Observation of the ⁶⁰Fe nucleosynthesis-clock isotope in galactic cosmic rays.
Iron-60 ((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 mega-electron volts per 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 million years and that the (60)Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍1 kiloparsec. A natural place for (60)Fe origin is in nearby clusters of massive stars.
Washington University, St. Louis, MO 63130, USA. wrb@wustl.edu mhi@wustl.edu.
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Binns
Washington University, St. Louis, MO 63130, USA. wrb@wustl.edu mhi@wustl.edu.
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Israel
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
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Christian
California Institute of Technology, Pasadena, CA 91125, USA.
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Cummings
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
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de Nolfo
Washington University, St. Louis, MO 63130, USA.
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Lave
California Institute of Technology, Pasadena, CA 91125, USA.
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Leske
California Institute of Technology, Pasadena, CA 91125, USA.
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Mewaldt
California Institute of Technology, Pasadena, CA 91125, USA.
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Stone
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
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von Rosenvinge
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
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Wiedenbeck