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

Abstract

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 ~500mega'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,