Evidence For Triaxial Deformation Near N=86: Collective Bands In Dy-152,Dy-153 And Ho-153

The N=86,87 isotopes of dysprosium and holmium have been investigated using the Eurogam II gamma-ray spectrometer. A new collective rotational band has been observed in Ho-153 and the previously observed nui(13/2) band in Dy-153 has been extended to much higher spin. Comparing these bands and similar bands in Dy-152 with the collective bands in the N=90 isotopes of Dy, Ho, and Er suggests that the first h(11/2) proton crossing is not observed at N=86,87. This observation has led to the reinterpretation of four of the previously observed bands in Dy-152. It is proposed that the low deformation collective bands in Dy-152,Dy-153 and Ho-153 are associated with a triaxial shape with large positive gamma and that they have a similar origin to the recently observed triaxial superdeformed bands in Lu163-165.