Beta-Decay Experiments along the R-Process Path

Most of the chemical elements in the universe have been produced by nuclear reactions in stars through a series of different nucleosynthesis processes. The basic features of most of these processes are well understood. However, one of the major open questions is the nature of the rapid neutron capture process (r-process), which is responsible for the synthesis of roughly half of the chemical elements heavier than iron, such as gold and uranium. Astronomical observations indicate that the r-process has been active since the early stages of galactic history, and takes place in explosive stellar environments, such as supernovae explosions or mergers between two neutron stars, but the details are not known. A major difficulty to understanding the r-process is that it involves unstable isotopes. The main activities supported by this award are a series of nuclear physics experiments to measure the radioactive decay properties of isotopes directly relevant to r-process studies. The experimental setup combines state-of-the-art radiation detectors for beta-decay measurements (BRIKEN), and unstable isotope beams at leading accelerator facilities in Japan (RIBF) and the US (NSCL). The nuclear data obtained in this experiments will allow for a more precise comparison between astronomical observations and astrophysical models for the r-process. Another objective of this proposal is to provide students, especially at the undergraduate level, with opportunities to become involved in research activities at the forefront of nuclear physics, and contribute to expanding the number of students pursuing scientific careers in the US. For this reason, the proposal includes research projects that will take place at the campus of Central Michigan University (CMU), such as the development of radiation detectors and computational work with astrophysical models of the r-process.

The main scientific outcome of this award will be the measurement of beta-delayed neutron emission probabilities (Pn values) with the new BRIKEN detector setup at the Radioactive Ion Beam Factory in RIKEN, Japan. The BRIKEN setup consists of an active stopper detector for radioactive ion beams, surrounded by a high-efficiency neutron detector composed of He-3 proportional counters. The experiments currently planned will measure Pn values, as well as decay half-lives, for neutron-rich isotopes spanning the regions from the N=50 shell closure up to the rare-earth elements. These experiments will provide new nuclear data for r-process models in regions of the nuclear chart relevant to address important questions of r-process nucleosynthesis, such as the astrophysical conditions that lead to the formation of elements in the A=130 abundance peak. The award will also support the participation of CMU researchers in r-process experiments at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. This includes a project to develop reaccelerated beams of heavy isotopes around the N=82 region, with the potential to open new opportunities for r-process measurements.