Loss of Collectivity at High Spin inW172and a Three-Band Interpretation of First Yrast Upbends

A measurement of lifetimes in ${}_{}{}^{172}{}_{}{}^{}\mathrm{W}$ by the Doppler-shift recoil-distance technique indicates a reduction of the transition quadrupole moments by about 15% for spins ≥ 12. This can be understood if the rotation-induced structure change is due not only to $i_{\frac{13}{2}}$ neutrons, which is the standard theory for this region of nuclei, but also to $h_{\frac{9}{2}}$ protons. The resulting three-band interpretation of the "upbend" in the yrast level sequence is supported by energy-level systematics suggesting that it is valid for a wide range of nuclei in this region.