Context : 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 ^ { 22 } Ne/ ^ { 20 } Ne , which is \sim 5 times higher in GCR than in the Sun .
This ratio provides key information on the GCR origin .

Aims : 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 .

Methods : 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 .

Results : We find that the observed GCR ^ { 22 } Ne/ ^ { 20 } Ne 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 10 ^ { -6 } -10 ^ { -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 ^ { 22 } Ne/ ^ { 20 } Ne ratio in GCR .

Conclusions :