Observation of the ^(60)Fe nucleosynthesis-clock isotope in galactic cosmic rays

^(60)Fe is a radioactive isotope in cosmic rays that serves as a clock to infer an upper limit on the time between nucleosynthesis and acceleration. We have used the ACE-CRIS instrument to collect 3.55 × 10^5 iron nuclei, with energies ~195 to ~500 MeV/nucleon, of which we identify 15 ^(60)Fe nuclei. The ^(60)Fe/^(56)Fe source ratio is (7.5 ± 2.9) × 10^(−5). The detection of supernova-produced ^(60)Fe in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the ^(60)Fe half-life of 2.6 Myr and that the ^(60)Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍ 1 kpc. A natural place for ^(60)Fe origin is in nearby clusters of massive stars