The six superheavy elements with atomic numbers 107-112 were discovered at GSI. They were produced in nuclear fusion reactions using shell-stabilized target nuclei at and around doubly-magic 208Pb. These reaction partners allow compound nuclei containing all nucleons of the projectile and target nuclei to be produced at low excitation energy. This favors their survival on the evolution to a superheavy nucleus, containing all protons of the two reaction partners. The low energy led to these reactions being termed “cold fusion reactions”. With increasing number of protons in the compound nuclei – i.e., towards ever heavier elements – the rate at which such nuclei can be produced in cold fusion reactions has been observed to decrease rapidly. Only one element beyond 112 (named copernicium) has been identified in a cold fusion reaction: element 113 (nihonium), the discovery of which at RIKEN was accepted by IUPAC in 2015. Along with nihonium, also three further even heavier elements, including elements 115 (moscovium) and 117 (tennessine) that were also studied at the TASCA separator at GSI, were accepted, thus completing the seventh row of the periodic table. Beyond nihonium, all elements were produced in reactions using doubly-magic 48Ca projectiles, reiterating the advantage of using such shell-stabilized reaction partners.
While it had been recognized decades ago that production rates in cold fusion reactions decrease steeply the heavier the fusion product is, the dynamical mechanisms underlying this trend remained only partially understood. To improve our understanding, detailed studies of three cold fusion reactions using 48Ca, 50Ti, and 54Cr projectiles on 208Pb targets, leading to the elements 102 (nobelium), 104 (rutherfordium), and 106 (seaborgium), have been performed at the Heavy Ion Accelerator Facility at the Australian National University (ANU) in Canberra, Australia, in a collaboration with scientists from GSI Darmstadt, HIM Mainz, and Johannes Gutenberg University Mainz. In contrast to the experiments at GSI, which focus on the observation of the fusion products, the present experiments aimed at studying the complementary reaction outcome – i.e., the fission-like process when projectile and target nuclei do not fuse, but reseparate a short time after coming into close contact, thus suppressing fusion. The process leads to the correlated emission of two nuclei at a wide range of angles. Studying the details of this process is key to obtaining an improved understanding of the rare cases where fusion occurs, leading to superheavy elements. For the studies to be most efficient and to ensure registration of as many reaction outcomes as possible, the unique large-area CUBE detection system at ANU, operated by the nuclear reaction dynamics group, was used, which offers superior acceptance for such events. The system registers the angle at which the events are recorded, the ratio of the masses of the two fragments, and their total kinetic energy. This allows reconstructing reconstructing the reaction outcome at the moment when the fragments separate. Current theoretical descriptions of the cold fusion reactions assume that the two fusing nuclei follow a thermal diffusion-like process to evolve into a compound nucleus, which is the basis for superheavy element production. If true, the fusion suppression would be highest at the lowest excitation energies, as then, the least amount of energy is available for this diffusion process. In contrast, the measured data suggest the suppression in this step to become smaller with decreasing energy, indicating that cold fusion is not driven by a thermal diffusion process. The results call for microscopic approaches to describe this reaction step, hence stimulating further theoretical work. The evolution of the fusion mechanism when going from 48Ca towards heavier projectiles is also relevant for the production of new elements beyond element 118 (oganesson): whereas this element could be produced using a 48Ca beam, this will no longer be possible for heavier elements, due to a lack of target materials with a sufficient number of protons to reach elements 119, 120 or beyond.
The work was published in Physics Review Letters and is the latest example of the successful collaboration between the SHE chemistry department at GSI and HIM and the Institute of Nuclear Chemistry at JGU with the Nuclear Reaction Dynamics Group at the ANU, which started in 2013 and has led to numerous joint publications.