and the highest light conversion efficiency of all organic scintillators (see Table 6.3)—which is still only about one-third of the light conversion efficiency of Nal(Tl). Its decay time ( 30 ns) is much shorter than that of inorganic crystals. Anthracene can be obtained in different shapes and sizes. 

Plastic scintillators have a density of about 10 kg/m. Their light output is lower than that of anthracene (Table 6.3). Their decay time is short, and the wavelength corresponding to the maximum intensity of their emission spectrum is between 350 and 450 nm. Trade names of commonly used plastic scintillators are Pilot B, Pilot Y, NE 102, and NE 110. The characteristics of these phosphors are discussed in Refs. 11-13. Plastic scintillators loaded with tin and lead have been tried as X-ray detectors in the 5-100 keV range. " Thin plastic scintillator films (as thin as 20 X 10 kg/m = 20 p,g/cm ) have proven to be useful detectors in time-of-flight measurements " (see Chap. 13). 

Charged particles. Experiments have shown that organic crystal scintillators (e.g., anthracene) exhibit a direction-dependent response to alphas and protons. An adequate explanation of the direction-dependent characteristics of the response does not exist at present. The user should be aware of the phenomenon to avoid errors. 

In addition to sodium iodide crystals, a number of organic scintillators such as stilbene, anthracene, and terphenyl have been used. In crystalline form, these compounds have decay times of 0.01 and O.l ps. Organic liquid scintillators have also been developed and are used to advantage because they exhibit less selfabsorption of radiation than do solids. An example of a liquid scintillator is a solution ofp-terphcnyl in toluene. 

ZnS(Ag) is a traditional phosphor for a-detection while anthracene and stilbene can be used for /3-particle detection. For y-rays, sodium iodide with a small amount of thallium impurity, Nal(Tl), is the most common phosphor. CsI(Tl) is another often used scintillator because it can be formed to special shapes, e.g. thin sheets, much easier than Nal(Tl). Plastics with incorporated organic scintillators are often used in nuclear physics experiments because they produce short light pulses and can be made in various shapes. 

Solid scintillators such as anthracene have been tested as alternatives for continuous measurements (ICRU 1972). The scintillation material and the radionuclide collector are combined, as discussed above for scintillation beads (Winn 1993). The operating characteristics of these devices, such as the sample volume, count accumulation period, energy resolution, detection efficiency, and background must be arranged to satisfy radionuclide concentration limit specifications. 

The scintillating system may be either in the solid or the liquid phase. The former is now rarely used except as a flowthrough monitoring material such as detergent coated blue grade anthracene or even POPOP. Although the latter is more efficient, it is normally in a poor crystalline habit for easy flow of liquids. 

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