Electromagnetic M1 transition strengths from inelastic proton scattering: The cases of Ca 48 and Pb 208

Inelastic proton scattering at energies of a few hundred million electron volts and extreme forward angles selectively excite the isovector spin-flip M1 (IVSM1) resonance. A method based on isospin symmetry is presented to extract its electromagnetic transition strength from the (p,p′) cross sections. It is applied to Ca48, a key case for an interpretation of the quenching phenomenon of the spin-isospin response, and leads to a M1 strength consistent with an older (e,e′) experiment excluding the almost two times larger value from a recent (γ,n) experiment. Good agreement with electromagnetic probes is observed in Pb208, suggesting the possibility of extracting systematic information on the IVSM1 resonance in heavy nuclei.