Gold (79Au) has one stable isotope, 197Au, and known radioisotopes ranging from 169Au to 210Au, with 195Au being the most stable with a half-life of 186.01 days, followed by 196Au at 6.165 days. Isotopes heavier than the stable mass number 197 generally decay by beta emission to mercury isotopes, while those lighter decay by electron capture to platinum isotopes or alpha emission to iridium isotopes; 196 decays both to platinum and to mercury. Of the meta states the most stable is 198m2Au at 2.27 days. Gold is currently the heaviest monoisotopic element (and is also mononuclidic). Bismuth formerly held that distinction until alpha decay of the 209Bi isotope was observed. All isotopes of gold are either radioactive or, in the case of 197Au, observationally stable, meaning that 197Au is predicted to be radioactive but no actual decay has been observed.
List of isotopes
Gold-198
Gold-198 (198Au) is a radioactive isotope of gold, normally made by neutron capture on natural gold (entirely gold-197). It undergoes exclusively beta decay to stable 198Hg with a half-life of 2.6946 days. The decay of 198Au (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%). 198Au 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 197Au and was ascribed a half-life of approximately 2.7 days.
Applications 198Au 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, 198Au nanoparticles are being investigated as an injectable treatment for prostate cancer. Sediment and water flow can be investigated using radioactive tracers such as 198Au. 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, 198Au 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 197Au irradiated by the intense neutron flux from an exploding thermonuclear weapon, would be transmuted into 198Au, 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, 198Au was created during the SL-1 accident in 1961 when the reactor went prompt critical, causing 197Au in a reactor operator's wedding ring to transmute into 198Au. 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.
See also Daughter products other than gold
Isotopes of mercury Isotopes of platinum Isotopes of iridium
References