5. Summary and conclusions
We have presented evidence from the Voyager and ISEE spacecraft data that the decay of certain K-capture isotopes has occurred in interstellar space. This is obtained from a study of the relative abundances of cosmic ray Vanadium isotopes, 49V decaying into 49Ti and 51Cr decaying to V51. These measurements show that 25% of these isotopes have decayed. The interpretation of this decay in terms of interstellar energy changes during propagation is not completely straight-forward but if it is assumed that interplanetary energy loss is correctly described in the current models for solar modulation, then this data would imply an interstellar energy difference between attachment of the K-electron during propagation and the present time of up to 100 MeV/nuc. This value is in the nature of an upper limit. To expand this study further this decay needs to be confirmed for the other K-capture isotopes and needs to be carried out at different solar modulation levels and different energies - but this technique holds considerable promise for providing answers to two fundamental questions in the study of cosmic rays. 1) How much post-acceleration or re-acceleration actually occurs after the initial cosmic ray acceleration in astrophysical sources? and 2) Does the interplanetary energy loss predicted in the present solar modulation theories actually occur, thus transforming the interstellar spectrum?
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998