Evolution of collectivity to very high spins in 160 Yb N. R. Johnson, F. K. McGowan, D. F. Winchell,* C. Baktash, J. D. Garrett, and I. Y. Lee † Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 J. C. Wells Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 and Tennessee Technological University, Cookeville, Tennessee 38505 L. Chaturvedi, ‡ W. B. Gao, § and W. C. Ma  Vanderbilt University, Nashville, Tennessee 37235 S. Pilotte ¶ and C.-H. Yu** University of Tennessee, Knoxville, Tennessee 37996 Received 11 September 1995 Lifetimes of 160 Yb yrast states at high rotational frequencies have been measured by the Doppler broadened line shape technique. Excited states in 160 Yb were populated by the reaction 120 Sn 44 Ca,4 n  160 Yb at a beam energy of 200 MeV and the experimental measurements were carried out in the coincidence mode with an array of Compton-suppressed germanium detectors. The results for the 24 + to 32 + yrast states led to values of the transition quadrupole moments similar to those obtained for the lower members 0.29 MeV of the ground band in previous recoil-distance measurements. The trend of reducing Q t values found for the I =34 + and 36 + states are suggestive of band termination. The results are discussed in the light of current theoretical predictions. PACS numbers: 21.10.Re, 21.10.Tg, 23.20.Lv, 27.70+q I. INTRODUCTION During the past several years the properties of ytterbium nuclei have been investigated by numerous groups. We have carried out many of these experiments where we have con- centrated on the assessment of changes in the nuclear collec- tivity as a function of rotational frequency in ytterbium nu- clei between masses of 158 and 165. These studies have involved both spectroscopy measurements and lifetime deter- minations, but it is the latter that we have emphasized be- cause the lifetime of a nuclear state provides the dynamic electromagnetic multipole moment which contains key infor- mation on the nuclear collectivity. It is well known that the family of Yb nuclei possesses some rather interesting and unusual features. One striking pattern of behavior is found in an examination of a plot of the transition quadrupole moment ( Q t ) vs mass number A for the 2 + →0 + and 17/2 + →13/2 + transitions in Yb nuclei from mass 158 through mass 170. A steady and almost linear increase in collectivity in this family up to 165 Yb is seen, at which point the Q t values begin to level off. This is an in- teresting pattern of behavior when it is compared with that for other nuclei in this region, e.g., for the Gd or the Er nuclei. For the latter two families, there is a sharp increase in the collectivity going from neutron number 88 to N =90, followed by a rather gradual but nonlinear increase in collec- tivity as one approaches midshell. Not only do we find that the quadrupole deformation in these Yb nuclei shows no dramatic increase at neutron num- ber 90  160 Yb, but an examination of the predicted 1 total Routhian surface TRS of 160 Yb reveals that this nucleus is extremely shallow in both the  and  degrees of freedom, even more so than are its N =90 isotonic neighbors. Thus, we expect this very soft nucleus to be quite susceptible to polarization effects that may well change both its collectivity and its shape at high rotational frequencies. Several years ago we 2–4 carried out recoil-distance RD lifetime measurements on 160 Yb and its near isotopic neighbors as a testing of these ideas. The results of those studies are summarized in Fig. 1 where we have plotted tran- sition quadrupole moments vs rotational frequency  for transitions in 159 Yb, 160 Yb, and 161 Yb. These results indicate a rather significant loss of collectivity above a rotational fre- quency of about 0.25 MeV. For these nuclei, where the Fermi surface lies near the bottom of the i 13/2 shell, the lifetime results may be understood—at least qualitatively—in terms of cranked shell model CSM and cranked Hartree-Fock- Bogoliubov CHFB calculations e.g., see Refs. 3,5. After the backbend, these nuclei undergo a shape change driven by the aligned i 13/2 quasineutrons to a triaxial shape that is oriented so as to reduce the collectivity of the rotation i.e., * Present address: Department of Physics and Astronomy, Univer- sity of Pittsburgh, Pittsburgh, PA 15260. † Present address: Lawrence Berkeley National Laboratory, Berke- ley, CA 94720. ‡ Present address: Physics Department, Banaras Hindu University, Varanasi-221005, India. § Present address: University of Oklahoma, Norman, OK 73019.  Present address: Mississippi State University, Mississippi, MS 39762. ¶ Present address: Universite ´ Louis Pasteur, F-67037, Strasbourg CEDEX, France. ** Present address: Oak Ridge National Laboratory, Oak Ridge, TN 37831. PHYSICAL REVIEW C FEBRUARY 1996 VOLUME 53, NUMBER 2 53 0556-2813/96/532/6718/$06.00 671 © 1996 The American Physical Society