NaITl

NaI(Tl) is a thallium-doped sodium iodide scintillator crystal widely used for gamma-ray detection and spectroscopy. The host lattice is sodium iodide (NaI), while thallium ions (Tl+) act as activators that enhance scintillation light production when exposed to ionizing radiation. The material is commonly referred to as NaI(Tl).

Physically, NaI(Tl) crystals are hygroscopic, requiring hermetic sealing to prevent moisture damage. They have a density of about 3.67 g/cm3 and a refractive index near 1.85 in the blue region. Under gamma irradiation, NaI(Tl) emits scintillation light with a peak wavelength around 415 nm. The light yield is approximately 38,000 photons per MeV, and the scintillation decay is on the order of about one microsecond, with a typical emission spectrum suited for conventional photomultiplier tubes.

Crystals are produced by standard halide crystal growth techniques, often with Tl doping levels of a few tenths of a percent by weight. Sizes range from small, compact detectors to large crystals paired with photomultiplier tubes or silicon photomultipliers. The crystal’s excellent optical transparency and high light output make NaI(Tl) a popular choice for gamma-ray spectroscopy and radiation-detection instruments, including gamma cameras and handheld detectors.

Applications include basic research, medical imaging (such as SPECT), nuclear security, and environmental monitoring. While NaI(Tl) offers good energy resolution and broad gamma sensitivity, its hygroscopic nature and relatively slower decay compared with some modern scintillators limit its use in very high-rate or harsh environments. Handling typically requires sealed detector assemblies and moisture protection.