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Source- NASA.gov: A popular method for the detection of gamma rays involves the use of crystal scintillators. A scintillator is a material that emits low-energy photons (usually in the visible range) when they are struck by a high-energy charged particle. When used as a gamma-ray detector, the scintillator does not directly detect the gamma-rays. Instead, the gamma-rays produce charged particles in the scintillator crystals, which interact with the crystal and emit photons. These lower-energy photons are subsequently collected by photomultiplier tubes (PMTs). Scintillators can be made of a variety of materials, depending on the intended applications. The most common scintillators used in gamma-ray detectors are made of inorganic materials, and are usually an alkali halide salt, such as sodium iodide (NaI) or cesium iodide (CsI). To help these materials do their job, an impurity, called an "activator," is often added. Thallium and sodium are often used for this purpose, so detectors are usually described as NaI(Tl), which means it is a sodium iodide crystal with a thallium activator, or as CsI(Na), which is a cesium iodide crystal with a sodium activator.