Speaker
Description
N.S. Martorana$^{1}$, C. Cardella$^{1}$, E.G. Lanza$^{1}$, L. Acosta$^{2}$, M.V. Andrés$^{3}$, F. Camera$^{4,5}$, A. Castoldi$^{5,6}$, E. De Filippo$^{1}$, E. Geraci$^{1,7,8}$, A. Giaz$^{5}$, B. Gnoffo$^{1,7}$, C. Guazzoni$^{5,6}$, C. Maiolino$^{9}$, E.V. Pagano$^{9}$, L. Pellegri$^{10,11}$, S. Pirrone$^{1}$, G. Politi$^{1,7}$, F. Risitano$^{1,8,12}$, F. Rizzo$^{7,8,9}$, P. Russotto$^{9}$, M. Trimarchi$^{1,12}$, G. Verde$^{1}$, O. Wieland$^{5}$, C. Zagami$^{8,9}$
and the NUSDAF collaboration
1 INFN-Sezione di Catania, Catania, Italy
2 Instituto de Estructura de la Materia, CSIC, Spain
3 Departamento de FAMN, Facultad de Física, Sevilla, Spain
4 Università degli studi di Milano, Milano, Italy
5 INFN Sezione di Milano, Milano, Italy
6 DEIB Politecnico di Milano, Milano, Italy
7 Università degli Studi di Catania, Catania, Italy
8 CSFNSM, Catania, Italy
9 INFN-LNS, Catania, Italy
10 School of Physics, University of the Witwatersrand, South Africa
11 iThemba Laboratory for Accelerator Based Sciences, South Africa
12 Dipartimento MIFT, Università di Messina, Messina, Italy
The Pygmy Dipole Resonance (PDR) is a low-lying electric dipole (E1) excitation observed in neutron-rich nuclei, located around the nucleon binding energy. It is macroscopically interpreted as an oscillation of the neutron skin against the nuclear core, although its precise nature, whether collective or arising from a series of no-coherent single-particle excitations, remains an open question. A notable feature of the PDR is its isospin mixing, which allows it to be excited by both isoscalar and isovector probes [1–3]. Experimental studies have shown that the PDR can indeed be populated with both types of probes. In stable nuclei, a splitting is observed in the excitation region below the neutron separation threshold: in the low-lying region the dipole states are excited by both isoscalar and isovector probes, while in the higher-lying one only the excitation by the isovector probes is present [1–3]. This phenomenon, referred to as isospin splitting, appears to be absent in unstable nuclei, where the PDR has been measured above the neutron separation threshold [4–5]. The investigation of the PDR is of great interest not only for nuclear structure studies, but also for constraining the neutron-skin thickness, probing the symmetry energy term of EoS, and improving models of astrophysical processes such as r-process nucleosynthesis [1]. A deeper understanding of the PDR requires systematic investigations with both isoscalar and isovector probes over a wide range of nuclear isotopes and both below and above the neutron emission threshold. In this contribution, an overview of recent results on PDR studies in unstable nuclei is presented, together with future perspectives aimed at improving our understanding of this mode through the use of radioactive ion beams. In particular, future experimental campaigns at FRIB, proposed within the NUSDAF initiative, will be discussed, focusing on advanced studies of the PDR along the Ni isotopic chain through the investigation of both γ and neutron decay channels. First theoretical calculations for Ni isotopes, as well as simulation studies on the feasibility of detecting both neutron and γ decay channels, will also be presented. In addition, benefiting from the high quality and intensity of radioactive beams at FRIB, the program plans to use some existing equipment. Possible perspectives that may be offered by doing experiments at FRIB with more efficient high energy gamma detectors (such as PARIS, among others) will also be discussed.
[1] A. Bracco et al., Progr. in Particle and Nuclear Physics 106, 360-433 (2019) and references therein
[2] D. Savran et al., Progr. In Particle and Nuclear Physics 70, 210-245 (2013) and references therein
[3] E.G. Lanza et al., Progr. in Part. and Nucl. Phys.,129, 104006, (2023) and references therein
[4] O. Wieland et al., Phys. Rev. Lett. 102, 092502 (2009)
[5] N.S. Martorana et al. In: Physics Letters B 782, 112 116 (2018)