Half-lives have been determined in laboratories for many radioisotopes (or radionuclides). Some radioactive fission products can be converted into shorter-lived radioisotopes by transmutation. Radium has been replaced by much cheaper cobalt-60 and other radioisotopes in radiation treatment. Longer-life radioisotopes, typically caesium-137 and strontium-90, present a long-term hazard. Today the plant makes tritium and radioisotopes, not plutonium. The other radioisotopes of iodine are never used in brachytherapy. In addition, they bombarded aluminium and magnesium with neutrons to make new radioisotopes. Radon decays to form solid radioisotopes that form coatings on the inside of pipework. The radon decays to form solid radioisotopes which form coatings on the inside of pipework. The bomb casing can be a significant sources of neutron-activated radioisotopes. Rn decays to Pb and other radioisotopes. A number of different gamma-emitting radioisotopes are used. The other iodine radioisotopes have much shorter half-lives, no longer than days. The other main method used to synthesize radioisotopes is proton bombardment. Numerous synthetic radioisotopes of naturally occurring elements have also been produced in laboratories.