Synthesis and Nuclear Structure of Heaviest Nuclei
Uranium is the heaviest element that exists on earth in substantial quantities. Whereas natural trace quantities of neptunium and plutonium have been found as well, all heavier elements are only available via artificial production in nuclear reactions. Neutron-capture reactions leading to neutron-rich isotopes undergoing β--decay to the next heavier element allow forming elements up to fermium in this nuclear-reactor driven pathway, which allows producing macroscopic quantities. Yet heavier elements are produced in the fusion of two lighter nuclei, in heavy-ion induced reactions using projectile beams accelerated to about 10% of the speed of light. With increasing atomic number, the lifetimes of the artificial nuclei tend to decrease to the order of hours, minutes, seconds or even less the heavier the nuclei are. Soon after the nuclear shell model was conceived, the existence of vastly more long-lived nuclei in a region of superheavy nuclei, far removed from the uranium region that contains the heaviest very long-lived nuclei, was theorized. This region form an Island of Stability of Superheavy Nuclei in the midst of very unstable nuclei; its center is located around the position of the closure of nuclear shells and commonly predicted to be in the region of elements 114-126, at a neutron number of around 184. The race for the production of ever heavier elements and the hunt for the “end of the periodic table” was and still is a main driving force for superheavy element research. Nowadays, the concept of the island of stability has been confirmed, e.g., by observing increasing nuclear lifetimes when the neutron number along an isotopic sequence increases, approaching 184. However, the location of the center of the island as well as its extension are still unclear. The search for new elements requires accelerator runs lasting many months or even years because the production rate of the heaviest isotopes are so small.
One current research focus is on the study of the optimum production pathway, e.g., for the yet unknown element 120.
On the other hand, only few properties are known for the heaviest known nuclei, where often only a handful of events have been registered. Initial information includes the decay mode – typically alpha decay or spontaneous fission – and nuclear lifetime. For alpha emitters, the alpha particle energy is known as well. Our studies aim at extending knowledge of the nuclear structure by registering not only alpha particles and fission fragments, but also electrons and photons. This allows us to pinpoint the atomic number of new nuclei and hence perform a safe Z-identification, as well as to obtain detailed structure information, which relates to the role of the shell effects, which are at the heart of the nuclei on the island of stability.
Experiments are performed by different collaborations, some led by our own group and other led by the Lund University, Sweden, see. e.g. our recent results on flerovium decay chains.