  The gamma-decay of the anti-analog of the giant dipole resonance (AGDR) has
been measured to the isobaric analog state excited in the p(124Sn,n) reaction
at a beam energy of 600 MeV/nucleon. The energy of the transition was also
calculated with state-of-the-art self-consistent random-phase approximation
(RPA) and turned out to be very sensitive to the neutron-skin thickness
(\DeltaR_(pn)). By comparing the theoretical results with the measured one, the
\DeltaR_(pn) value for 124Sn was deduced to be 0.175 \pm 0.048 fm, which agrees
well with the previous results. The energy of the AGDR measured previously for
^(208)Pb was also used to determine the \DeltaR_(pn) for ^(208)Pb. In this way
a very precise \DeltaR_(pn) = 0.181 \pm 0.031 neutron-skin thickness has been
obtained for 208Pb. The present method offers new possibilities for measuring
the neutron-skin thicknesses of very exotic isotopes.

Nuclear Experiment