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cosmic-ray track The interaction of ionizing cosmic rays (see cosmic radiation) with mineral surfaces produces solid-state damage which, when etched with acid, is revealed as tracks of varying length. Most tracks are produced by iron group nucleii (nuclear charge (Z) 18–28, VH ions). Track lengths due to galactic cosmic rays may extend to depths of 20cm. In contrast, solar flare tracks are mostly less than one millimetre in length. Track densities, typically about 1012/m2 on unshielded lunar mineral surfaces, provide a measure of exposure age.