All you need for radiation shielding is enough mass that isn't itself dangerously radioactive that stays where you want it. If you're trying to make a mass-eff

All you need for radiation shielding is enough mass that isn't itself dangerously radioactive that stays where you want it. If you're trying to make a mass-efficient radiation shield, the choice of elements matters (lighter elements are better than heavy; polyethylene is one of the better ones because it incorporates a lot of hydrogen), but if you have a big enough asteroid, just start digging...