RTs: Aumann, Balabanski, Enders, Isaak, Martinez-Pinedo, Matei, Pietralla, Tsoneva, Werner

Project Area Overview: Photonuclear reactions will be applied for contemporary nuclear science from nuclear structure to astrophysics, topics to which some of the RTs have previously made transformative contributions. The research projects range from photonuclear disintegrations of light nuclei and tests of nuclear structure theory, to the properties of nuclear resonances and fission processes of heaviest nuclei. These topics are relevant for our understanding of cosmic objects such as binary neutron-star mergers and for the assessment of nuclear reaction rates that are relevant for the s-, p- and r-processes of nucleosynthesis or even for primordial Big Bang Nucleosynthesis.

Research Area B addresses studies of photonuclear reactions with quasi-monoenergetic photon beams, provided currently at the HIγS facility at the Triangle Universities Nuclear Laboratory (TUNL), Durham, NC, and with advanced parameters by the VEGA system of ELI-NP in the near future, and with continuous-energy bremsstrahlung at the S-DALINAC. The structure of nuclear dipole modes ranging from light isotopes in the A~10 mass regime towards heavy deformed nuclei in the rare-earth and actinides region will be investigated experimentally by scanning the nuclear photoresponse at the continuous-energy bremsstrahlung beams at TU Darmstadt. These experiments will be complemented by detailed studies of regions of particular interest with high energy resolution and sensitivity at the quasi-monochromatic photon beams. For these studies, different simulation tools have been developed which take into account the characteristics of the photon beams and the available detector arrays.

The experimental data will be compared to microscopic nuclear structure theory based on an extended quasiparticle-random-phase approximation with multiphonon coupling, which was used to predict new modes of nuclear excitation. Exploiting the linear polarization of the provided quasi-monochromatic gamma-ray beams, magnetic dipole excitations of heavy deformed nuclei will be investigated for clarifying the distribution of orbital and spin-M1 strength as a function of nuclear deformation. Electric dipole strength will be studied with total photoabsorption measurements at the photon tagger NEPTUN at TU Darmstadt below and above the particle-separation threshold contributing information on the nuclear dipole polarizability which itself contains precious information on the density dependence of the nuclear symmetry energy. The latter is an important prerequisite for the understanding of objects with extreme neutron excess as, for instance, occurring in the crust of neutron stars. Especially, the evolution of the GDR as a function of nuclear deformation has become of particular interest due to recent data from inelastic proton and real-photon scattering experiments. Furthermore, photonuclear disintegration processes will be studied above the particle-separation threshold.

The modelling of the observed elemental abundances depends, among other quantities, on nuclear photon strength functions. Intense quasi-monoenergetic linearly-polarized photon beams enable the disentanglement of different contributions of nuclear photon strength functions with new techniques including the method of nuclear self-absorption. The experimental data will be interpreted with respect to nuclear structure phenomena and their relevance for r-process nucleosynthesis in binary neutron-star mergers. Studies of key photonuclear reactions will shed light on the p-process nucleosynthesis.

While extensively used on a large-scale for the generation of electricity, the nuclear fission process still cannot be described accurately on a macroscopic level. Hence, the fine-structure of photo-induced fission processes of transuranium actinides will be studied in dedicated photofission experiments with the quasi-monoenergetic photon beams at ELI-NP and HIγS exploiting correlations between fragment observables using a position-sensitive ionization chamber. The experimental nuclear fission data will be interpreted in microscopic descriptions. Sub-barrier fission studies will provide deeper understanding of the fission barriers.