A little bit of everything, actually. Cosmic rays, neutrons, protons, gamma rays, electrons. Granted, the particles are mostly stopped by the hull, but the gammas and cosmic rays come zipping right through the exterior of the spacecraft. You'd need a few feet of lead or concrete to give real gamma protection.
Space travel is far from routine...
I don't have the ref. chart in front of me. Aren't gamma rays about 10^15 Hertz?