First Exploration of Neutron Shell Structure Below Lead and Beyond $\boldsymbol{N=126}

The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical r process in producing nuclei heavier than A∼190. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region, neutron excitations in Hg207 have been probed using the neutron-adding (d,p) reaction in inverse kinematics. The radioactive beam of Hg206 was delivered to the new ISOLDE Solenoidal Spectrometer at an energy above the Coulomb barrier. The spectroscopy of Hg207 marks a first step in improving our understanding of the relevant structural properties of nuclei involved in a key part of the path of the r process.