Natural tantalum consists of two stable isotopes: 181Ta and . There are also 35 known artificial radioisotopes, the longest-lived of which are 179Ta with a half-life of 1.82 years, 182Ta with a half-life of 114.43 days, 183Ta with a half-life of 5.1 days, and 177Ta with a half-life of 56.56 hours. All other isotopes have half-lives under a day, most under an hour. There are also numerous isomers, the most stable of which is 178m1Ta with a half-life of 2.36 hours. Tantalum has been proposed as a "salting" material for nuclear weapons. A jacket of 181Ta, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope with a half-life of 114.43 days and produce approximately 1.12 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several months. Such a weapon is not known to have ever been built, tested, or used.
The nuclide has sufficient energy to decay in three ways: isomeric transition to the ground state of, beta decay to Tungsten|, and electron capture to Hafnium|. However, no radioactivity from any decay mode of this nuclear isomer has ever been observed. Only a lower limit on its half-life of over 1015 years has been set, by observation. The very slow decay of is attributed to its high spin and the low spin of lower-lying states. Gamma or beta decay would require many units of angular momentum to be removed in a single step, so that the process would be very slow. The very unusual nature of 180mTa is that the ground state of this isotope is less stable than the isomer. This phenomenon is exhibited in bismuth-210m and americium-242m, among other nuclides. has a half-life of only 8 hours. is the only naturally occurring nuclear isomer. It is also the rarest primordial nuclide in the Universe observed for any element that has any stable isotopes. In an s-process stellar environment with a thermal energy kT = 26 keV, the nuclear isomers are expected to be fully thermalized, meaning that 180Ta rapidly transitions between spin states and its overall half-life is predicted to be 11 hours. As of October 3, 2016 the half life of 180mTa is calculated from experimental observation to be least years.
