What is the origin of the elements in the cosmos?
What are the nuclear reactions that drive stars and stellar explosions?
How do simple patterns originate in complex nuclei?
How can the knowledge and technical progress provided by nuclear physics be used to benefit society?
To answer these questions requires an understanding of the physics of exotic nuclei – unstable nuclei, which play an important role in nucleosynthesis and hence the chemical evolution of the universe. Though the stable nuclides have been well studied for decades, the structure of unstable nuclei is only recently coming into reach.
All terrestrial matter has a cosmic origin. Following the Big Bang, the nucleosynthesis was primarily limited to isotopes of hydrogen, helium and lithium. All heavier elements were subsequently formed in the crucibles of stars, and stellar explosions such as novae, x-ray bursts, supernovae and neutron-star mergers. In these cataclysmic events, the energy generation and nucleosynthesis are driven by networks of reactions on unstable nuclei, formed by the rapid captures of protons, alpha particles or neutrons. The unstable nuclei eventually decay back to stability, affecting the chemical evolution of the universe, while leaving an elemental or isotopic ‘fingerprint‘ of their origins on the final abundances. These reaction networks are governed by the nuclear structure of these short-lived nuclei.
Understanding these nucleosynthesis processes, and the astrophysical scenarios in which they occur, is an interdisciplinary process. One crucial component is studying the structure of the hard-to-access radioactive nuclei involved, via experiments at radioactive ion beam facilities. In some cases, the reaction of astrophysical interest can be studied directly, or specific experiments may target the strengths and branching ratios of nuclear resonances and bound states important for a specific nuclear reaction. Other experiments probe the single-nucleon and collective structures of these short lived nuclei, in order to understand how nuclear structure evolves for nuclei with unstable combinations of protons and neutrons.
A specific challenge is understanding reactions of a radioactive nucleus with a neutron, which itself is unstable. Many of the nuclear processes that create the elements are driven by neutron-induced reactions. Neutron-induced reactions also drive the nuclear power plants that light our homes. Ensuring our national nuclear security requires knowledge of neutron-induced reactions on radioactive nuclei. Surrogate reactions and other indirect techniques are required to inform these neutron-induced reaction rates.
The ORRUBA silicon detector array, along with GODDESS (the coupling of ORRUBA to large germanium detector arrays, Gammasphere and GRETINA) are optimized for reaction measurements using radioactive beams on light targets, providing high resolution and efficiency for charged particle detection. GODDESS is designed to operate with GRETA at the nascent Facility for Rare Isotope Beams (FRIB), which will provide a quasi-4pi particle gamma spectromenter for RIB physics.