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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Nuclear shapes and resonances

Alternative title: Deformasjoner og resonanser i atomkjernen

Awarded: NOK 10.2 mill.

The atomic nucleus consists of protons and neutrons that are bound together by the strong nuclear force. Many nuclear properties can be understood by describing the nucleus as an electrically charged droplet of nuclear matter. Other properties indicate that nuclei have a microscopic structure in which the protons and neutrons are organized in shells that are characterized by specific quantum numbers. The shape of a nucleus is governed by a delicate interplay between the liquid-drop properties and its shell structure: nuclei with closed proton and neutron shells have spherical shape, whereas nuclei with partially filled shells tend to deform and assume ellipsoidal shapes. If a nucleus is excited by transferring energy to it in nuclear reactions, its response to this excitation depends crucially on its shape, similar to how the shape of a bell determines its sound with characteristic frequencies and overtones. Such characteristic modes of excitation are called nuclear resonances. The properties of such nuclear resonances are important to understand the forces that bind the nucleus together and to determine the likelihood of reactions between different nuclei. Obtaining information about nuclear shapes and the properties of nuclear resonances is therefore important to understand how elements are created in astrophysical processes, for example when a star explodes in a supernova or two neutron stars collide. The research project aims at measuring nuclear shapes and properties of nuclear resonances and comparing experimental results to theoretical models. A particular emphasis of the project is to provide opportunities for young researchers to conduct doctoral and postdoctoral studies. The Oslo Scintillator Array OSCAR, a national research infrastructure hosted at the Oslo Cyclotron Laboratory (OCL) at UiO, has played a key role for the success of the project. OSCAR was installed at OCL in 2017 and fully commissioned in 2018. Many experiments were performed with OSCAR over the course of the project, which have contributed important results on nuclear resonances and reaction rates. Several experiments helped to explain an unexpected enhancement for the emission of low-energy photons from highly excited states. This enhancement affects the probability for a nucleus to capture neutrons, with important consequences for our understanding of the formation of heavy elements in the universe. Using a data analysis method that is known internationally as the Oslo Method, a novel technique to determine neutron capture rates was established. These quantities are also important for reactor applications, and several experiments on actinide nuclei provided neutron capture rates that are important for example for thorium-based reactor designs. Such experiments have also given new insight into the fission process itself. A further highlight was the precise determination of the electromagnetic decay probability of the so-called Hoyle state, a resonance in carbon-12. This process is responsible for the fusion of helium into carbon in stars, and hence for the production of all chemical elements beyond carbon in the universe. The research program at OCL was complemented by experiments at international accelerator laboratories such as ISOLDE at CERN, GANIL (France), RIKEN (Japan), Michigan State University (USA), and iThemba LABS (South Africa). In several cases, the Oslo Method was applied to short-lived nuclei that cannot be reached at OCL. Other experiments investigated the shape of atomic nuclei, which can deviate significantly from spherical. A particular focus was placed on nuclei that can assume different shapes, a phenomenon known as shape coexistence. The project has contributed to more than 80 publications in peer-reviewed scientific journals, and the results were presented at international conferences. In addition to three PhD projects, this research program has resulted in 18 Master theses. The research performed within the project was the subject of two international conferences held in Oslo in 2017 and 2019 with more than 90 participants from around the world. The 2021 edition of the conference, which was planned for 2021, had to be postponed to 2022 because of the covid-19 pandemic.

Results obtained within this project were published in prestigious journals, including Nature, Physical Review Letters, and Physics Letters B, and many are already well cited. The results are also included in the IAEA database on photon strength functions. Leading theorists in the fields of nuclear structure and nuclear astrophysics, are using our experimental data to benchmark and improve theoretical models. Results obtained with the Oslo Method are important inputs for calculations of neutron capture cross sections. The demand for such data has increased after the discovery of neutron star mergers as a site for nucleosynthesis by rapid neutron capture. The project has laid the foundation for the center of excellence initiative "Bright Matter" currently under review. There is a high demand for employees with background in nuclear physics, both in Norway and internationally. Through the education of many Master and PhD students the project has contributed to fulfilling this demand.

Resonances in the photon strength function (PSF) reveal fundamental information about the structure of a nucleus, its modes of excitation, and underlying shape. They also have a large impact on nuclear reaction rates. Experimental measurements of resonances in the PSF therefore provide data that can be used to refine theoretical nuclear structure models and that are at the same time highly relevant for nuclear astrophysics and nuclear energy applications. The Oslo Group has developed an experimental method to simultaneously measure the nuclear level density and PSF. The experimental program will greatly benefit from the new Oslo Scintillator Array (OSCAR), which will be available as a national infrastructure at the Oslo Cyclotron Laboratory from 2017. One of the key questions addressed by the project is the evolution of resonances in the PSF when moving from stable towards neutron-rich nuclei. To investigate this it is necessary to apply the Oslo method with radioactive ion beams (RIB). A first pilot experiment with a stable heavy ion beam was successfully performed at iThemba Labs. A first experiment with RIB and OSCAR detectors is approved at the HIE-ISOLDE facility at CERN. Another key question addressed by the project is to study resonances in the PSF as a function of the underlying nuclear deformation. The deformation itself can be investigated by measuring quadrupole moments and electromagnetic transition probabilities between discrete nuclear states. Experimental measurements of these observables provide important benchmarks for theoretical calculations. Two experiments, using Coulomb excitation with RIB at HIE-ISOLDE and lifetime measurements with AGATA, have been approved at CERN and GANIL, respectively. The experimental studies of this project will enhance the visibility of the Oslo group internationally and provide unique opportunities for two PhD students and a postdoctoral researcher to perform cutting-edge research at world-leading facilities.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek