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HK: Fachverband Physik der Hadronen und Kerne

HK 10: Struktur und Dynamik von Kernen

HK 10.1: Gruppenbericht

Montag, 17. März 2014, 16:30–17:00, HZ 4

Dipole Polarizability and Neutron Skin in ⁶⁸Ni — Dominic Rossi^1,2,3, Thomas Aumann⁴, and •Konstanze Boretzky¹ for the R3B collaboration — ¹GSI Darmstadt, Germany — ²Univ. Mainz, Germany — ³NSCL, MSU, USA — ⁴TU Darmstadt, Germany

The symmetry energy term E_sym of the nuclear equation-of-state describes fundamental phenomena both in nuclear physics and in astrophysics. The electric dipole (E1) response of nuclei as a function of the isospin asymmetry is driven by E_sym. Studies of the Pygmy Dipole Resonance (PDR) in exotic nuclei have been used to constrain E_sym or the neutron skin thickness Δ R_n,p. The electric dipole polarizability α_D, being very sensitive to the low-lying E1 strength, is correlated to Δ R_n,p in a robust and only moderately model-dependent manner [PRC 81, 051303 (2010)]. Here, a first experimental determination of α_D in an unstable nucleus and the derivation of its Δ R_n,p will be reported [PRL 111, 242503 (2013)]. From the E1 strength distribution in ⁶⁸Ni measured using the R3B-LAND setup at GSI, the resonance parameters for the observed PDR at 9.55(17) MeV and the giant dipole resonance at 17.1(2) MeV are determined. In combination with results from Wieland et al. [PRL 102, 092502 (2009)] an unexpectedly large direct photon-decay branching ratio of 7(2)% is observed for the PDR. The measured α_D of 3.40(23) fm³ is compared to relativistic RPA calculations yielding Δ R_n,p of 0.17(2) fm for ⁶⁸Ni. This work was supported by HIC for FAIR, EMMI, the GSI-TU Darmstadt cooperation agreement, the Helmholtz-CAS Joint Research Group HCJRG-108 and BMBF grants 06MZ222I, 05P12RDFN8 and 06MT9156.