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Why is thallium-added sodium iodide used in scintillation detectors?
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radiation measurement
Figure 1: (A) A simple equivalent circuit for the development of a voltage pulse at the output of a detector. R represents the resistance and C the capacitance of the circuit; V(t) is the time (t)-dependent voltage produced. (B) A representative current pulse due to the interaction of a single quantum in the detector. The total charge Q is obtained by integrating the area of the current, i(t), over the collection time, tc. (C) The resulting voltage pulse that is developed across the circuit of (A) for the case of a long circuit time constant. The amplitude (Vmax) of the pulse is equal to the charge Q divided by the capacitance C.
Thallium-activated sodium iodide, denoted as NaI(Tl), is used in scintillation detectors primarily because it acts as an effective inorganic phosphor that produces an unusually large light yield when exposed to radiation.12
The addition of a trace amount of thallium to the sodium iodide crystal lattice is essential for creating the luminescence properties required for radiation detection. This material is favored for several key reasons:2
- High Light Output: It possesses a scintillation efficiency of approximately 13 percent, which is among the highest for inorganic scintillators.1
- Spectral Matching: Its emission spectrum peaks in the blue region of the electromagnetic spectrum, making it highly compatible with the spectral response of standard photomultiplier tubes.1
- Gamma-Ray Spectroscopy: Because it contains iodine—an element with a high atomic number—it has a large photoelectric cross section, making it highly effective for detecting gamma rays.1
- Scalability: It can be grown into large, optical-quality crystals economically, allowing for high intrinsic peak efficiency in gamma-ray detection.1