The nuclide (
m denotes a
metastable state) is one of a very few
nuclear isomers which are more stable than their ground states. Although it is not unique in this regard (this property is shared by
bismuth-210m (210mBi) and
americium-242m (242mAm), among other nuclides), it is exceptional in that it is
observationally stable: no decay has ever been observed. In contrast, the ground state nuclide has a half-life of only 8 hours. has sufficient energy to decay in three ways:
isomeric transition to the
ground state of ,
beta decay to Tungsten|, or
electron capture to Hafnium|. However, no radioactivity from any of these theoretically possible decay modes has ever been observed. As of 2023, the half-life of 180mTa is calculated from experimental observation to be at least (290 quadrillion) years. The very slow decay of 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. Similar suppression of gamma or beta decay occurs for
210mBi, a rather short-lived alpha emitter. Because of this stability, is a
primordial nuclide, the only naturally occurring
nuclear isomer (excluding short-lived radiogenic and cosmogenic nuclides). It presents one of two apparent violations of the
Mattauch isobar rule, the other involving
tellurium-123. It is also the rarest primordial nuclide in the Universe observed for any element which has any stable isotopes. In an
s-process stellar environment with a thermal energy
kBT = (i.e. a temperature of 300 million kelvin), the nuclear isomers are expected to be fully thermalized, meaning that 180Ta is equilibrated between spin states and its overall half-life is predicted to be 11 hours. It is one of only
five stable nuclides to have both an odd number of protons and an odd number of neutrons, the other four stable
odd-odd nuclides being
2H,
6Li,
10B and
14N. == See also ==