Even if you managed to avoid matter, at some point, photons will start to become a problem. As you continue to accelerate, eventually, the cosmic microwave background itself will become deadly X-rays.
It's particles that make things radioactive, mostly neutrons as they aren't repelled by the nucleus and thus have a much easier time getting in. And while it would be hard to construct a shield out of it helium is effectively immune to becoming radioactive under neutron bombardment. And while lead isn't immune the reaction sequence produces nothing that won't be contained by the lead and it self-regenerates, enough bombardment returns it to where it started.