The nuclear dipole response
The nuclear dipole response has an electric and a magnetic part. Both contributors exhibit a variety of features with a multitude of implications, such as the violation of fundamental symmetries, extreme astrophysical objects such as neutron stars, the synthesis of heavy elements, or the dynamics of supernova type-II explosions.
The electric dipole response (E1R) can be classified in low-lying quadrupole-octupole coupled excitations, which are particular interesting if the nucleus has quadrupole but also (so far established in less than a handful of cases) octupole correlations in its ground state. The E1R is dominated by the Giant Dipole Resonance (GDR) at comparably high excitation energy beyond the particle thresholds. On the GDR’s low-energy tail near the proton and neutron evaporation threshold a less strong additional enhancement of E1 strength is found. This, so called Pygmy Dipole Resonance (PDR) is often visualized as an out-of-phase motion of a neutron skin formed by excess neutrons and an isospin saturated core composed of an almost equal number of protons and neutrons. This mode provides an additional wide-open doorway in particle-capture reactions in astrophysical scenarios as the slow and rapid neutron capture processes. Furthermore, the skin picture of ‘pure’ neutron matter suggests connections via the symmetry energy, which enters the nuclear Equation-of-State, to neutron stars.
The magnetic dipole response splits into the scissors resonance near 3 MeV and the Gamow-Teller Resonances near the particle threshold. The latter has strong implications for beta decay probabilities and even more so neutrino-nucleus scattering cross sections. The latter are deemed to be the mechanism driving the explosion of type-II Supernova.
Our group has become interested to benchmark these excitations in nuclei with equal proton and neutron number. For example, in the case of the PDR, this allows to disentangle shell effects (in our nuclei) from neutron-skin+shell effects in more neutron rich nuclei. We have an ongoing experimental program for the lightest stable Ni isotopes 58 and 60 and aim to investigate 24-Mg and 28-Si. As experimental tools we use the scattering of real photons, bremsstrahlung for the excitation strength and fully-polarized laser-Compton backscattered photons to assign the E1 or M1 character. Furthermore, we employ inelastic alpha-particle scattering experiments to assign an isospin character to the observed E1 excited levels. Recently, our group has proposed to use high-Q value beta decay of mothers with a low ground state spin to gain an alternative insight in the levels of the PDR. In a first campaign we were able demonstrate that the PDR plays even a role in reactor physics.