RIBSS

Science Topics

The two main research thrusts of the RIBSS Center are nuclear reaction and decay studies with radioactive ion beams. These thrusts have strong bonds to instruments that are designed and built by the Center and theory including modern nuclear reaction models. Central to all the work of the Center are the people, and in particular the students and postdocs.

Our science relates to the synthesis of the elements in stellar and explosive environments, the structure of the atomic nucleus itself, and the way nuclei decay and react with each other. The information about the structure of the nucleus extracted from reaction and decay experiments can be applied to astrophysical studies as well as stewardship science. As astrophysical nucleosynthetic pathways are often defined by the competition between capture reactions and decays, having both components as active research fields in the Center allows us to investigate problems from two sides.

Nuclear Reactions

Nuclear reactions that add or remove one or two particles are powerful probes of nuclear structure. In the Center we use reactions of RIBs with deuterons, or other light ions, to transfer a neutron, or proton, onto the beam. By detecting the particles emitted and reconstructing the kinematics, we can learn about the states in the nuclei involved. Performing these types of reactions in "inverse kinematics", with beams of heavier nuclei and targets of light nuclei, creates an opportunity to study exotic short-lived nuclear species, but also present significant technical challenges. Within the Center we develop detectors whose properties are specifically matched to the challenges of these experiments and we develop reaction theory to interpret the results of experiments and to provide modern frameworks for understanding nuclear reactions.

Some of our highest profile reaction measurements have been performed on beams of reaccelerated fission fragments around the shell closure at neutron number 50, and close to the short-lived doubly magic nucleus 132Sn. These experiments relate to nuclear structure as well as heavy element production in stellar collisions or explosions. The Center also performs experiments with neutron-deficient nuclei to understand nova nucleosynthesis and the rapid proton capture (rp-) process.

Nuclear Reaction Theory

Reaction theory is crucial to disentangle the desired nuclear structure information from experiments and to connect that information to the big questions facing our field. The Center's theory effort is working on including state-of-the-art many-body theory in reaction calculations. One aspect of this is to include non-local effects in the optical potential used in the calculation, reflecting the sub-structure of the nucleus. Our goal is to generate a global non-local optical potential that works for a wide range of nuclei, over a wide range of energies and including insight from microscopic theory.

Quantifying uncertainties in nuclear reaction theory is critical to understanding the results of calculations and to making comparisons with experimental data. We have started to explore uncertainty quantification for elastic, inelastic, and transfer reactions. Using χ2 fits to elastic scattering, we have extracted optical potentials and propagated confidence bands to determine uncertainties in elastic and transfer cross-section predictions.

The reation theorists in the group work in close collaboration with the experimentalists in concieving, planning, and interpreting the results from experiments, as well as training early career experimentatlists on reaction theory methods.

Nuclear Decay

One of the most effective ways to explore short-lived nuclei is by studying their decay. Decay experiments can explore the most exotic nuclear species as every single atom will decay; whereas the probability of a reaction on a target is often very small. Understanding the decays themselves is important. At the same time, measuring decay products, including charged particles, neutrons, and gamma rays, can reveal nuclear structure that is critical to understand the fundamental properties of nuclei far from stability. These studies very often offer us the first glimpse into unexplored regions of the nuclear chart.

A particular focus in the Center's decay studies is the measurement of β-delayed neutron emission. As we move away from the stable nuclei adding more and more neutrons, the probability to emit a neutron, or sometimes multiple neutrons, grows. It is therefore essential to perform neutron spectroscopy in decay studies of neutron rich nuclei. Beta-delayed neutron emission is important in the astrophysical rapid neutron capture process which involves neutron-rich nuclei around iron up to uranium and also in understanding the decay of fission fragments.