Abstract
A detailed study of the positive parity yrast and yrare rotational bands in
77Rb is presented. Using the reaction
40Ca(
40Ca, 3
p) and the EUROGAM I spectrometer, γγγ 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 (
ϵ
2 ≈ 0.38) is crossed by a less deformed one (
ϵ
2 ≈ 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.