The Time Delay between Nucleosynthesis and Acceleration Based on ACE Measurements of Primary Electron-Capture Nuclides

Supernovae should produce the radioactive nuclide ^(59)Ni, and in the ejecta of the explosions these particles will decay by electron capture with a halflife of 7.6 x 10^5 yr to produce ^(59)Co. However, if the ^(59)Ni nuclei are accelerated to cosmic-ray energies on a time scale short compared to this halflife, they are stripped of their electrons and decay is prevented. Thus the abundances of ^(59)Ni and ^(59)Co can be used to determine whether the time between nucleosynthesis and cosmic-ray acceleration is short or long compared to the 59Ni halflife (Soutoul, Casse, & Juliusson 1978). We have used the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) to measure the abundances of 59Ni and 59Co in galactic cosmic rays, and find that the data are consistent with complete decay of 59Ni indicating a time delay 2 10^5 yr. We present the observations and discuss their significance for models of cosmic ray origin and acceleration