I've written a SF story where a twenty million ton metallic asteroid is moved by antimatter propulsion. Apparently antimatter engines emit gamma rays. At the moment I'm planning to stick to pure antiproton annihilation 'cause it's easier to explain (so not a hybrid engine). For plot purposes (and because I've already written this in) I'm hoping a huge sustained gamma emission (i.e. annihilating the ludicrous amount of antimatter needed to propel such a large mass) might result in some residual contamination. I'm hoping the engine blast cones will become (at least slightly) radioactive. Could this happen?