Observation of the $^{\mathrm{60}}$Fe Nucleosynthesis-Clock Isotope in Galactic Cosmic Rays~

We have measured the abundance of $^{\mathrm{60}}$Fe, a radioactive isotope in cosmic rays that serves as a clock to infer an upper limit on the time between nucleosynthesis in supernovae and cosmic ray acceleration. The ACE-CRIS instrument has collected 3.55 \texttimes 10$^{\mathrm{5}}$ iron nuclei from which we have resolved 15 $^{\mathrm{60}}$Fe nuclei. From this a $^{\mathrm{60}}$Fe/$^{\mathrm{56}}$Fe source ratio of (7.5 \textpm 2.9) \texttimes 10$^{\mathrm{-5}}$ is obtained. The detection of supernova-produced $^{\mathrm{60}}$Fe in cosmic rays implies that the time required for acceleration and transport to Earth does not greatly exceed the $^{\mathrm{60}}$Fe half-life of 2.6 Myr and that the $^{\mathrm{60}}$Fe source distance does not greatly exceed the distance cosmic rays can diffuse over this time, \textless \textasciitilde 1 kpc. A natural place for $^{\mathrm{60}}$Fe origin is in nearby clusters of massive stars.