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Created page with "== Introduction == Radioactivity, also known as radioactive decay, is a process by which an unstable atomic nucleus loses energy by radiation. This process is a fundamental concept in nuclear physics and is the method by which certain isotopes undergo decay, leading to the emission of ionizing radiation. thumb|center|A close-up image of a Geiger counter measuring radioactivity.|class=only_on_mobile Image:Detail-44560.jp..."