Scintillators as Gamma-ray Detectors
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
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).
When gamma-rays pass through matter, they can undergo one of three basic
production. Each of these
processes can create high-energy electrons or anti-electrons
that interact in the scintillator as charged particles. By adding up the
energy collected in the surrounding photomultiplier tubes, the energy
of the detected gamma-ray can be determined.
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.
Inorganic scintillators have been used as gamma-ray detectors
aboard many space-based missions to observe sources of cosmic gamma rays.
These missions include: the Compton Gamma-Ray Observatory (CGRO), the first High Energy
Astrophysical Observatory (HEAO-1), and the Rossi X-Ray Timing Explorer (RXTE).
The Fermi Gamma-Ray Burst
uses 12 NaI scintillators and 2 bismuth germanate (BGO) detectors to
monitor the entire sky and is sensitive to gamma-rays between a few
and 25 MeV.
Last Modified: November 2010