The tracer principle states that radioactive isotopes have the same chemical properties as nonradioactive isotopes of the same element. Isotopes of the same element differ only in the number of neutrons in their atoms, which leads to nuclei with different stabilities. Unstable nuclei gain stability by radioactive decay which leads to different types of radioactivity. One type is gamma radiation which is useful in medicine because it penetrates the body without causing damage and can then be detected easily.
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