Pygmy dipole resonance in stable nuclei

P VON NEUMANN-COSEL

Abstract. Two examples of recent work on the structure of low-energy

electric dipole modes are presented. The first part discusses the

systematics of the pygmy dipole resonance (PDR) in stable tin

isotopes deduced from high-resolution ($\gamma,\gamma'$)

experiments. These help to distinguish between microscopic QRPA

calculations based on either a relativistic or a non-relativistic

mean-field description, predicting significantly different

properties of the PDR. The second part presents a novel approach to

measure the complete electric dipole strength distribution from

excitation energies starting at about 5 MeV across the giant dipole

resonance (GDR) with high-resolution inelastic proton scattering

under $0^\circ$ at energies of a few 100 MeV/nucleon. The case of

$^{208}$Pb is discussed in detail and first result from a recent

experiment on $^{120}$Sn is presented.

Keywords. $^{112,120}$Sn$(\gamma,\gamma')$; deduced systematics of

the pygmy dipole resonance in $^{112-124}$Sn; $^{120}$Sn;

$^{208}$Pb($\vec{p},\vec{p}\,'$) at $0^\circ$; deduced complete

$B(E1)$ strength.

PACS Nos 24.30.Gd; 25.20.Dc; 21.60.Jz; 27.60.+j