宇宙線
Cosmic ray spallation is thought to be responsible for the abundance in the universe of some light elements—lithium, beryllium, and boron—as well as the isotope helium-3. This process (cosmogenic nucleosynthesis) 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 nucleosynthesis. Cosmic ray spallation was investigated as a possible process to generate deuterium. As it turned out, spallation could not generate much deuterium, but the new studies of spallation showed that this process could generate lithium, beryllium and boron; indeed, isotopes of these elements are over-represented in cosmic ray nuclei, as compared with solar atmospheres (whereas hydrogen and helium are present in about primordial ratios in cosmic rays).