Why are alkali-metal iodide crystals used in neutron detectors and scintillation counters?

Alkali-metal iodide crystals, such as sodium iodide and cesium iodide, are widely used in scintillation counters and radiation detectors primarily because they contain iodine, an element with a high atomic number of 53.1

This high atomic number results in a large photoelectric cross section, which makes these materials highly effective at absorbing gamma-ray radiation. When these crystals absorb radiation, they emit light—a process known as scintillation—which can then be detected by devices like photomultiplier tubes or photodiodes. Because these crystals can be grown in large, optical-quality sizes, they offer high intrinsic efficiency for detecting gamma rays compared to other detector types.12

In the context of neutron detection, specific alkali-metal iodides like lithium iodide are used because they incorporate elements that interact with neutrons, allowing the crystal to function as a scintillator for recording neutron interactions.3