NaI(Tl) Scintillation Counter

Scintillation_Counter - Photomultiplier Tube

A NaI(Tl) scintillation counter is a radiation detector which uses the effect known as scintillation. Scintillation, which occurs in the NaI(Tl) crystal, is a flash of light produced in a transparent material by the passage of a particle (an electron, an alpha particle, an ion, or a high-energy photon). Scintillation occurs in the scintillator, which is a key part of a scintillation detector. In general, a scintillation detector consists of:

  • Scintillator. A scintillator generates photons in response to incident radiation.
  • Photodetector. A sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode), which converts the light to an electrical signal and electronics to process this signal.

The basic principle of operation involves the radiation reacting with a scintillator, which produces a series of flashes of varying intensity. The intensity of the flashes is proportional to the energy of the radiation. This feature is very important. These counters are suited to measure the energy of  gamma radiation (gamma spectroscopy) and, therefore, can be used to identify gamma emitting isotopes.

Scintillation counters are widely used in radiation protection, assay of radioactive materials and physics research because they can be made inexpensively yet with good efficiency, and can measure both the intensity and the energy of incident radiation. Hospitals all over the world have gamma cameras based on the scintillation effect and, therefore, they are also called scintillation cameras.

The advantages of a scintillation counter are its efficiency and the high precision and counting rates that are possible. These latter attributes are a consequence of the extremely short duration of the light flashes, from about 10-9  (organic scintillators) to 10-6 (inorganic scintillators) seconds. The intensity of the flashes and the amplitude of the output voltage pulse are proportional to the energy of the radiation. Therefore, scintillation counters can be used to determine the energy, as well as the number, of the exciting particles (or gamma photons). For gamma spectrometry, the most common detectors include sodium iodide (NaI) scintillation counters and high-purity germanium detectors.

Thallium-doped Sodium Iodide – NaI(Tl) scintillators


Photomultiplier Tube


Components of Photomultiplier Tube



Radiation Protection:


Nuclear and Reactor Physics:


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Scintillation Counter