Nuclear Reaction Studies for Nuclear Structure and Nucleosynthesis

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. This project will serve to advance our understanding of the structure of and reactions on atomic nuclei away from the valley of stability and their synthesis in stars and their explosions. It will exploit state-of-the-art instruments and accelerator facilities in the U.S. Central to this effort is enhancing the education of students and preparing them for careers in education and fundamental and applied research. The anticipated nuclear physics results 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 fission fragments.

The structure of the proposed activities is designed to have a positive impact on the education and training of graduate students. The project will also serve to enhance the diversity of the nuclear science workforce by seeking out early career scientists who are women or come from other under-represented backgrounds. The participation of these early career scholars in the forefront research and the development of arrays of instruments would prepare them for careers in higher education and fundamental and applied research, at national laboratories and in industry.

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 and are important in calculating the direct component in neutron capture reactions that are responsible for the synthesis of elements heavier than iron. Light-ion transfer reactions will be studied with radioactive ion beams with energies near the Coulomb barrier and about 40-MeV per nucleon. Studies will concentrate on neutron-rich nuclei near the N=50 neutron shell closure, accelerated fragments following spontaneous 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 radioactive ion beams at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University, the ATLAS accelerator facility at Argonne National Laboratory and the Nuclear Science Laboratory at the University of Notre Dame. The focus at NSCL is to constrain the shape of the potential that binds neutrons in a neutron-rich nucleus and, therefore, extract spectroscopic strengths with reduced dependence on theoretical model parameters. Both charged particles and coincident gamma radiation will be measured with flexible configurations of detectors. At ATLAS, Gammasphere-ORRUBA: Dual Detectors for Experimental Structure Studies will be exploited 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 large, highly segmented germanium gamma-ray detector arrays). The Rutgers-led efforts will be complemented by efforts led by other members of the ORRUBA collaboration. The results will be compared with theoretical predictions of nuclear structure away from stability and disseminated as input into calculations of nucleosynthesis in stars and their explosions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.