The isotopes ${}_{}{}^{206}{}_{}{}^{}\mathrm{Fr}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Fr}$ were produced by the reactions $\mathrm{Ir}({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne},xn){}_{}{}^{206,208}{}_{}{}^{}\mathrm{Fr}$ and mass separated on-line. The electron-capture and positron decays to ${}_{}{}^{206}{}_{}{}^{}\mathrm{Rn}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Rn}$ were studied by collecting $\gamma$ ray and internal conversion electron singles spectra as a function of decay time as well as $\gamma -\gamma$, $\gamma -e^{-}$, and $\gamma -\mathrm{x}$ ray coincidence spectra. The energies and many of the spins were determined for 18 excited, even parity states in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Rn}$ and for 10 excited, even parity states in ${}_{}{}^{206}{}_{}{}^{}\mathrm{Rn}$. These nuclei appear to be excellent candidates for interpretation in terms of a weak coupling shell model. The energy levels were also compared to the predictions of the interacting boson approximation model.

RADIOACTIVITY ${}_{}{}^{206,208}{}_{}{}^{}\mathrm{Fr}$ from mass separated products of ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ir}({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne},xn){}_{}{}^{206,208}{}_{}{}^{}\mathrm{Fr}$. Measured $E_{\gamma }$, ICC, $\gamma -\gamma -t$, $e^{-}\gamma -t$. Deduced levels $E$, $J$, $\pi$ in ${}_{}{}^{206,208}{}_{}{}^{}\mathrm{Rn}$.

• Received 15 September 1980

DOI:https://doi.org/10.1103/PhysRevC.23.1717