The first superdeformed states to be observed were the fission isomers, low-spin states of elements in the actinide series. The strong force decays much faster than the Coulomb force, which becomes stronger when nucleons are greater than 2.5 femtometers apart. For this reason, these elements undergo spontaneous fission. In the late 1980s, high-spin superdeformed rotational bands were observed in other regions of the periodic table. Specific elements include ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, and mercury.