Dear Michelaf,
Welcome to the Geant4 forum
Alpha capture may be possible in silicon. Particularly, Si29(alpha,n) reaction has a threshold of 1.7MeV, and Si30(alpha,n) of 3.9 MeV. These neutrons may induce subsequent reactions that lead to higher total energy than initial energy of the alpha.
However, there was a post few months ago pointing to the possibility that Geant4 did not include the Si29(a,n) reaction.
To investigate it further, one may save the events where total energy is higher than expected, or check the production of secondary neutrons via alpha-n reaction in Si29/30.
I am not expert on the subject, so I would be glad to learn from others thanks to this question 
Best,
Alvaro