Excitation of the isovector giant dipole resonance by inelastic α scattering and the neutron skin of nuclei

The cross section of the isovector giant dipole resonance (GDR) excited by inelastic scattering of 120-MeV α particles for 0°≤Θα’≤3° has been measured using the ${}_{}{}^{116,124}{}_{}{}^{}\mathrm{Sn}$(α,α’ ${\gamma }_0$) and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$(α,α’ ${\gamma }_0$) reactions. The results are compared with distorted-wave Born-approximation calculations with a GDR form factor depending on the relative difference of proton and neutron radii (Δ${\mathit{R}}_{\mathit{p}\mathit{n}}$/${\mathit{R}}_0$). Within the model used, the Δ${\mathit{R}}_{\mathit{p}\mathit{n}}$/${\mathit{R}}_0$ values for ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{124}{}_{}{}^{}\mathrm{Sn}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ deduced from the comparison of measured and calculated cross sections are (0.$7_{\mathrm{-}0.7}^{+2.3\mathrm{}}$)%, (4.$1_{\mathrm{-}2.5}^{+2.1\mathrm{}}$)%, and (3.0±1.3)%, respectively.