Suppression of band crossing in the neutron-rich nuclei Yb-172,Yb-173 due to the absence of a static pair field

High-spin states in the neutron-rich nuclei Yb-172,Yb-173 have been populated in a Er-170(Li-7,(p,d,t)chi n) incomplete-fusion reaction and the emitted gamma-radiation was detected with the GASP array. The signature partners of the 7/2(+)[633] rotational band of the odd-N Yb-173 isotope have been newly established and were observed up to spin values of (45/2(+)) and (43/2(+)), respectively. The ground-state band of the even-even nucleus Yb-172 has been observed up to a spin value of (22(+)). No band crossings were found in these bands. To explain this observation; it is proposed that the static pair field is absent; considering that the neutron odd-even mass differences reach for these nuclei very small values and that the band crossing is absent in cranked shell modell calculations without pairing. The results indicate, however, that strong dynamic correlations are still present.