Study of the Pygmy Dipole Resonance in 124Sn by means of the (a,a′g ) reaction

In recent years alpha-gamma coincidence experiments at 136MeV incident energy on 48Ca, 140Ce, 138Ba and 124Sn were performed at the KVI in Groningen to study the isospin character of electric dipole excitations below the particle threshold, frequently called Pygmy Dipole Resonance (PDR). An array of HPGe gamma-detectors has been used in coincidence with the Big-Bite Spectrometer and a resolution of about 10 keV in the gamma-ray energy has been achieved. The results show that the excitation patterns of the PDR in the (alpha,alpha′) reaction seem to differ significantly from the results obtained in Nuclear Resonance Fluorescence (NRF) (gamma,gamma′) measurements. The PDR, which until now has been assigned to one excitation mode, splits up into two parts: One that is excited in (alpha, alpha′ gamma) and (gamma,gamma') reactions (denoting a dominant isoscalar character), and one that is only excited in (gamma, gamma′) (denoting a dominant isovector character). This indicates that two different excitation mechanisms produce these low-lying E1 excitations. The preliminary results of the latest measurements on the N = 82 nucleus 138Ba and the Z = 50 nucleus 124Sn show that this break up into two parts is a common feature of the PDR in semi-magic nuclei.