Oversimplified sketch of the formation of cosmogenic-isotope records in terrestrial (panel a) and lunar (panel b) archives. Primary cosmic-ray particles CR₁ and CR₂ with energies E₁ and E₂ < E₁ are denoted by blue dots, nuclear reactions by stars, secondaries by arrows, and produced cosmogenic isotopes by red dots. Two different moments of time t₁ and t₂ > t₁ are shown on the left- and right-hand sides, respectively.