Broken pairs and evolution of collectivity in the A

An in-beam \ensuremath{\gamma}-ray study performed with the $^{121}\mathrm{Sb}$${(}^{19}$F,4n) reaction has established the high spin level structure of the N=78 nucleus $^{138}\mathrm{Nd}$. States up to I=21\ensuremath{\Elzxh} and 8.5 MeV excitation energy have been indentified. High spin states are described in the framework of the cranking model and of the interacting boson model with broken pairs. The calculations reproduce levels up to I=18\ensuremath{\Elzxh} including the two ${10}^{+}$ states which, from the feeding cascades, are identified as the \ensuremath{\nu}${\mathit{h}}_{11/2}^{\mathrm{\ensuremath{-}}1}$ and \ensuremath{\pi}${\mathit{h}}_{11/2}^{2}$ excitations. Cranked Strutinsky type calculations predict opposite shapes for the two different types of excitations.