A scintillation counter or scintillation detector is a radiation detector which uses the effect known as scintillation. Scintillation 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. The scintillator consists of a transparent crystal, usually a phosphor, plastic or organic liquid that fluoresces when struck by ionizing 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.
Phosphors – Scintillation Detectors
In general, a phosphor is a substance that exhibits the phenomenon of luminescence. These materials are also used in scintillation detectors in various forms. 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 because the transparence is important to a good light transmission to reach the electrode, and the short decay time allows fast response.
Inorganic Scintillation Phosphors
Inorganic scintillation phosphors 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. 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.