Inorganic Crystal Scintillators

Most of the inorganic scintillators are crystals of the alkali metals, in particular alkali iodides, that contain a small concentration of an impurity. Examples are NaI(Tl), CsI(Tl), CaI(Na), Lil(Eu), and CaF2(Eu). The element in parentheses is the impurity or activator. Although the activator has a relatively small concentration—e.g., thallium in Nal(Tl) is 10 on a per mole basis—it is the agent that is responsible for the luminescence of the crystal. 

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

All SPAs are based upon the phenomenon of scintillation. Scintillation is an energy transfer that results from the interactions of particles of ionizing radiation and the de-localized electrons found in conjugated aromatic hydrocarbons or in inorganic crystals. When the decay particle collides with... 

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. 

Table 8.3 lists the properties of some common scintillators. The data indicate that the greater density of inorganic crystals makes them preferable for y-ray counting. The resolving time is shorter for the organic systems whether liquid or solid. When large detector volumes are necessary a liquid solution system is the simplest and most economical. 

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. 

Scintillation detectors consist of a scintillator (usually inorganic crystals such as Nal) and a photomultiplier tube. The scintillator emits light upon the absorption of radiation. This light... 

Radioactive particles can be applied as tracers, used to study solids mixing, particle trajectory, or RTD (residence time distribution) of gas or solids within a fluidized bed, as well as to measure the suspension density. Inorganic salt scintillators, i.e., crystals of inorganic salts containing trace quantities of activators to enhance the emission have been used to detect the... 

Scintillation detectors, especially in the case of inorganic crystals, constitute a seeond very important class of gamma ray detectors. In fact, they possess the requirements of high efficiency, fast response, low cost, good linearity whieh fulfill the experimental needs. The major drawback of scintillator detectors is their energy resolution that in the case of NaI Tl (one of the best and most used scintillators) is of the order of 40 keV at 662 keV, namely 40 times worse than that of HPGe detector. 

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... 

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