This didn't work in 'How stuff works' so I'll try it here. Some time back a friend (PhD Nuclear Engineer), told me that the energies a neutron needed to form C14 from nitrogen were (and this is where my memory gets real fuzzy) around 15 MEV. Furthermore that neither nuclear explosions or the Sun produced neutrons of this energy. The implications were they were formed in supernovas and survived due to relativistic time dialation. This just seems to tickle my BS meter, but having no knowledge of nuclear physics, to speak of, I'd rather hear some confirmation of this before I go accepting it as likely or dismissing it.