CCD PrimerBinning |
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. The three key properties of a cosmic ray particles:
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. Source is still not certain |
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