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

Iron-60 (⁶⁰Fe) 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 × 10⁵ iron nuclei, with energies ~195 to ~500 mega-electron volts per nucleon, of which we identify 15 ⁶⁰Fe nuclei. The ⁶⁰Fe/⁵⁶Fe source ratio is (7.5 ± 2.9) × 10^-5. The detection of supernova-produced ⁶⁰Fe in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the ⁶⁰Fe half-life of 2.6 million years and that the ⁶⁰Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍1 kiloparsec. A natural place for ⁶⁰Fe origin is in nearby clusters of massive stars.