Organic Crystal Scintillators

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. 

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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. 

There are three classes of solid state scintillation phosphors organic crystals, inorganic crystals, and plastic phosphors. 

Figure 18.18 Schematic view of the scintillation mechanism in organic crystals. (From Wang et al., 1975.)...

The details of the scintillation process are complicated and depend very much on the molecular structure of the scintillator. In organic crystals, the molecules of the organic solid are excited from their ground states to their electronic excited states (see Fig. 18.18). The decay of these states by the emission of photons occurs in about 10-8 s (fluorescence). Some of the initial energy absorbed by the molecule is dissipated as lattice vibrations before or after the decay by photon emission. As a result, the crystal will generally transmit its own fluorescent radiation without absorption. 

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. 

Owing to the presence of organic solvent, the mechanism of luminescence for the liquid scintillator is much more complicated than that for a crystal scintillator. The energy successively transfers to generate finally the luminescence, as follows ... 

In both scintillator and gas detectors, the absorption of radiation causes excitation and ionization however with the scintillation process, the absorbed energy produces a flash of light, rather than a pulse of current. The principal types of scintillation detectors found in the clinical chemistry laboratory are the sodium iodide crystal scintillation detector and the organic liquid scintillation detector. Because of the crystal detector s relative ease of operation and economy of sample preparation, most clinical laboratory procedures have been developed to measure nucfides, such as which can be counted efficiently in a crystal detector. A liquid scintillation detector is used to measure pure (3-emitters, such as tritium or C. 

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. 

Scintillation Counter. Photomultiplier detectors, discussed in Chapter 5, are very sensitive to visible and UV light, but not to X-rays, to which they are transparent. In a scintillation detector the X-radiation falls on a compound that absorbs X-rays and emits visible light as a result. This phenomenon is called scintillation. A PMT can detect the visible light scintillations. The scintillating compound or phosphor can be an inorganic crystal, an organic crystal or an organic compound dissolved in solvent. 

Sn, 1 1 Eu, 1 1 Dy and i Tm. Second, scintillation detectors may be considered. Thin organic (crystal or plastic) scintillators are used for detecting electrons. Gas scintillation proportional counters with a good energy resolution (e.g. R 8% at 6 keV) may also be constructed for CEMS as well as semiconductor detectors. 

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. 

Uses Dyes starting material for the preparation of alizarin, phenanthrene, carbazole, 9,10-anthraquinone, 9,10-dihydroanthracene, and insecticides in calico printing as component of smoke screens scintillation counter crystals organic semiconductor research wood preservative. 

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. 

The scintillation process in inorganic scintillators differs from that in organic scintillators. Consider the structure of an ionic crystal, as shown in Figure 18.19. When an energetic electron passes through the crystal, it may raise valence electrons from the valence band to the conduction band. The electron vacancy in the valence... 

Use Dyes, alizarin, phenanthrene, carbazole, anthra-quinone, calico printing, a component of smoke screens, scintillation counting crystals, organic semiconductor research. 

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). 

Figure 9.13 Four examples of response functions (a) 5-MeV Alpha particles detected by a silicon surface barrier detector (Chap. 13), or 20-keV X-rays detected by a Si(Li) reactor (Chap. 12). ib) 1-MeV Gamma ray detected by a NaI(Tl) crystal (Chap. 12). (c) 1-MeV Electrons detected by a plastic scintillator (Chap. 13). (

Stilbene scintillators were used as early as 1957. Stilbene as a crystal is very sensitive to mechanical and thermal shock and shows an anisotropic response to neutrons—i.e., neutrons incident from different directions, with respect to the crystal lattice, produce different light output. Liquid organic scintillators have none of these problems in addition, they have higher H/C ratio, and light production from carbon recoils relatively lower than in stilbene. For all these reasons, liquid organic scintillators are almost exclusively used for detecting fast neutrons. 

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