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.
Last modified 2 July 1998,
Return to ACE homepage.