Lifetime measurements in Dy 162

Background: The nature of oscillations or excitations around the equilibrium deformed nuclear shape remains an open question in nuclear structure. The Dy162 nucleus is one of the most extensively studied nuclei with the (n,γ), (n,e-), (α,2n) reactions and most recently the (p,t) pickup reaction adding 11 0+ states to an excitation energy of 2.8 MeV to an already-well-developed level scheme. However, a major shortfall for a better understanding of the nature of the plethora of bands and levels in this nucleus has been the lack of lifetime measurements. Purpose: To determine the character of the low-lying excited bands in this Dy162 nucleus, we set out to measure the level lifetimes. Method: Lifetimes were measured in the Dy162 nucleus following neutron capture using the Gamma-Ray-Induced Doppler (GRID) broadening technique at the Institut Laue-Langevin in Grenoble, France. Results: In total, we have measured the lifetimes of 12 levels belonging to a number of excited positive- and negative-parity bands in the low-lying spectrum of the Dy162 nucleus. The lifetime of the Kπ=2+ bandhead at 888.16 keV was previously measured. We confirm this value and measure lifetimes of the 3+ and 4+ members of this band yielding B(E2) values that are consistent with a single γ-vibrational phonon of several Weisskopf units. The first excited Kπ=4+ band, with a bandhead at 1535.66 keV, is strongly connected to the Kπ=2+ band with enhanced collective B(E2) values and it is consistent with a double phonon vibrational (γγ) excitation. Lifetime of Kπ=0+ band members have also been measured, including the 4Kπ=02++ state at 1574.29 keV and the 2Kπ=03++ state at 1728.31 keV. This latter state also displays the characteristics of a double phonon excitation built on the Kπ=2+ band. Conclusions: We discuss our findings in terms of the presence or absence of collective quadrupole and octupole vibrational excitations. We find two positive-parity excited bands at 1535.66 keV (Kπ=4+) and the 1728.312-keV 2+ state of a Kπ=0+ band at 1666 keV connected with sizably collective B(E2) values to the (Kπ=2+)γ band at 888 keV.