Orbital magnetic dipole strength inSm148,150,152,154and nuclear deformation

Nuclear-resonance-fluorescence spectra have been measured in the chain of ${}_{}{}^{148,150,152,154}{}_{}{}^{}\mathrm{Sm}$ isotopes. Together with supplementary information from inelastic electron scattering and other reaction studies, orbital M1 transition strengths have been deduced for a number of $1^{+}$ states located around an excitation energy of 3 MeV. The systematic study, carried out for the first time, for nuclei within a large range of the deformation parameter δ shows that the orbital M1 strength varies quadratically with δ. This result is interpreted in terms of models containing explicitly neutron and proton degrees of freedom.