Decay through a doorway state and the puzzle of ${}^{180}\mathrm{Ta}$

The decay of a very long-lived state, such as the famous $J=9^{-}$ isomeric state in ${}^{180}\mathrm{Ta}$, when irradiated by $\gamma$ rays or subjected to Coulomb excitation, can be accelerated by the existence of a doorway state for mixing an excited state of the isomeric band with the states of other bands decaying to the ground state. We describe the mechanism of such a decay similar to the chaos-assisted tunneling.