  Charge exchange spin-dipole (SD) excitations of 90Zr are studied by the
90Zr(p,n) and 90Zr(n,p) reactions at 300 MeV. A multipole decomposition
technique is employed to obtain the SD strength distributions in the cross
section spectra. For the first time, a model-independent SD sum rule value is
obtained: 148+/-12 fm^2. The neutron skin thickness of 90Zr is determined to be
0.07+/-0.04 fm from the SD sum rule value.
