A New Type of Band Crossing at Large Deformation

A detailed study of the positive parity yrast and yrare rotational bands in Rb-77 is presented. Using the reaction Ca-40(Ca-40,3p) and the EUROGAM I spectrometer, gamma gamma gamma coincidences enabled us to follow both bands over a large spin range and to measure many E2 strengths. The moments of inertia and transition quadrupole moments indicate that a more deformed band (epsilon(2) approximate to 0.38) is crossed by a less deformed one (epsilon(2) approximate to 0.29), Since the yrare band starts at the extremely low spin value of I = 9/2, the conventional band crossing mechanism of two aligning high-j quasiparticles is excluded. The frequency-dependent equilibrium shapes were calculated with Nilsson-Strutinsky type calculations using a diabatic tracing of configurations near the neutron Fermi level, This is the first observation of such a band crossing.