E2 decay strength of the M1 scissors mode of ¹⁵⁶Gd and its first excited rotational state

The E2/M1 multipole mixing ratio δ_1→2 of the 1⁺_sc→2⁺₁ γ-ray decay in ¹⁵⁶Gd and hence the isovector E2 transition rate of the scissors mode of a well-deformed rotational nucleus has been measured for the first time. It has been obtained from the angular distribution of an artificial quasimonochromatic linearly polarized γ-ray beam of energy 3.07(6) MeV scattered inelastically off an isotopically highly enriched ¹⁵⁶Gd target. The data yield first direct support for the deformation dependence of effective proton and neutron quadrupole boson charges in the framework of algebraic nuclear models. First evidence for a low-lying J^π=2⁺ member of the rotational band of states on top of the 1⁺ band head is obtained, too, indicating a significant signature splitting in the K=1 scissors mode rotational band.