Scintillators are kinds of materials that provide detectable photons in the visible part of the light spectrum, following the passage of a charged particle or a photon. The scintillator consists of a transparent crystal, usually a phosphor, plastic or organic liquid that fluoresces when struck by ionizing radiation. The scintillator must also be transparent to its own light emissions and it must have a short decay time. The scintillator must be also shielded from all ambient light so that external photons do not swamp the ionization events caused by incident radiation. To achieve this a thin opaque foil, such as aluminized mylar, is often used, though it must have a low enough mass to minimize undue attenuation of the incident radiation being measured.
There are primarily two types of scintillators in common use in nuclear and particle physics: organic or plastic scintillators and inorganic or crystalline scintillators.
Inorganic Scintillators
Inorganic scintillators are usually crystals grown in high temperature furnaces. They include lithium iodide (LiI), sodium iodide (NaI), cesium iodide (CsI), and zinc sulfide (ZnS). The most widely used scintillation material is NaI(Tl) (thallium-doped sodium iodide). The iodine provides most of the stopping power in sodium iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and pulse decay times of approximately 1 microsecond (~ 10-6 sec). Scintillation in inorganic crystals is typically slower than in organic ones. They exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. This feature is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays and X-rays than organic scintillators. This is due to their high density and atomic number which gives a high electron density. A disadvantage of some inorganic crystals, e.g., NaI, is their hygroscopicity, a property which requires them to be housed in an airtight container to protect them from moisture.
Thallium-doped Sodium Iodide – NaI(Tl) scintillators
The most widely used scintillation material is NaI(Tl) (thallium-doped sodium iodide). NaI(Tl) as the scintillator is used in scintillation detectors, traditionally in nuclear medicine, geophysics, nuclear physics, and environmental measurements. The iodine provides most of the stopping power in sodium iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and pulse decay times of approximately 1 microsecond (~ 10-6 sec). The wavelength of maximum emission is 415 nm. Scintillation in inorganic crystals is typically slower than in organic ones. They exhibit high efficiency for detection of gamma rays and are capable of handling high count rates. Inorganic crystals can be cut to small sizes and arranged in an array configuration so as to provide position sensitivity. This feature is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays and X-rays. This is due to their high density and atomic number which gives a high electron density. A disadvantage of some inorganic crystals, e.g., NaI, is their hygroscopicity, a property which requires them to be housed in an airtight container to protect them from moisture. The crystals are usually coupled with a photomultiplier tube, in a hermetically sealed assembly.
Thallium-doped Caesium Iodide – CsI(Tl) scintillators
Caesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles. Pure CsI is a fast and dense scintillating material with relatively low light yield that increases significantly with cooling. The drawbacks of CsI are a high temperature gradient and a slight hygroscopicity.
Thallium-doped Caesium Iodide has an effective atomic number of 54 and a density of 4.51 g/cm3. CsI(TI) is also hygroscopic and should not be subjected to high humidity or water. CsI(TI) has different decay times for different particles (680ns and 3340ns) and can be used to determine between different kinds of radiation. The emission spectrum is peaked at 540-560 nm.
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