Cosmic
Rays
Cosmic Rays are sub-atomic particles arriving from outer space, which have high energy as a result of their rapid motion.
They were discovered when the electrical conductivity of the earth's atmosphere was traced to ionisation caused by energetic radiation. The Austrian-American physicist Victor Franz Hess showed
in 1911-1912 that atmospheric ionisation increases with altitude and he
concluded that the radiation must be coming from outer space. The discovery
that the intensity of the radiation depends on latitude implied that the
particles composing the radiation are electrically charged and are deflected
by the earth's magnetic field.
Scientists have discovered that cosmic rays can effect
the earth by causing changes in the weather. Cosmic rays
can cause clouds to form in the upper atmosphere after the particles collide
with other atmospheric particles in our troposphere. The process of a
cosmic ray particle colliding with particles in our atmosphere and disintegrating
into smaller pions, muons and the like, is called a cosmic ray shower.
About 87% of cosmic rays are protons and about 12%
are alpha particles. The light elements compose 0.25% of the cosmic rays.
The source of cosmic rays is still not certain.
The sun emits cosmic rays of low energy at the time of large solar flares,
but these events are far too infrequent to account for the bulk of cosmic
rays. If other stars are like the sun, they are not adequate sources either.
Supernova explosions are responsible for at least the initial acceleration
of a significant fraction of cosmic rays. Supernovas are believed to be
the sites at which the nuclei of heavy elements are formed, so it is understandable
that the cosmic rays should be enriched in heavy elements if supernovas
are cosmic ray sources. However no direct evidence exists that supernovas
contribute significantly to cosmic rays.
- They are thought
to be produced by our Sun (when they are called the solar wind), stars,
supernovae, neutron stars and black holes.
The most well-known
source of cosmic rays is the Sun, with its solar wind. These particles
have a characteristic energy limit and so we know that the super high
energy ones are coming from something more powerful than our Sun.
In 1960 a scientist
called suggested that lower-energy cosmic rays come mainly from inside
our own galaxy (the Milky Way), whereas those of higher energy come
from more distant sources. He expected to see different numbers of low
energy cosmic rays coming from various directions at us from within
our galaxy because the galaxy is not arranged symmetrically around us.
He found that the lower-energy rays had directions are scattered in
a similar pattern and his idea was right.
- They travel at
very fast speed indeed - very close to the speed of light.
- Cosmic rays are
made up from a stream of high energy particles which are generally ionized
atoms (ions), ranging
from a single proton, up to an iron nucleus and sometimes an even heavier
one!
- When charged particles
move they create a magnetic field (you should know this from your GCSE
electricity work) and because there is a small magnetic field permeating
the Galaxy (and an even smaller field permeating inter-galactic space)
they travel in curved paths as their field interacts with that around
them.
- The earth's atmosphere
protects us from being exposed to many of these particles. As a cosmic
ray enters the atmosphere, it will collide with a particle in the atmosphere
(usually a nitrogen or oxygen molecule) and cause ionisation of that.
The collision produces matter out of the energy of the cosmic particle
(you've heard of Einstein's equation E = mc2 haven't you? …. but this is way beyond the scope of GCSE!) and the ramifications
of that are studied to tell us more about the origins of the particles.
If you live at
a high altitude or go up in an aeroplane your radiation
dose increases because you are hit by more cosmic rays!
If you live nearer
the poles you will receive a higher dose of cosmic radiation because
the earth surface rotates more quickly at the equator than at the poles… also the atmosphere is deeper at the equator than at the poles.
- Alternatively,
as these particles travel through the atmosphere, they interact with
nitrogen molecules in the atmosphere, exciting the electrons in them
and causing them to emit light (rather like the way a fluorescent light
works).
- Cosmic rays have
been detected with very high energies, we believe that they originate
from quasars and active galactic nuclei. We do not know where these
particles are coming from but a lot of exciting research is going on
in that area. We have not got detectors that are able to detect cosmic
rays of even higher energy but scientists are sure that super-energy
Cosmic rays exist…. we just can't 'see' them with any instrument
we have devised yet.