  The composition of Galactic Cosmic Rays (GCR) presents strong similarities to
the standard (cosmic) composition, but also noticeable differences, the most
important being the high isotopic ratio of Ne22/Ne20 which is about 5 times
higher in GCR than in the Sun. This ratio provides key information on the GCR
origin. We investigate the idea that GCR are accelerated by the forward shocks
of supernova explosions, as they run through the presupernova winds of the
massive stars and through the interstellar medium. We use detailed wind and
core yields of rotating and non-rotating models of massive stars with mass
loss, as well as simple models for the properties of the forward shock and of
the circumstellar medium. We find that the observed GCR Ne22/Ne20 ratio can be
explained if GCR are accelerated only during the early Sedov phase, for shock
velocities >1500-1900 km/s. The acceleration efficiency is found to be of the
order of 1.e-6 - 1.e-5, i.e. a few particles out of a million encountered by
the shock escape the SN at GCR energies. We also show quantitatively that the
widely publicized idea that GCR are accelerated in superbubbles fails to
account for the high Ne22/Ne20 ratio in GCR
