Urbana Campus Research Calendar (OVCRI)

The r-process, a series of rapid neutron-capture reactions in cataclysmic astrophysical events such as neutron star mergers or supernovae explosions, is responsible for the creation of roughly half of the heavy nuclei in our universe. The conditions present in these events are such that the neutron-capture reactions occur in a time scale much shorter than the lifetime of the nuclei involved and as such

the process proceeds mainly through reactions on short-lived very neutron-rich nuclei. In fact, most of the nuclei involved are either very poorly known or have never been observed in the laboratory. Sensitivity studies have looked at various scenarios for the r-process conditions and identified regions around neutron numbers 82 and 126 where basic nuclear properties would have the largest impact on the distribution of produced nuclei. At ANL, a program centered around the ATLAS facility is aimed at improving access to these nuclei and developing the tools to measure the most critical quantities to constrain r-process scenarios.

Over the last decade, the CAlifornium Rare Ion Breeder (CARIBU) addition to the ATLAS superconducting linac facility provided access to neutron-rich nuclei around the N=82 neutron shell closure that allowed us to gather data that, together with simulations that reverse-engineered the r-process, provided a better understanding of the astrophysical conditions necessary to reproduce the main r-process abundance peak. Additional information is needed to confirm these findings and that requires access to isotopes outside the range of those accessible by current facilities. We have therefore undertaken an upgrade of CARIBU, called nuCARIBU, which makes use of a novel high-intensity-cyclotron based neutron generator irradiating a highly enriched 235U target to increase the fission fragment yield and allow these studies to be extended to even more exotic nuclei in the N=82 region. In addition, the sensitivity studies highlighted that an other region of high interest is the neutron-rich region "east" of 208Pb which is a particularly sensitive probe for the N=126 abundance peak. This region has proven to be very difficult to access so far and data from high-energy fragmentation indicate that this will remain so even when the next generation facilities such as FRIB. At ATLAS, we are building a new facility, the N=126 factory, which will use a different production mechanism to access this region. It will take advantage of the unique high-intensity heavy-ion beams at around 10 MeV/u available at ATLAS to produce these nuclei by multi-nucleon transfer reactions and separate them using the techniques developed at CARIBU for fission fragments.

The talk will present the basic nuclear physics inputs required to understand the r-process, together with the existing ATLAS, nuCARIBU and N=126 facilities. The constraints on astrophysical r-process conditions obtained in the CARIBU campaign will also be presented, together with a brief overview of the current research programs at these facilities.

This work is supported by the U.S. Department of Energy, Office of Nuclear Physics, under contract No. DE-AC02-06CH11357, and uses resources from ANL’s ATLAS facility, an Office of Science National User Facility.