Neutron skin thickness of 90Zr determined by charge exchange reactions

Charge exchange spin-dipole (SD) excitations of Zr-90 are studied by the Zr-90(p,n) and Zr-90(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 Zr-90 is determined to be 0.07 +/- 0.04 fm from the SD sum rule value.