Excitation of the isovector GDR by inelastic α-scattering as a measure of the neutron skin of nuclei

The cross section of the isovector giant dipole resonance (GDR) has been measured in the 116,124Sn(α, α'γ0), 150Nd(α, α'γ0) and 208Pb(α, α'γ0) reactions at Eα = 120 MeV and 0 ≤ Θα' ≤ 3 . The results are compared with DWBA calculations performed with nuclear GDR form factors that depend on the relative difference between the radii of proton and neutron distributions (ΔRPN/R0). Taking a linear combination of form factors provided by the Goldhaber-Teller and Steinwedel-Jensen models in a ratio that fits the GDR systematics, the ΔRPN values for 116Sn, 124Sn and 208Pb were deduced by comparing measured and calculated cross sections to be 0.02 ± 0.12 fm, 0.21 ± 0.11 fm, and 0.19 ± 0.09 fm, respectively. For the deformed 150Nd, the neutron-skin thickness along and perpendicular to the symmetry axis was found to be 0.22 ± 0.16 and 0.50 ± 0.15 fm, respectively, yielding βn2/βp2 = 0.92 ± 0.08. (orig.)