Gamma decay of pygmy states from inelastic scattering of ions

An overview of relevant results on the study of 1⁻ states focusing on their excitation with nuclear probes is given. Results obtained for the ⁹⁰Zr , ¹²⁴Sn , and ²⁰⁸Pb nuclei using the (¹⁷O,¹⁷O′γ) reaction are compared with available data obtained with the (γ, γ′, and (p, p′) reactions. These comparisons allow to learn on the nature of the populated states, particularly the E1 states, whose isospin character is presently poorly known. The DWBA description of the data is discussed in terms of different form factors, standard collective form factor and form factors obtained by folding microscopically calculated transition densities. The relevant aspects related to the used theoretical approach are also presented. The main objective of the analyses is the extraction of the values of the fraction of the energy weighted sum rule strength for the isoscalar dipole excitation. For completeness, in all cases, the DWBA analysis was made also for the excitations of 2⁺ and 3⁻ states.