The primary $E1\gamma$-ray transition strength from the thermal neutron capture of ${}^{18}\mathrm{O}$ to the $3/2^{-}$ state, 18 keV below the neutron threshold of ${}^{19}\mathrm{O}$, was found to be about $2\times {10}^5$ stronger than that to the $1/2^{-}$ excited state, 729 keV below the threshold. In addition, the $E1$ strength from the $3/2^{-}$ state leading to the $3/2^{+}$ state with three ($d_{5/2}){}^3$ neutrons was more than six times stronger compared to that to the $5/2^{+}$ ground state with a single ($d_{5/2}$) neutron. These anomalous γ-ray decay patterns give the first clear experimental evidence for the predicted property of the $3/2^{-}$ state as mainly having a one-hole-four-particle configuration. Unique features of the ${}^{18}\mathrm{O}$ ground state with a two-hole-four-particle configuration and of the $3/2^{+}$ state combined with a discrete prompt γ-ray spectroscopy following the thermal neutron capture of ${}^{18}\mathrm{O}$ enabled us to exclusively identify the configuration of the $3/2^{-}$ state.

• Received 23 July 2007

DOI:https://doi.org/10.1103/PhysRevC.76.051301