Nuclear reactions in massive stars that explode as supernovae produce a wide range of heavy nuclei, including both stable and radioactive species. A recent study from ACE using one of the radioactive isotopes is providing new information on the time that elapses before some of these particles are accelerated to relativistic energies to become cosmic rays.

In the laboratory, the isotope ⁵⁹Ni decays by capturing an orbital electron with a halflife of 7.5 x 10⁴ years. However, at cosmic ray energies the atomic electrons are rapidly stripped away by collisions with the interstellar gas leaving a bare nucleus which is no longer able to decay. Thus, if the ⁵⁹Ni in a supernova's ejecta is accelerated to high energy in a short time (compared with the halflife) after it was synthesized, it should be present in the cosmic rays observed at Earth millions of years later. If the time before acceleration is much longer, however, the ⁵⁹Ni will have decayed and one should observe it as ⁵⁹Co, the daughter product of this decay.

The curves in the figure show the results of calculations of how the abundances of ⁵⁹Ni (upper panel) and ⁵⁹Co (lower panel) are expected to depend on the time that elapses between the supernova explosion and the acceleration of cosmic rays. In addition to the nuclei made in the explosion, there are small additional contributions (dashed lines) produced by the breakup of heavier cosmic rays when they collide with atoms of the interstellar gas. The different curves indicate the distribution of the synthesized mass 59 nuclei between ⁵⁹Ni and ⁵⁹Co.

The Cosmic Ray Isotope Spectrometer (CRIS) on ACE has measured the relative abundances of the isotopes of cobalt and nickel. The hatched bands show the ranges of ⁵⁹Ni and ⁵⁹Co abundances (relative to the stable isotope ⁶⁰Ni) that are consistent with the data. The measurements agree with the calculated curves for elapsed times of approximately 10⁵ years or longer, corresponding to nearly-complete decay of the supernova-synthesized ⁵⁹Ni to ⁵⁹Co before cosmic ray acceleration.

An alternative explanation is that in cosmic rays the fraction of mass 59 material which was synthesized as ⁵⁹Ni was no more than ~25%. This is also an interesting possibility, since it would indicate that the population of supernovae which is the source of cosmic rays may differ from that which contributes to the general interstellar gas.

Contributed by Mark Wiedenbeck Jet Propulsion Laboratory

See The CRIS Home Page at Caltech for more information on this ACE instrument.

ACE News Archives

Last modified 2 July 1998, Andrew Davis
Return to ACE homepage.
/ACE/