Sodium Iodide (NaI) Detectors

Sodium Iodide is a type of detector within a class known as scintillation detectors. Gamma rays penetrate through the detector housing and into the NaI crystal where they may experience interactions with the atoms inside the crystal, as shown in Figure 1.

Figure 1 - A Sodium Iodide crystal with Thallium as a dopant. A gamma ray is shown interacting with one of the NaI molecules.

Figure 1 - A diagram of a Sodium Iodide  crystal, doped with Thallium, inside a detector housing with a Gamma-ray striking one of the atoms.

These interactions can cause photons of visible light to be released from the atom and travel short distances within the crystal. Thallium (Tl) is used as a dopant for NaI because it helps to increase the number of visible photons being output by the crystal and also helps to improve the crystal's transparency to those emitted photons so they can be measured by equipment outside the crystal. As shown in Figure 2 these photons of visible light are detected by a photocathode to produce an electrical signal, which is then amplified and processed to create a gamma ray spectrum.

Figure 2 - Sodium Iodide detector assembly. Visible light is created inside the NaI crystal due to incoming gamma rays. This visible light is converted to an electrical signal by the photocathode. This electrical signal is amplified by the Photomultiplier Tube (PMT) and then processed into a spectrum by the Multichannel Analyser (MCA).

Figure 2 - A typical detector assembly showing the relationship between the major components: the NaI crystal, the Photocathode, the Amplifier and the electronics used for processing the signal after amplification.
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