Gold-198 (<sup>198</sup>Au) is a radioactive isotope of gold, normally made by neutron capture on natural gold (entirely gold-197). It undergoes exclusively beta decay to stable <sup>198</sup>Hg with a half-life of 2.6946 days.
The decay properties of <sup>198</sup>Au have led to interest in its potential use in radiotherapy for cancer treatments. This isotope has also found use in nuclear weapons research and as a radioactive tracer in hydrological research.
<sup>198</sup>Au was possibly observed for the first time in 1935 by Enrico Fermi et al., though it was not correctly identified at the time. This isotope was conclusively identified in 1937 following neutron irradiation of stable <sup>197</sup>Au and was ascribed a half-life of approximately 2.7 days.
The decay of <sup>198</sup>Au (shown) is relatively simple, involving only three levels of the daughter product nucleus. 99% of decaying atoms follow the path to the middle level, and thus emit a beta particle with maximum energy 961 keV (the rest goes to the neutrino) and a single gamma ray of energy 412 keV (converted about 4%).
<sup>198</sup>Au is used for radiotherapy in some cancer treatments. Its properties may be favorable for use in medicine because the 4 mm penetration range of its beta particles in tissue allows it to destroy tumors without nearby non-cancerous tissue being affected by radiation, and its half-life is short but not so short as to create problems in handling or delivery. For this reason, <sup>198</sup>Au nanoparticles are being investigated as an injectable treatment for prostate cancer.
Sediment and water flow can be investigated using radioactive tracers such as <sup>198</sup>Au. This has been used extensively since artificial radioisotopes became available in the 1950s, as a supplement to millennia of investigations using other tracing techniques.
Inside coker units at oil refineries, <sup>198</sup>Au 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.
Gold has been proposed as a material for creating a salted nuclear weapon (cobalt is another, better-known salting material). A jacket of natural (the only stable gold isotope), irradiated by the intense neutron flux from an exploding thermonuclear weapon, would be transmuted into <sup>198</sup>Au, whose gamma emission would significantly increase the radioactive hazard of the weapon's fallout for days. Such a weapon is not known to have ever been built, tested, or used. However, was created during the SL-1 accident in 1961 when the reactor went prompt critical, causing in a reactor operator's wedding ring to transmute into .
The highest amount of <sup>198</sup>Au detected in any United States nuclear test was in shot "Sedan" detonated at Nevada Test Site on July 6, 1962.