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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - gRESONANT (Resonant Nuclear Gamma Decay and the Heavy-Element Nucleosynthesis)

Teaser

Summary

\"What is the nature of resonances in the gamma-decay probability in atomic nuclei? What is the impact of such resonant gamma decay on unknown neutron-capture reaction cross sections for the heavy-element nucleosynthesis? How can the most important of these unknown cross sections be predicted with a sufficient precision, so that they donâ€™t represent a major source of uncertainty in astrophysics applications, for next-generation nuclear reactors, and transmutation of nuclear waste?

The questions above are the key scientific issues of the ERC Starting Grant \"\"Resonant Nuclear Gamma Decay and the Heavy-Element Nucleosynthesis\"\" (gRESONANT). After all, we live in a Universe consisting of a great variety of elements and their isotopes. The elements are the building blocks of all visible matter, from spectacular supernova remnants to life on Earth. We are all made from debris of the Big Bang, as well as stardust from stars long gone. However, exactly how the heavy elements were and are created is still not well understood. This puzzle has been identified as one of the \"\"Eleven Science Questions for the New Century\"\" by the US National Research Council Committee. The grand challenge of nuclear astrophysics is therefore to explain how the elements were, and still are, made. This project aims at finding new pieces to this big picture.

With the recent neutron-star collision discovery by the LIGO and Virgo gravitational-wave detectors, and the corresponding measurements of its electromagnetic counterparts, it was finally confirmed that the heaviest elements such as gold and platinum are created while two neutron stars merge together. For the first time, there was a live detection of the rapid neutron-capture process!

Because neutron star mergers provide a \"\"cold\"\" environment for the rapid neutron-capture process, it means that neutron-capture rates are key to understanding the details of the process that \"\"cooks\"\" about half of the elements heavier than iron. This ERC StG project deals with novel methods for constraining neutron-capture rates with experimental data on fundamental, nuclear quantities governing nuclear gamma-decay, i.e. the nuclear level density and the gamma-emission strength. The overarching hypothesis is that gamma-decay resonances will increase neutron-capture rates, with implications both for nuclear structure (what is the nature of these resonances?) and for nuclear astrophysics (increased neutron-capture rates may change the predicted r-process reaction flow and final abundances, as well as impacting s-process branch points).\"

Work performed

We have performed experiments at the Oslo Cyclotron Laboratory, Department of Physics, University of Oslo, to measure nuclear level densities and gamma-emission strengths of nuclei in the tungsten-rhenium-osmium mass region. A detailed study of the data will reveal new information on the gamma-emission strength, and probe the possible existence of pygmy resonances close to neutron threshold. The results will be used to constrain the (n,gamma) reaction cross section on the s-process branch-point nuclei 185W and 186Re. Typically, the neutron-capture branch is ignored in s-process calculations. Our goal is to include experimentally-constrained rates for the branch points to obtain improved s-process simulations.

Further, we have completed a fusion-evaporation experiment at the University of Jyvaskyla Accelerator Laboratory, to study the total gamma-ray spectra of the superheavy nucleus nobelium-254 and to look for the so-called scissors mode, a resonance that is expected to appear for non-spherical nuclei. The data are currently being analyzed.

We have also performed experiments with the newly developed beta-Oslo method at the National Superconducting Cyclotron Laboratory, Michigan State University, to find out whether unstable, neutron-rich nuclei as those involved in the rapid neutron-capture process have a large gamma-emission probability for low-energy gamma transitions. We have for the first time constrained the neutron-capture rate of two neutron-rich nickel isotopes.

Theoretical studies on uncertainties in r-process neutron-capture reaction rates are also ongoing.

Final results

The new beta-Oslo method is a very promising technique to experimentally constrain neutron-capture rates for unstable nuclei. Studying the scissors mode in superheavy nuclei has never been done before and opens new aspects in this field. Moreover, forthcoming experiments are expected to reveal whether the scissors mode is also present in exotic, neutron-rich nuclei. Improved reaction-network simulations are expected both for the s-process (explicit inclusion of branch points) and the r-process (realistic and improved neutron-capture rate uncertainties, indirect constraints from data).