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Given a scientific passage and an answer, generate a question for the given answer.

Passage: To balance a nuclear reaction. The two general kinds of nuclear reactions are nuclear decay reactions and nuclear transmutation reactions. In a nuclear decay reaction, also called radioactive decay, an unstable nucleus emits radiation and is transformed into the nucleus of one or more other elements. The resulting daughter nuclei have a lower mass and are lower in energy (more stable) than the parent nucleus that decayed. In contrast, in anuclear transmutation reaction, a nucleus reacts with a subatomic particle or another nucleus to form a product nucleus that is more massive than the. Answer: radioactive decay
Output:
When an unstable nucleus emits radiation and is transformed into the nucleus of other elements this is called?