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Resonant Nuclear Gamma Decay and the Heavy-Element Nucleosynthesis

Periodic Reporting for period 4 - gRESONANT (Resonant Nuclear Gamma Decay and the Heavy-Element Nucleosynthesis)

Reporting period: 2019-09-01 to 2020-02-29

"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 don’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). 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)."
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 tungsten-185, rhenium-186, and osmium-191. 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. Our first result on the osmium-191 case was published in 2019.

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 presence of this resonance may enhance neutron-capture rates and thus increase the chance of producing super-heavy nuclei in neutron-star collisions. The data clearly show that the scissors mode is indeed present in nobelium-254. The results are now prepared for publication.

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: nickel-68 and nickel-69. We have proven the existence of the upbend in neutron-rich nuclei for the first time in neutron-rich nickel-69,70, and we have performed large-scale shell-model calculations showing that the upbend is indeed a general feature for many nuclei in the nickel region.

The publications connected to this project are available through the University of Oslo's repository: https://www.duo.uio.no/?locale-attribute=en
Moreover, data on level densities and gamma-ray strength functions are available through the website of the Oslo Cyclotron Laboratory: http://ocl.uio.no/compilation
The results of the project have also been presented through various international conferences and workshops.
We have shown that the Oslo method can provide excellent results on reaction rates for unstable s-process branch point nuclei. Moreover, the novel beta-Oslo method has been established, enabling indirect measurements of neutron-capture reaction rates on very exotic, neutron-rich nuclei.We have also developed a new method to search for the magnetic scissors resonance in short-lived, superheavy nuclei. The results have triggered theoretical studies within nuclear physics, as well as an extensive investigation of the impact on astrophysical processes where heavy elements are formed. Nuclear astrophysics is a very exciting research field, and this project has contributed to improving our understanding of how heavy elements have been created in stars and neutron-star collisions.
Nuclear gamma-decay resonances may impact the element production in neutron star collisions.