Structure of superheavy hydrogen H-7

The properties of nuclei with extreme neutron-to-proton ratios reveal the limitations of state-of-the-art nuclear models and are key to understand nuclear forces. H-7, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron-to-proton ratio ever known, but its sheer existence and properties are still a challenge for experimental efforts and theoretical models. We report here the first measurement of the basic characteristics and structure of the ground state of H-7; they depict a system with a triton core surrounded by an extended four-neutron halo, built by neutron pairing, that decays through a unique four-neutron emission with a relatively long half-life. These properties are a prime example of new phenomena occurring in almost pure-neutron nuclear matter, beyond the binding limits of the nuclear landscape, that are yet to be described within our current models.