Scintillators as Gamma-ray Detectors
A popular method for the detection of
involves the use of crystal
scintillators. The general description of a scintillator is a material that
emits low-energy (usually in the
photons when 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 three basic processes:
scattering, photoabsorption, or
production. Each of these
processes can create high-energy electrons or anti-electrons
interact in the scintillator as charged particles. By adding up the energy
collected in the surrounding photomultiplier tubes, we can determine the energy
of the gamma-ray detected.
Scintillators can be made of a variety of materials, depending on the
intended applications. The most common scintillators used in gamma-ray
detectors which are made of inorganic materials are usually an alkali halide
salt, such as sodium iodide (NaI) or cesium iodide (CsI). To help these
materials do their job, a bit of impurity is often added. This material is
called an 'activator'. Thallium and sodium are often used for this purpose.
So one often sees detectors 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-radiation.
These missions include: the
Observatory (CGRO), the
first High Energy
Astrophysical Observatory (HEAO-1), and the
Rossi X-Ray Timing
The GLAST Gamma-Ray Burst Monitor will use 12 NaI
scintillators and 2 bismuth germanate (BGO) detectors to cover the
entire sky and be sensitive to gamma-rays between a few keV and 25 MeV.