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 (He-3, t) charge-exchange reaction at 450 MeV for 0 degrees less than or equal to theta(t) less than or equal to 1.15 degrees have been measured. The results are compared with the predicted sum-rule st rengths related to the difference of neutron and proton root-mean-square ra dii. Within the model used, the radii differences (neutron-skin thicknesses ) deduced for Sn-114, Sn-116, Sn-118, Sn-122, and Sn-124 are less than or e qual to 0.09, 0.12 +/- 0.06, 0.13 +/- 0.06, 0.22 +/- 0.07, and 0.19 +/- 0.0 7 fm, respectively. This result suggests a new method for the measurement o f the neutron-skin thicknesses of unstable nuclei. [S0031-9007(99)08966-8].