Radioactive Rays: In 1896, the French scientist Henry Becquerel first discovered that the nucleus of a uranium metal emits spontaneously continuous radiation with special cutting power. It has since been observed that isotope isotopes spontaneously radiate different types of rays and change their nuclei to become isotopes of other elements. This behavior or religion is called radioactivity and such isotopes are called radioactive isotopes. Notable radioactive isotopes of uranium-238 , strontium- 90 , iodine-130 , plutonium-239. Radioactivity is the nuclear phenomenon of the atom , so it cannot be controlled by any ordinary external physical process.