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Published May 6, 2016 | Supplemental Material
Journal Article Open

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


^(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 MeV/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 Myr and that the ^(60)Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, ⪍ 1 kpc. A natural place for ^(60)Fe origin is in nearby clusters of massive stars.

Additional Information

© 2016 American Association for the Advancement of Science. Received 3 November 2015; accepted 18 March 2016. Published online 21 April 2016. This work was supported by NASA Grants NNX08Al11G and NNX13AH66G at the California Institute of Technology, the Jet Propulsion Laboratory, and Washington University in St. Louis. Goddard Space Flight Center was funded by NASA through the ACE Project. We thank S. Woosley and J. Brown at the University of California Santa Cruz for providing modeling calculation data and for discussions about uncertainties in their calculations. We thank A. Chieffi at the Institute for Space Astrophysics and Planetology, Rome, Italy, for discussions on the uncertainties in the C&L calculations. Accelerator testing of the CRIS detectors was made possible by N. Anantaraman, R. Ronningen, and the staff at the National Superconducting Cyclotron Laboratory at Michigan State University, while H. Specht, D. Schardt, and the staff of the GSI heavy ion accelerator in Darmstadt, Germany made possible the heavy ion calibrations of the completed CRIS instrument. The data used are archived at NASA's Space Physics Data Facility (http://spdf.gsfc.nasa.gov) as dataset ac_h2_cris and can be retrieved from this site by direct download or through the SPDF's CDAWeb data service.

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