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Radioisotopes are unstable forms of matter that undergo radioactive decay through the emission of particles and gamma rays. There are over 800 known radioisotopes, and their uses often depend on their half-life, which is the time it takes for half the mass of a radioactive material to decay into another material. Medical radioisotopes such as iodine-131 and iodine-125 are used as tracers or in cancer treatment. Uranium-235 can be forced to decay faster through neutron bombardment, resulting in fission.
Radioisotopes, or radionuclides, are unstable forms of elementary matter created by man or found in nature. All undergo a spontaneous process of radioactive decay through the emission of alpha and beta particles, gamma rays and more. All elements in the periodic table with atomic numbers greater than 83 are radioisotopes. There are over 800 known radioisotopes that have been identified, with a further 275 isotopes existing overall from the 81 stable elements on the periodic table.
Isotopes are forms of an element with varying numbers of neutrons in the nucleus of the atom. As radioisotopes decay, they slowly transform into other isotopes of the same element by gaining or losing neutrons, and eventually become other elements entirely. This depends on their decay rate, which is known as their half-life. The uses of radioisotopes often depend on their half-life, which is the time it takes for half the mass of a radioactive material to decay into another material. Carbon, which is stable at 12C and 13C, is a radioisotope at 8C or 14C, with carbon-14 having the slowest decay rate with a half-life of 5,700 years. For this reason, and because it occurs naturally, 14C is used for carbon dating of human fossils and artifacts from ancient societies.
In an unstable atom the proton/neutron equilibrium is slightly different from its stable form, resulting in an imbalance in the binding energy of the nucleus. As elements get heavier, more neutrons must exist in the nucleus to balance the proton-proton repulsion forces. For example, uranium-238 is stable because it has 92 protons and 146 neutrons in a nucleus. Nuclear radioisotopes such as uranium-235 are unstable, however, at 92 protons and 143 neutrons, so they decay very slowly, with a half-life of 700 million years. Forcing uranium-235 to decay at a faster rate by neutron bombardment results in the creation of a very unstable nucleus which essentially blows itself apart and starts a chain reaction known as fission.
Medical radioisotopes such as iodine are also found outside what is known as the stability band, but, in this case, offer beneficial characteristics. Iodine-131 has four more neutrons than its stable counterpart and has a half-life of eight days. Because iodine can be safely ingested, it is used medicinally as a form of tracer or imaging agent. Iodine-125 is also used by injecting it directly into tumors in a procedure known as brachytherapy, to slowly destroy cancer cells through radiation, with a half-life of 60 days. Radioisotopes in medical applications also include iridium-192 with a half-life of 72 days and palladium-103 with a half-life of 17 days.
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