Cosmogenic nuclide
A cosmogenic nuclide is an isotope created when cosmic rays strike atoms in a rock exposed in space or at the surface.
A cosmogenic nuclide is an isotope made by cosmic rays: high-energy particles, mainly protons, that stream through space from the Sun and from distant stellar explosions. When they hit atoms in a rock, they can break nuclei into smaller pieces or add particles, producing new isotopes.
Meteoroids in space are fully exposed, so cosmogenic nuclides build up in them steadily. Stable ones, such as helium-3, neon-21 and argon-38, accumulate continuously, while radioactive ones, such as beryllium-10, aluminium-26 and chlorine-36, reach a balance between production and decay.
Measuring them answers several questions. How long was the meteoroid travelling as a small body? That is the cosmic-ray exposure age. How long has it been on Earth? That is the terrestrial age. How big was it before entering the atmosphere, and how deep inside it was this sample? That comes from comparing nuclide amounts that depend on shielding depth. Cosmogenic nuclides also date rock surfaces on Earth.
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