Our group uses a variety of physical, analytical, and nuclear techniques to study the chemical and physical properties of the heaviest elements. These fall into three broad categories: the chemistry of heavy elements, nuclear forensics, and the study of nuclear reactions.
Chemistry of Heavy Elements
Our medium-term goal is to start performing “online” chemistry experiments, where radioactive material is produced in a nuclear reaction and used within seconds of production. We have designed and characterized a cutting-edge “gas stopper” that will be used to “thermalize,” or slow down, heavy atoms produced in nuclear reactions to the energy necessary for chemical experiments. This gas stopper will be used to deliver these products to a new chemistry laboratory where we will measure their chromatographic properties, and is described in a recent paper. A schematic of the gas stopper is shown below. These experiments will provide information on the complexation behavior of these elements, and will help determine whether trends in the periodic table are maintained for the heaviest elements. (There are reasons to believe that the periodicity of the elements may not hold for extremely high atomic numbers).
Any online chemistry experiment is preceded by “offline” experiments, where ultra-trace (pipcomolar to femtomolar) concentrations of radioactive elements are studied in a traditional chemistry laboratory. These extremely low concentrations are needed to mimic the “atom-at-a-time” nature of transactinide experiments, where there is never more than one atom present at a time. We have studied the interaction of the nihonium (atomic number 113) homologs indium and thallium with ionic liquids and other “designer” molecules. We have developed mechanisms for the extraction, and we are using this to design systems that will potentially interact with superheavy elements in the way that we want.
In collaboration with Prof. Sunil Chirayath in the department of Nuclear Engineering, we are using nuclear forensics to determine the origin and history of nuclear samples. Prof. Chirayath and his group have used state-of-the-art models to simulate the irradiation of uranium pellets in a reactor, and then arranged to perform the actual irradiation. The radioactive samples have been transported to Texas A&M University, where our group is now conducting destructive and non-destructive radiochemical analyses of it for comparison with the model calculations. Additionally, we are working to identify specific signatures which could allow other forensic information to be determined.
Nuclear Reaction Studies
We study nuclear reactions to provide insight into the mechanism of nuclear fusion reactions. Currently, experiments worldwide are trying to discover the next new element, which will likely be element 120. Previous experiments relied on using the very neutron-rich isotope 48Ca as a projectile, but element 120 will require a heavier projectile because of a lack of appropriate targets. This product will be near the predicted closed shells at Z = 120 and N = 184. As an analog for these very challenging experiments, we are studying the production of nuclides near the Z = 82 and N = 126 shells, which have must higher production cross sections. We have measured excitation functions for the reactions of 48Ca, 45Sc, 50Ti, and 54Cr projectiles with lanthanide targets. In addition, we have a developed a simple theoretical model that describes the experimental data well. The results suggest that the production of new elements with projectiles heavier than 48Ca will be heavily suppressed because of two effects: a significant decrease in the probability of forming the compound nucleus, and a significant decrease in the probability that the compound nucleus will survive against fission. This work is helping to guide the discovery of new elements worldwide.