Gold (Au) has one stable isotope, 197Au, and 18 radioisotopes, with 195Au being the most stable with a half-life of 186 days.
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Gold is currently considered the heaviest monoisotopic element (bismuth formerly held that distinction, but bismuth-209 has been found to be slightly radioactive).
Relative atomic mass: 196.966569(5).
Radioactive particle tracking
Inside coker units at oil refineries, gold-198 is used to study the hydrodynamic behavior of solids in fluidized beds and can also be used to quantify the degree of fouling of bed internals.
Nuclear medicine
Gold-198 is a beta emitter with range in tissue of about 11 mm and half life 2.7 days. It is used in some cancer treatments and for treating other diseases. Gold-198 nanoparticles are being investigated as an injectable treatment for prostate cancer.
Nuclear weapons
Gold has been proposed as a material for creating a salted nuclear weapon (cobalt is another, better-known salting material). A jacket of natural 197Au, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 198Au with a half-life of 2.697 days and produce approximately .411 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used. Gold has been used in thermonuclear weapons as radiation mirrors within the secondary assembly. Ivy Mike used a thin layer of gold on the secondary casing walls to enhance the blackbody effect, trapping more energy in the foam to enhance the implosion.
The highest amount of 198Au detected in any United States nuclear test was in shot "Sedan" detonated at Nevada Test Site on July 6, 1962.