Processes that Create
Cosmic Gamma Rays
There are several
physical processes that generate
cosmic gamma
rays:
- A high-energy particle can collide with another
particle
- A particle can collide and annihilate with its
anti-particle
- An element can undergo radioactive decay
- A charged particle can be accelerated
Particle-Particle Collisions
In gamma-ray astronomy, "particle-particle collision" usually means a
high-energy proton,
or cosmic ray, strikes another proton or atomic nucleus. This collision
produces, among other things, one or more neutral pi mesons (or pions).
These are unstable particles that decay into a pair of gamma rays.
Since the pion is usually moving at a high velocity as a result of its
violent birth, the gamma rays are projected forward in a slight "V"
formation. This process gives rise to gamma rays with a broad
spectrum
of energies (all greater than 72 mega-electron-volts,
which is a measurement of the kinetic energy in the incident
particles).
Matter-antimatter annihilation
A particle and its anti-particle, such as an electron
and a positron,
will undergo something called an
annihilation process. In physics, this process produces neutral pions
that quickly decay into gamma rays.
The results from electron-positron annihilations were seen by the OSSE
experiment
aboard the CGRO satellite.
The colors in this map represent the intensity of gamma-ray emission
from positron-electron annihilation in the plane of our galaxy near the
galactic center. The emission is at 511 keV, which is the rest-mass
energy of the positron. The map is of a model that fits the OSSE 511
keV observations. OSSE has discovered that the radiation
is mostly contained in a region of about 10 degrees diameter centered
on the center of the galaxy . The line plot
superimposed on the map
represents an
OSSE observation of the 511 keV emission line.
Radioactive Decay
Radioactive decay results when an element changes to another element by virtue of changes within the atom's nucleus. These changes leave the nucleus in an excited state. The atom emits a gamma ray as it decays into the ground state. We not only observe these gamma rays, but their fluxes
and spectra
identify the specific nuclei and the rate of their excitation.
Extreme physical conditions are required to produce excited nuclei, thus
allowing us to probe unique physical environments with these
observations. Radioactive gamma-ray sources in space are associated
with events of nucleosynthesis, such as supernovae.
Acceleration of Charged Particles
A
magnetic field exerts a force on a charged particle that is moving in it.
This causes the particle to radiate, with the emitted power being
proportional to the square of the force divided by the square of the mass
of the particle. For electrons, this radiation
is often in the gamma-ray region of the
electromagnetic spectrum. The character of the radiation (and the
name given to it) depends on the nature of the accelerating force. If
the electron is accelerated in the electrostatic field around a
nucleus, the resulting radiation is called bremsstrahlung;
it is synchrotron
radiation (sometimes also called cyclotron radiation) when the acceleration
takes place in a static
magnetic field; and the process is called
or Compton
scattering (sometimes also called Thomson scattering) when the acceleration
occurs in the electromagnetic
field of a photon.
Last Modified: October 2010
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