Yrast states ofRn216and the extent of a region of possible static intrinsic reflection asymmetry

We report the results of a study of the yrast excitation spectrum for ${}_{}{}^{216}{}_{}{}^{}\mathrm{Rn}$ via the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ${(}^{14}$C,α2n${)}^{216}$Rn reaction. Measurements of γ-γ coincidences, γ-ray angular distributions, and γ-x-ray coincidences were made. We propose a level spectrum up to $J^{\pi }$ ${=(10\mathrm{}}^{+}$) which does not exhibit the alternating parity structure observed in several isotopes of radium and thorium. From an examination of information on ${}_{}{}^{216}{}_{}{}^{}\mathrm{Rn}$ and the neighboring nuclei, we can tentatively determine that the radon isotopes of masses 216 to 222 and the N=128 isotones ${}_{}{}^{216}{}_{}{}^{}\mathrm{Ra}$ and ${}_{}{}^{218}{}_{}{}^{}\mathrm{Th}$ define a lower mass boundary of a region beyond which static intrinsic reflection asymmetric shapes may exist in the nuclear ground states.