Nuclear Reactions to Probe Structure of Exotic Nuclei

Experimental nuclear physics helps us to understand the properties of the nuclei at the center of atoms and how the elements are synthesized in stars and their explosions. This project enhances our understanding of these fundamental questions by accelerating nuclei to high energies and measuring gamma and charged particle radiation that is emitted when they interact with other atomic nuclei. The focus is on accelerating beams of atomic nuclei that are unstable to beta decay, living for times as short as seconds. The interactions between atomic nuclei informs our understanding of the properties of atomic nuclei, especially those far from stability that are more weakly bound, as well as helping us to understand how the parents of the stable elements were synthesized. The project exploits state-of-the-art instruments and accelerator facilities in the U.S. Central to this effort is enhancing the education of students and postdoctoral scholars and preparing them for careers in education and fundamental and applied research. The nuclear physics results from this project are also of importance in astronomy, to understand the abundance of elements observed in the cosmos, and for nuclear energy and national security, to understand properties of and reactions on fragments from fission of actinides. To expand the reach of this project, the principal investigator presents workshops to undergraduate students on how to prepare for graduate studies and gives lectures to the public and schools on this research.

One of the frontier areas of nuclear physics is the study of the structure of atomic nuclei far from the valley of stability. In atomic nuclei the single-particle orbitals are expected to change as a function of neutron and proton number, and in addition are very sensitive to the presence of deformation. Single-particle characteristics can be probed in single-particle transfer reactions. Light-ion transfer reactions will be studied with beam energies near the Coulomb barrier and about 35-MeV per nucleon. Studies will concentrate on neutron-rich nuclei near the N=50 neutron shell closure, accelerated fragments following fission of 252Cf and light nuclei important for understanding the synthesis of nuclei in stars and their explosions. These studies will be carried out with accelerated beams of rare isotopes at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University and the Argonne Tandem Linac Accelerator System (ATLAS) at Argonne National Laboratory. The NSCL focus is to extract spectroscopic strengths with reduced dependence on theoretical model parameters. At ATLAS the group will exploit Gammasphere-ORRUBA: Dual Detectors for Nuclear Structure Studies to probe the fragmentation of single-particle strength with coincidence measurements between charged particles (measured with ORRUBA, the Oak Ridge Rutgers University Barrel Array of position-sensitive silicon strip detectors) and gamma rays (measured with Gammasphere, the array of 110 Compton-suppressed Ge gamma-ray detectors). The group will also further develop capabilities to measure particle-gamma-ray coincidences with scintillator detectors coupled to ORRUBA.