In nature, there are some nuclides nuclei are not stable enough to spontaneously decay, from a nuclide nuclei decay into another nuclide nuclei, this change occurs at the same time to emit rays, this phenomenon is called radioactive decay, these nuclides are called radionuclides.
There are many radionuclides. Can be divided into two categories, one is the natural existence of spontaneous decay of nuclides, known as natural radionuclides. They are mainly heavy nuclides with atomic numbers greater than 82 and some nuclides with atomic numbers not too large. For example, potassium ( ), rubidium ( ), samarium ( ) and so on. Another kind of radioactive isotope is obtained by artificial method (nuclear reaction), which is called artificial radioactive isotope. For example, plutonium ( ) used in the manufacture of atomic bombs, americium ( ) commonly used as a source of excitation for radioactive instruments, cobalt ( ) used in medical treatment, etc. are all artificial radionuclides.
Natural radionuclides emit rays when they decay, which can be categorized into alpha rays (alpha particles), beta rays (electrons), and gamma rays (gamma photons) according to their nature.