Cosmic ray
For the 1962 Bruce Conner film, see
Cosmic Ray (film)
Cosmic rays are energetic particles originat-
ing from outer space that impinge on Earth’s
atmosphere. Almost 90% of all the incoming
cosmic ray particles are protons, about 9%
are helium nuclei (alpha particles) and about
1% are electrons (beta minus particles). The
term ray is a misnomer, as cosmic particles
arrive individually, not in the form of a ray or
beam of particles.
The variety of particle energies reflects
the wide variety of sources. The origins of
these particles range from energetic pro-
cesses on the Sun all the way to as yet un-
known events in the farthest reaches of the
visible universe. Cosmic rays can have ener-
gies of over 1020 eV, far higher than the 1012
to 1013 eV that man-made particle accelerat-
ors can produce. (See Ultra-high-energy cos-
mic rays for a description of the detection of
a single particle with an energy of about 50 J,
the same as a well-hit tennis ball at 42 m/s
[about 94 mph].) There has been interest in
investigating cosmic rays of even greater en-
ergies.[1]
The energy spectrum for cosmic rays.
Composition
Cosmic rays may broadly be divided into two
categories, primary and secondary. The cos-
mic rays that arise in extrasolar astrophysical
sources are primary cosmic rays;
these
primary cosmic rays can interact with inter-
stellar matter to create secondary cosmic
rays. The sun also emits low energy cosmic
rays associated with solar flares. The exact
composition of primary cosmic rays, outside
the Earth’s atmosphere, is dependent on
which part of the energy spectrum is ob-
served. However, in general, almost 90% of
all the incoming cosmic rays are protons,
about 9% are helium nuclei (alpha particles)
and about 1% are electrons. The remaining
fraction is made up of the other heavier nuc-
lei which are abundant end products of stars’
nuclear synthesis. Secondary cosmic rays
consist of the other nuclei which are not
abundant nuclear synthesis end products, or
products of the Big Bang, primarily lithium,
bery