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Cosmic ray spallation is a form of naturally occurring nuclear fission and nucleosynthesis. It refers to the formation of elements from the impact of cosmic rays on an object. Cosmic rays are highly energetic charged particles from outside of Earth ranging from stray electrons to alpha particles. These cause spallation when a cosmic ray (e.g. a proton) impacts with matter, including other cosmic rays. The result of the collision is the expulsion of large members of nucleons (protons and neutrons) from the object hit. This process goes on not only in deep space, but in our upper atmosphere due to the impact of cosmic rays.

Cosmic ray spallation produces some light elements such as lithium and boron. This process was discovered somewhat by accident during the 1970s.

Models of big bang nucleosynthesis suggested that the amount of deuterium was too large to be consistent with the expansion rate of the universe and there was therefore great interest in processes that could generate deuterium after the big bang.

Cosmic ray spallation was investigated as a possible process to generate deuterium. As it turned out, spallation could not generate much deuterium, and the excess deuterium in the universe could be explained by assuming the existence of non-baryonic dark matter. However, studies of spallation showed that it could generate lithium, beryllium and boron, and indeed these isotopes are over-represented in cosmic ray nuclei, as compared with solar atmospheres (whereas H and He are present in about primordial ratios in cosmic rays).

Isotopes of aluminum, carbon (carbon-14), chlorine, iodine and neon, are also formed through cosmic ray spallation.

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