γdecay of excited states in198Rnidentified using correlated radioactive decay

The low-lying level structure of the neutron-deficient isotope ${}^{198}\mathrm{Rn}$ has been studied for the first time, using the ${}^{166}\mathrm{Er}{(}^{36}\mathrm{Ar},4n)$ reaction at a beam energy of 175 MeV. Evaporation residues were selected using an in-flight gas-filled separator, RITU, and implanted at the focal plane into a 16-element position-sensitive, passivated ion-implanted planar silicon detector. Prompt $\ensuremath{\gamma}$ rays in ${}^{198}\mathrm{Rn}$ were observed at the target position using the JUROSPHERE array of 24 Compton-suppressed germanium detectors, and were identified by the subsequent radioactive decay of associated recoiling ions in the silicon detector. Isotopic assignments of the nuclei produced were made on the basis of the energy and half-life of their $\ensuremath{\alpha}$ decay. The estimated production cross section for ${}^{198}\mathrm{Rn}$ in this reaction was 180 nb. The excitation energy of the ${2}_{1}^{+}$ state indicates inconsistency with theoretical predictions of large stable ground-state deformation in this isotope. The systematics of other low-lying excited states suggests the presence of deformed intruder states falling in energy with increasing neutron deficiency.