First observation of a rotational band and the role of the proton intruder orbital pi 1/2(+)[431] in very neutron-rich odd-odd Nb-106

A rotational band was observed for the first time in Nb-106 by means of gamma-gamma-gamma and gamma-gamma-gamma-gamma measurements of prompt fission gamma rays from Cf-252 by using the Gammasphere multidetector array. Projected shell model and potential-energy surface calculations were performed and were compared to the experimental data of Nb-106 and the previously reported Nb-104. Configurations and spin-parity were assigned to the ground level, low-lying levels, and rotational bandheads of the two Nb isotopes. The new rotational band in Nb-106 was interpreted as a K-pi = 2(-)band with a configuration pi 1/2(+)[431] Chi nu 5/2(-)[532]. The same spin-parity and configuration were assigned to the analogous band in Nb-104. pi 1/2(+)[431] Chi nu 5/2(+)[413], K-pi = 3(+) were assigned to the 0.66-mu s isomer and explained the M2 isomeric decay to the ground in 106Nb. The proton intruder orbital pi 1/2(+)[431] plays an important role in shape evolution with regard to triaxial deformation in these neutron-rich Nb isotopes.