Hexadecapole transition moments in strongly deformed nuclei and the validity of the sd-boson approximation in the IBA

The low-lying spectrum of 156Gd up to 3 MeV has been investigated with proton scattering at Ep = 40 MeV. Considerable hexadecapole transition strength is found at Ex>1.5 MeV indicating the necessity for the inclusion of a g-boson in the IBA to explain properties of states above this energy. An IBA calculation including a g-boson reproduces qualitatively the experimentally observed hexadecapole transition strength below 3 MeV