An alternative to magic numbers

In addition to "magic numbers", there are nuclear isomers that are exceptionally stable. The June 2005 edition of Physics Today (sorry, subscribers only) discussed several elements that have metastable nuclear states. Metastable states by themselves are unusual because they are excited states that take a long time to decay to the ground state, and hence the term meta-stable. They take a long time because a direct transition from the the metastable state to the ground state is often forbidden by quantum mechanical selection rulesrule, so the system has to make a transition to a state that does allow the transition to the ground state. An interesting example is the curious case of ^180mTantalum. For Tantalum, the ground state itself is unstable, with a half-life of only 8 hours. The metastable state has a half-life of approx. 10¹⁵ years, which is only a lower limit since no actual decays have been observed. Specifically, the metastable state has a spin-parity of 9^-, while the ground state has a spin-parity of 1⁺, so the transition is forbidden. However, it could transition from 9^- to 2⁺ and then to 1⁺, but the transition requires a multipoleexpansion transition with E7 or M8 character, which have never been seen. (E1, E2, E3, … dipole, quadrupole, octopole, … E refers to electric, M to magnetic.)