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Masses, isomers and decay studies for elemental nucleosynthesis

Periodic Reporting for period 2 - MAIDEN (Masses, isomers and decay studies for elemental nucleosynthesis)

Reporting period: 2019-12-01 to 2021-05-31

The origin of chemical elements heavier than iron, such as gold, has been a long-standing puzzle. About half of the heavy-element abundances are expected to be produced by the astrophysical rapid neutron capture process, the r process. Its astrophysical site has been one of the biggest outstanding questions in physics. The observation of the binary neutron-star merger GW170817 and the associated kilonova in August 2017 gave the first direct evidence that the r process takes place at least in such neutron-star mergers. However, there are still several open questions related to the r process. Can neutron-star mergers explain all observed r-process abundances? What is the role of supernovae in the production of heavier elements? How to interpret the observed kilonova? In order to better model the r process, accurate nuclear physics input data are needed. Nuclear masses are one of the most important nuclear physics inputs for the r process and for studying the chemical evolution in the Cosmos. In this project, high-precision mass measurements are performed for the r process employing novel production and measurement techniques. Long-living isomeric states, which also play a role in the r process, are resolved from the ground states to obtain accurate mass values. Post-trap decay spectroscopy will be performed to confirm which state has been measured in order to avoid systematic uncertainties in the mass values. Decay properties of neutron-rich nuclei are also relevant for the r-process calculations. The new data gathered in this project will be compared with theoretical nuclear models and included in the r-process calculations performed for different astrophysical conditions. The improved r-process calculations are essential to fully benefit from the anticipated new multimessenger observations from neutron-star mergers. This project will advance our knowledge of nuclear structure far from stability and reduce nuclear data uncertainties in the r-process calculations, which can potentially constrain the astrophysical site for the r process and help to understand the origin of heavier elements.
In this project we have performed high-precision mass measurements for around 90 radioactive nuclides, including almost 30 isomeric states, with the JYFLTRAP Penning trap at the IGISOL facility in the JYFL-ACCLAB accelerator laboratory at the University of Jyväskylä. The measurements have provided crucial data for the r-process calculations. For example the rare-earth mass measurements have yielded a better agreement between the r-process calculations and the observed r-process abundances. A new gas cell and target platform have been designed and manufactured to produce heavier neutron-rich nuclei via multinucleon-transfer reactions at IGISOL. These nuclei are particularly relevant for the heaviest r-process abundance peak and the origin of gold. Decay properties of radioactive nuclei have been studied for nuclear structure and astrophysics using various detector setups. In addition, ion-trapping techniques have been further developed for faster measurements and to obtain a higher resolving power, both for the mass measurements and decay spectroscopy experiments.
The project has progressed extremely well with many new technical developments done and experimental results obtained. The focus in the latter part of the project will be to complete the planned experimental programme and study the impact of the experimental results on the r-process calculations. The expected results will provide more accurate input data for the r-process calculations and therefore reduce the nuclear-physics related uncertainties. This is essential to better interpret the anticipated multimessenger observations from neutron star mergers. In addition, the results provide information on nuclear structure far from stability and help to develop theoretical nuclear models in the long run.
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