In recent years investigations have been made to study the electric Pygmy Dipole Resonance (PDR) systematically, mainly in semi-magic nuclei. For this purpose the well understood high resolution (γ ,γ ') photon scattering method is used. In complementary (α,α'γ ) coincidence experiments at Eα = 136 MeV a similar γ -energy resolution and a high selectivity to E1 transitions can be obtained at the Big-Bite Spectrometer (BBS) at KVI, Groningen. In comparison to the (γ ,γ') method a structural splitting of the PDR is observed in the N=82 nuclei 138Ba and 140Ce and in the Z=50 nucleus 124Sn. The low energy part is excited in (γ ,γ') as well as in (α,α'γ ) while the high energy part is observed in (γ ,γ') only. The experimental results together with theoretical QPM
and RQTBA calculations on 124Sn which are able to reproduce the splitting of the PDR qualitatively are presented. The low-lying group of 1− states seem to represent the more isoscalar neutron-skin oscillation of the PDR while the energetically higher-lying states seemingly belong to the transitional region between the PDR and the isovector Giant Dipole Resonance (IVGDR).
|Name||AIP Conference Proceedings|
|Conference||Frontiers in Nuclear Structure, Astrophysics, and Reactions: FINUSTAR 3|
|Period||2010-08-23 → 2010-08-27|