Important Properties of Certain Inorganic Scintillators

Nal(TI). Nal(Tl) is the most commonly used scintillator for gamma rays. It has been produced in single crystals of up to 0.75 m ( 30 in) in diameter and of considerable thickness (0.25 m 10 in). Its relatively high density (3.67 X 10 kg/m ) and high atomic number combined with the large volume make it a 

7-ray detector with very high efficiency. Although semiconductor detectors (Chap. 7 and 12) have better energy resolution, they cannot replace the Nal(Tl) in experiments where large detector volumes are needed. 

The emission spectrum of Nal(Tl) peaks at 410 nm, and the light-conversion efficiency is the highest of all the inorganic scintillators (Table 6.1). As a material, Nal(Tl) has many undesirable properties. It is brittle and sensitive to temperature gradients and thermal shocks. It is also so hygroscopic that it should be kept encapsulated at all times. Nal always contains a small amount of potassium, which creates a certain background because of the radioactive 

CsI(Tl). CsI(Tl) has a higher density (4.51 X 10 kg/m ) and higher atomic number than Nal therefore its efficiency for gamma detection is higher. The light-conversion efficiency of CsI(Tl) is about 45 percent of that for NaKTl) at room temperature. At liquid nitrogen temperatures (77K), pure Csl has a light output equal to that of Nal(Tl) at room temperature and a decay constant equal to 10 s. The emission spectrum of CsI(Tl) extends from 420 to about 600 nm. 

Csl is not hygroscopic. Being softer and more plastic than Nal, it can withstand severe shocks, acceleration, and vibration, as well as large temperature gradients and sudden temperature changes. These properties make it suitable for space experiments. Finally, Csl does not contain potassium. 

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