Excitation of Isovector Spin-Dipole Resonances and Neutron Skin of Nuclei

The cross sections of the isovector spin-dipole resonances (SDR) in the Sb isotopes excited by the ${(}^3\mathrm{He},t)$ charge-exchange reaction at 450 MeV for $0°\le {\theta }_t\le 1.15°$ have been measured. The results are compared with the predicted sum-rule strengths related to the difference of neutron and proton root-mean-square radii. Within the model used, the radii differences (neutron-skin thicknesses) deduced for ${}^{114}\mathrm{Sn}$, ${}^{116}\mathrm{Sn}$, ${}^{118}\mathrm{Sn}$, ${}^{122}\mathrm{Sn}$, and ${}^{124}\mathrm{Sn}$ are $\le 0.09$, $0.12\pm 0.06$, $0.13\pm 0.06$, $0.22\pm 0.07$, and $0.19\pm 0.07\mathrm{fm}$, respectively. This result suggests a new method for the measurement of the neutron-skin thicknesses of unstable nuclei.