Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: You may have seen this sign beforemaybe in a hospital. The sign means there is danger of radiation in the area. Radiation consists of particles and energy that are given off by radioactive isotopes, which have unstable nuclei. But you don't have to go to a hospital to be exposed to radiation. There is radiation in the world all around you. Answer: nuclei
Radioactive isotopes are characterized by an unstable what?