Sorry I can't distinguish when is a radioactivity question becomes a chemistry question :(
Is there any borderline to distinguish them from a physics question?
The good thing about the stackexchange network is, that if a question is well asked, but not in the right place, It can be migrated to a more appropriate site.
If your question has a link to chemistry it will most likely be on topic. If the community has a few experts on the topic you will find an answer. If no-one can give an answer it is wise to think about migration. It is really more of a case by case situation.
If in doubt, just go ahead and ask.
What is the application of the question you have in mind?
If you are thinking, "how would nuclear explosion effect the surface of the moon," probably go physics.
If you are thinking, "does radioactive carbon 13 have different bond characteristics compared to carbon 12," them chemistry might be your best bet.