gamma decay of excited states in Rn-198 identified using correlated radioactive decay

The low-lying level structure of the neutron-deficient isotope Rn-198 has b een studied for the first time, using the Er-166(Ar-36,4n) reaction at a be am 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-ele ment position-sensitive, passivated ion-implanted planar silicon detector. Prompt gamma rays in Rn-198 were observed at the target position using the JUROSPHERE array of 24 Compton-suppressed germanium detectors, and were ide ntified by the subsequent radioactive decay of associated recoiling ions in the silicon detector. Isotopic assignments of the nuclei produced were mad e on the basis of the energy and half-life of their ct decay. The estimated production cross section for Rn-198 in this reaction was 180 nb. The excit ation energy of the 2(1)(+) state indicates inconsistency with theoretical predictions of large stable ground-state deformation in this isotope. The s ystematics of other low-lying excited states suggests the presence of defor med intruder states falling in energy with increasing neutron deficiency. [ S0556-2813(99)07802-4].