Tantalum-180m

Main isotopes of tantalum

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct 177Ta syn 56.56 h ε 177Hf 178Ta syn 2.36 h ε 178Hf 179Ta syn 1.82 y ε 179Hf 180Ta syn 8.125 h ε 180Hf β− 180W 180mTa 0.012% stable 181Ta 99.988% stable 182Ta syn 114.43 d β− 182W 183Ta syn 5.1 d β− 183W
Standard atomic weight (Ar)	7002180947880000000♠180.94788(2)

Natural tantalum (₇₃Ta) consists of two stable isotopes: ¹⁸¹Ta (99.988%) and 180m
Ta
(0.012%).

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 (other than ^180mTa) is ^178m1Ta with a half-life of 2.36 hours.

Tantalum has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of ¹⁸¹Ta, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 182
Ta
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 180m
Ta
(m denotes a metastable state) has sufficient energy to decay in three ways: isomeric transition to the ground state of 180
Ta
, beta decay to 180
W
, and electron capture to 180
Hf
. 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 180m
Ta
is attributed to its high spin (9 units) 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.

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