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Giant resonance decay studies: a means to constrain fundamental properties of nuclear matter

Final Activity Report Summary - GIANT RESONANCES (Giant resonance decay studies: a means to constrain fundamental properties of nuclear matter)

Coincidence experiments on the systematic proton-decay studies of isoscalar overtone mode giant resonances, of the Isoscalar giant dipole resonance (ISGDR) in particular, were performed using inelastic alpha-scattering on 90Zr and 208Pb targets. The main experimental goal was to determine partial direct-decay branching ratios from the population of low-lying single-hole states in the daughter nuclei and to obtain a background-free definition of resonance strength distributions in a model-independent way in contrast to single measurements.

This was achieved by the effective separation of the dipole strength on a background of other multipolarities with the application of the difference-of-spectra method. Furthermore, the use of the coincidence detectors at backward angles helped to exclude contribution from forward-peaked quasi-free processes, while corrections for random coincidence events suppressed instrumental background components.

As overtone modes of isoscalar giant resonances, such as Isoscalar giant monopole resonance (ISGMR) and ISGDR, can be associated with a compression oscillation character their excitation energy is directly related to the compression modulus of nuclear matter. This research was at first motivated by the fact that, in addition to the fundamental interest of studying microscopic structures beyond collective excitations of giant resonances, a more precise and systematic determination of the compression modulus was required for appropriate and accurate descriptions of various astrophysical phenomena and heavy ion reactions through the application of the nuclear equation-of-state.

The results on the direct-decay branching ratios and giant resonance parameters from strength distribution were compared to recently performed self-consistent and continuum Random phase approximation (continuum-RPA) calculations. Moreover, as strength distributions could be studied for various particle-hole configurations, the revision of descriptions on coupling to collective behaviour might open new perspectives for theoretical discussions on the compression modulus.

The observation of direct-decay channels to single-hole states resulted in the first unambiguous evidence for a highly-selective disentanglement of giant resonance strengths and some signature for a new resonance mode with quadrupole character, probably related to the overtone of the Isoscalar giant quadrupole resonance (ISGQR), was obtained in the energy region above the ISGDR. The interpretation of the data remained a subject of discussions with theoretical experts by the time of the project completion.

Furthermore, the research line of studying compression modulus and microscopic structures of overtone giant resonances was extended towards a systematic investigation of the symmetry energy, which was also representing a key term of the nuclear equation-of-state. The appropriate application of the nuclear equation-of-state at saturation densities required the knowledge of the symmetry energy in terms of isospin-dependence. Experiments were planned to determine this quantity by the observation of the Spin-dipole resonance (SDR) in the long isotopic chain of Sn nuclei with higher precision compared to previous studies. The experiments employed charge-exchange reaction in inverse kinematics, for which a direction-sensitive neutron-detector array was constructed by our group, and construction work started as part of the project.