|
|
X-Ray Absorption
 Absorption by the
Earth's atmosphere restricts ground-based observations to radio, near infrared,
and visible wavelengths. X-rays are absorbed high above the Earth in the
following way:
X-ray photons--tiny high-energy packets of electromagnetic radiation--are
absorbed by encounters with individual atoms. Even though the atoms in the
atmosphere are widely spaced, the total thickness of the atmosphere is large and
the total number of atoms is enormous. An X-ray photon passing through the
atmosphere will encounter as many atoms as it would in passing through a 5 meter
(16 ft) thick wall of concrete!
What happens when an X-ray is absorbed
in the atmosphere?
The energy of the X-ray goes into tearing one of the electrons away from its
orbit around the nucleus of a nitrogen or an oxygen atom.
This process is called photo-electric absorption, because a photon is absorbed
in the process of removing an electron from an atom. The high-energy of X-rays is
necessary for photo-electric absorption to take place.
X-ray telescopes in orbit above the Earth's atmosphere can collect X-rays from
energetic sources billions of light years away. These cosmic X-rays are focused
by barrel-shaped mirrors onto an instrument especially designed to measure
properties such as the incoming direction and energy of the X-ray photon. A
gaseous or solid material in the instrument absorbs the X-rays by the
photo-electric effect.
|
