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

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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,Symbolkiloparsec. A natural place for 60Fe origin is in nearby clusters of massive stars.

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