Constraints on the time delay between nucleosynthesis and cosmic-ray acceleration from observations of ⁵⁹Ni and ⁵⁹Co

Measurements of the abundances of cosmic-ray ⁵⁹Ni and ⁵⁹Co are reported from the Cosmic-Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer. These nuclides form a parent-daughter pair in a radioactive decay which can occur only by electron capture. This decay cannot occur once the nuclei are accelerated to high energies and stripped of their electrons. The CRIS data indicate that the decay Of ⁵⁹Ni to ⁵⁹Co has occurred, leading to the conclusion that a time longer than the 7.6 x 10⁴ yr half-life of ⁵⁹Ni elapsed before the particles were accelerated. Such long delays indicate the acceleration of old, stellar or interstellar material rather than fresh supernova ejecta. For cosmic-ray source material to have the composition of supernova ejecta would require that these ejecta not undergo significant mixing with normal interstellar gas before ≃10⁵ yr has elapsed.