Nuclei lie at the center of every atom in the universe, provide 99.98% of its mass, and form the core of all matter. Learning how these tiny systems work teaches us about the hidden forces in nature that are only found inside nuclei, but provide almost all the energy that power the sun and stars, and which provides life and energy on earth. Nuclei play a crucial role in the history of our universe through element formation. One hundred years after the discovery of the nucleus, new experiments, new detector technologies, and new accelerators are being built to synthesize and study thousands of new totally unknown isotopes and deepen the universality of our understanding. Our group has programs in measuring and calculating the lightest nuclei from first principles, as well as in exploring the balance of forces that dictate what the heaviest nuclei might be, and how to synthesize them. We measure properties of nuclei far from stability, both on the neutron-rich side from fission-fragments as well as on the proton-rich side along the proton "drip-line" to understand stellar nucleosynthesis processes.
The understanding of nuclear structure is undergoing a period of renaissance, fueled by ab-initio approaches based on realistic nuclear forces. Currently, computational limitations restrict these calculations to the very lightest nuclei. However, insights gained from light nuclei have influence nuclear modeling across the nuclear chart. These include effects of three-body and higher correlations to describe the origin, mass and isospin dependence of the spin-orbit force, and on incorporating tensor forces that are now seen as essential to describe nuclei across the mass table. It is these “residual forces” which determine the shell-gaps across the nuclear landscape, and in turn, it is the shell bunching and gaps that dictate the observable properties of all heavier nuclei. The “new” magic numbers away from stability, the demise of traditional shell gaps, changes in pairing, and “islands of inversion” are all consequences of the isospin dependence of the residual forces. Understanding these effects is the core of the physics program for the Facility for Rare Isotope Beams (FRIB). The new approaches are tested and improved though benchmarking against precise nuclear data, specifically against data on the lightest nuclei. Precisely re-measuring some key transition rates in light nuclei, at the few percent level, is one of the goals of our proposed research. Experiments are carried out at Argonne and NSCL.
At the other end of the nuclear landscape, in very heavy nuclei, the situation is quite different. The nuclei are more classical in nature with higher level densities and liquid-drop-like properties strongly influenced by the ever-increasing Coulomb energy. However, the level densities are far from that of a Fermi gas, and gaps and bunching in the sequence of quantum states still modulate all observables. Information on the sequence and spacing of levels near the Fermi surface is sparse, especially in the domain of very heavy nuclei with Z > 100. However, this information is essential for understanding binding energies, decay rates, shapes, and pairing of the very heaviest systems. It is a convenient trick of nature that allows us to use deformation and rotation in Z ~ 100 nuclei, where production cross-sections are quite large, in order to populate some of the highest quantum states known in nuclei and then predict shell gaps in the true “Super Heavy” domain with Z > 120. Locating and identifying these states, and learning about correlations in these very heavy systems is the second thrust of our work. We have pioneered using actinide targets and multi-nucleon transfers to reach key states of interest. We have been using high-K isomer investigations and nucleon alignment techniques to learn about the deformed Nilsson states and their crossings. A key current issue is the influence of the shell gaps on pairing correlations which are very important, ill-understood, and far from constant across the region.
Decay properties of very neutron rich nuclei are of interest for nuclear structure, nuclear astrophysics, and for “applied” research in the nuclear energy domain. This research was originally motivated by the opportunities promised by the Californium Rare Isotope Breeder Unit (CARIBU) ion-source facility at Argonne National Laboratory, and in the longer term by the beams at FRIB. We have built a “Decay Beamline” at CARIBU and led the effort to design, build and operate the “X-Array” clover array system and the "SATURN" array of beta-decay and tape transport systems. We conduct experiments to study the beta and gamma decays of long-lived isomers in neutron-rich nuclei produced from the fission of the strong 252Cf source at CARIBU, and in “Decay Heat” and beta-delayed neutron studies of nuclei relevant to next-generation fuel cycles. We have also developed a novel scintillator array for detecting neutrons that is being tested for measuring beta-delayed neutrons emitted from neutron-rich nuclei in these experiments.
This research focuses on the study of neutron-deficient nuclei at the limits of nuclear stability, and the role they play in both unlocking the fundamental inner workings of the nucleus,as well as understanding nuclear processes that occur in extreme astrophysical environments. Although exotic isotopes live only for fractions of a second, their properties are critical for our understanding of processes that create the heavy elements in stars, in supernova explosions and in Type I X-ray bursts on the surface of neutron stars. Much of the work is centered on learning about the rapid proton-capture process, thought to take place during X-ray bursts. These periodic bursts can occur on timescales between 10 and 1,000 seconds and are highly energetic events. The characteristic shapes and intensities of their observed light curves are dictated by the properties of very neutron-deficient nuclei. Critical to this process are “waiting- point" nuclei, where a decrease in the proton capture rate is coupled with a long beta-decay half-life. Measuring the properties of key nuclides near the waiting points is one of the main goals of this research, primarily conducted at NSCL at present, and planned for FRIB in the future, where we will be able to produce and study exotic nuclei that are currently difficult or impossible to access, but are key to our understanding of the cosmos.