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Cosmic Rays - Samples of matter
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Galactic cosmic rays (GCRs) are the high-energy particles that
flow into our solar system from far away in the Galaxy. GCRs are mostly
atomic nuclei from which all of the surrounding electrons have been
stripped during their high-speed passage through the Galaxy.
Cosmic rays provide one of our few direct samples of matter from outside
the solar system. The magnetic fields of the Galaxy, the solar system,
and the Earth have scrambled the flight paths of these particles so much
that we can no longer point back to their source in the Galaxy. But the
"composition" of GCRs can still tell us a lot about their sources and
their passage through the Galaxy. They have probably been accelerated
in the tremendous blast waves (shocks) that occur in supernova remnants
such as the Crab nebula (pictured at left). |
All of the natural elements in the periodic table
are present in cosmic rays, in roughly the same proportion as they
occur in the solar system. But detailed differences provide a
fingerprint of the cosmic ray's source. Measuring the quantity of each
different element is relatively easy, since the different charge of each
element's nuclei give very different signatures. Harder to measure,
but a better fingerprint, is the isotopic composition (nuclei of the
same element but with different numbers of neutrons). To tell the
isotopes apart involves in effect weighing each atomic nucleus that enters the
detector.
How Do You Weigh a Particle Moving at Half the Speed of
Light?
What Else Makes Observing Cosmic Rays Hard?
Tell Me About Flying Cosmic Ray Detectors On
Balloons
Cosmic Ray Telescopes on ACE
The Advanced Composition
Explorer (ACE) was launched on a Delta II rocket in 1997. It is now
orbiting the Earth-Sun libration point, L1, studying energetic particles
coming from the Sun, interplanetary space, and the distant Galaxy. Of
the nine instruments on board, the Cosmic Ray Isotope Spectrometer (CRIS)
is specifically designed to look at Galactic Cosmic Rays.
Tell me more about the CRIS instrument on ACE
Future Projects
The CRIS instrument on ACE will measure the fingerprints of galactic
cosmic rays very well up to iron and nickel (the 26th and 28th elements
of the periodic table). The next steps will be to study the much rarer
elements heavier than nickel, and to study cosmic rays at higher energies.
An experiment,
ACCESS, is planned for the Space Station to study both these questions.
Beyond that, an experiment, OWL,
is being designed that will have a pair of
telescopes in orbit using the atmosphere to detect the very highest energy
cosmic rays, individual particles with more kinetic energy than a Nolan
Ryan fastball. An
ambitious detector system like this is needed because fewer than a thousand
of these particles hit the whole Earth per day.
Thank you to
Eric Christian for contributing to this article.
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