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

摘要

Iron-60 (Fe-60) 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 x 10(5) iron nuclei, with energies similar to 195 to similar to 500mega-electron volts per nucleon, of which we identify 15 Fe-60 nuclei. The Fe-60/Fe-56 source ratio is (7.5 +/- 2.9) x 10(-5). The detection of supernova-produced Fe-60 in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the Fe-60 half-life of 2.6 million years and that the Fe-60 source distance does not greatly exceed the distance cosmic rays can diffuse over this time, (less than or similar to) under bar1 kiloparsec. A natural place for Fe-60 origin is in nearby clusters of massive stars.