Neutron scatter is the main thing producing the dark spots, though there will certainly be some neutron capture too. In some materials, e.g. boron, neutron capture can be significant.

Still, it's always important to compare the induced radioactivity with the background radioactivity as a comment above did. It's easy to forget that we live in a field of natural low-dose ionizing radiation from cosmic rays and from the bowels of the Earth.