Pygmy resonances and symmetry energy

Abstract

I present a brief summary of the first three decades of studies of pygmy resonances in nuclei and their relation to the symmetry energy of nuclear matter. I discuss the first experiments and theories dedicated to study the electromagnetic response in halo nuclei and how a low energy peak was initially identified as a candidate for the pygmy resonance. This is followed by the description of a collective state in medium heavy and heavy nuclei which was definitely identified as a pygmy resonance. The role of the slope parameter of the symmetry energy in determining the properties of neutron stars is stressed. The theoretical and experimental information collected on pygmy resonances, neutron skins, and the numerous correlations found with the slope parameter is briefly reviewed. PACS. 25.20.-x Photonuclear reactions -26.60.+c Nuclear matter aspects of neutron starsTo the memory of Pier Francesco Bortignon

Expanding Nuclear Physics Horizons with the Gamma Factory

“…EC cannot occur in bare ions because of the absence of K-shell electrons. Furthermore, it was shown that for the cases of 140 59 Pr and 142 61 Pm, the nuclear lifetime of the one-electron Pr 58+ (Pm 60+ ) ion is shorter than the ones of the corresponding two-or many-electron cases [45,46]. This could be explained by conservation of the total angular momentum, since only particular spin orientations of the nucleus and of the captured electron can contribute to the allowed EC decay [47].…”

“…One interpretation is an oscillation of excess neutrons (the neutron skin) against the N = Z core. The PDR is of great interest because it provides information on the neutron skin and on the nuclear symmetry energy as a crucial ingredient to the nuclear matter equation-of-state [142]. On theoretical grounds, employing the relativistic quasiparticle random phase approximation (RQRPA) approach, a one-to-one correlation was found between the pygmy dipole strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skin [143].…”

Expanding Nuclear Physics Horizons with the Gamma Factory

“…The PDR is of great interest because it provides information on the neutron skin and on the nuclear symmetry energy as a crucial ingredient to the nuclear matter equation‐of‐state. [ 144 ] On theoretical grounds, employing the relativistic quasiparticle random phase approximation (RQRPA) approach, a one‐to‐one correlation was found between the pygmy dipole strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skin. [ 145 ]…”

A unified description of the structure and electromagnetic breakup of $^{\bf 11}$Be

“…11 Be [15] and 19 C [16]) into the continuum [12,19,20]. It is also reported that astrophysical aspects such as abundance pattern in the r-process nucleosynthesis could also be related to the presence of low-lying dipole strength present in neutron-rich nuclei [21,22].…”