Scintillators inorganic, response

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. 

One of the important differences between inorganic and organic scintillators is in the response time, which is less than 10 ns for the latter (response time of inorganic scintillators is 1 fis see Table 6.1) and makes them suitable for fast timing measurements (see Chap. 10). Table 6.3 lists important properties of some organic scintillators. 

Charged particles. For protons and deuterons, the response of the scintillator is proportional to the particle energy, at least for > 1 MeV. For alpha particles, the proportionality begins at about 15 MeV (Fig. 6.9). Theoretical aspects of the response have been studied extensively. Today, inorganic scintillators are seldom used for detection of charged particles. 

Response functions of two photocathode materials used in PMT construction (left scale) to spectral characteristics of four inorganic scintillators (right scale). See text for details (From Knoll 2000 used by permission of Wiley)... 

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. 

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