Visible light radiation has
wavelength range from about 4000 angstroms to 7000 angstroms. This corresponds
to an energy of two or three electron volts comparing to gamma-ray photons
energy domain which starts at about 100 keV.
The cosmic gamma rays constitute
the highest-energy domain of electromagnetic spectrum. Gamma-ray photons emitted
by celestial bodies are ten of thousand times more energetic than photons of
visible light.
Visible light radiation has
wavelength range from about 4000 angstroms to 7000 angstroms. This corresponds
to an energy of two or three electron volts comparing to gamma-ray photons
energy domain which starts at about 100 keV.
The earth's atmosphere absorbs
cosmic gamma radiation. So they can be studied only in space.
Because the wavelenths of gamma
photons are smaller than the distances between the atom matter, it is not
possible to reflect and concentrate a beam of gamma photons in lenses and
mirrors as it can be done with visible light in traditional optical
telescopes.
Gamma-ray photons can be detected
by collecting electrons they create when striking with detectors which are high
density materials as sodium iodide. Three major types of interaction play an
important role in radiation measurements and all these processes lead to the
partial or complete transfer of the gamma-ray photon energy to electron.
The atomic number of the
interaction medium and the energy of the gamma photon have a strong influence on
the relative probabilities of these three interactions :
Photoelectric absorption predominates
for low-energy gamma rays (up to several tens keV) for the heaviest elements.
Pair production predominates for
high-energy gamma rays (above 5-10 MeV).
Compton scattering is the most
probable process over the range of energies between these extremes.
The observational domain of the
SIGMA telescope extends from high-energy X-rays to low-energy gamma rays, in the
energy range 35 keV to 1.3 MeV.
When a gamma photon enters the
sodium iodide gamma-ray detector, it excites the molecules and induces them to
emit a flash of visible light.
Sensitive light-detecting
photomultiplier tubes detect that light and convert it in an electric pulse. The
height of the electric pulse is proportional to the gamma photon energy in the
sodium iodide crystal.
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