Cosmic Rays
Liquid nitrogen cooled CCD cameras are able to integrate for up to 6 hours. Realistically
integration times are reduced to about 60 minutes. Longer integrations are limited
by the influence of cosmic rays. Our spectroscopy software program offers a cosmic
ray filter to reduce their influence. Cosmic rays look like hot pixels and are
randomly distributed over the entire image. The nature of cosmic rays makes it
impossible to provide cosmic ray shielding for the camera housing or the CCD itself.
What are cosmic rays ?
Cosmic rays are subatomic particles arriving from outer space, which have high
energy as a result of their rapid motion.
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- The three key properties of a cosmic ray particles:
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- 1. Electric charge
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- 2. Rest mass
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- 3. Energy
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- About 87 percent of cosmic rays are protons (hydrogen nuclei) and about
12 percent are alpha particles (helium nuclei). Heavier elements are also
present, but in greatly reduced numbers. For convenience, scientists divide
the elements into light (lithium, beryllium and boron), medium (carbon, nitrogen,
oxygen and fluorine) and heavy (the remainder of the elements). The light
elements compose 0.25 percent of the cosmic rays. Because the light elements
constitute only about 1 billionth of all matter in the universe, it is believed
that light element cosmic rays are formed by the fragmentation of heavier
cosmic rays that collide with protons, as they must do in traversing interstellar
space. From the abundance of light elements in cosmic rays, it is inferred
that cosmic rays have passed through the material equivalent of a layer of
water 4 cm (about 1.5 in) thick. The medium elements are increased by a factor
of about 10 and the heavy elements by a factor of about 100 over normal matter,
suggesting that at least the initial stages of acceleration to the observed
energies occur in regions enriched in heavy elements. The energy of cosmic
ray particles is measured in units of giga (billion) electron volts (GeV)
per proton or neutron in the nucleus. The distribution of the proton energy
of cosmic rays peaks at 0.3 GeV, corresponding to a velocity two thirds that
of light and falls toward higher energy, although particles up to 1011 GeV
have been detected through showers of secondary particles created when they
collide with atmospheric nuclei. On average, about 1 electron volt of energy
per cubic centimeter of space is invested in cosmic rays in our galaxy. Even
an extremely weak magnetic field deflects cosmic rays from straight line paths;
a field of 3 × 10-6 gauss, such as is believed to be present
throughout interstellar space, is sufficient to force a 1-GeV proton to gyrate
with a radius of 10-6 light-year. A 1011-GeV particle gyrates with
a radius of 105 light years, about the size of the galaxy. Thus,
the interstellar magnetic field prevents cosmic rays from reaching the earth
directly from their points of origin, accounting for the directions of arrival
being isotropically distributed at even the highest energies.
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- Source 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 then they are not adequate sources either.
Supernova explosions are responsible for at least the initial acceleration
of a significant fraction of cosmic rays, as the remnants of such explosions
are powerful radio sources, implying the presence of energetic electrons.