Lifetime measurements in Dy162

Abstract

Background: The nature of oscillations or excitations around the equilibrium deformed nuclear shape remains an open question in nuclear structure. The $^{162}\mathrm{Dy}$ nucleus is one of the most extensively studied nuclei with the $(n,\ensuremath{\gamma})$, $(n,{e}^{\ensuremath{-}})$, $(\ensuremath{\alpha},2n)$ 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 $^{162}\mathrm{Dy}$ nucleus, we set out to measure the level lifetimes.Method: Lifetimes were measured in the $^{162}\mathrm{Dy}$ 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 $^{162}\mathrm{Dy}$ nucleus. The lifetime of the ${K}^{\ensuremath{\pi}}={2}^{+}$ 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 $\ensuremath{\gamma}$-vibrational phonon of several Weisskopf units. The first excited ${K}^{\ensuremath{\pi}}={4}^{+}$ band, with a bandhead at 1535.66 keV, is strongly connected to the ${K}^{\ensuremath{\pi}}={2}^{+}$ band with enhanced collective $B(E2)$ values and it is consistent with a double phonon vibrational $(\ensuremath{\gamma}\ensuremath{\gamma})$ excitation. Lifetime of ${K}^{\ensuremath{\pi}}={0}^{+}$ band members have also been measured, including the ${4}_{{K}^{\ensuremath{\pi}}={0}_{2}^{+}}^{+}$ state at 1574.29 keV and the ${2}_{{K}^{\ensuremath{\pi}}={0}_{3}^{+}}^{+}$ state at 1728.31 keV. This latter state also displays the characteristics of a double phonon excitation built on the ${K}^{\ensuremath{\pi}}={2}^{+}$ 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 $({K}^{\ensuremath{\pi}}={4}^{+})$ and the 1728.312-keV ${2}^{+}$ state of a ${K}^{\ensuremath{\pi}}={0}^{+}$ band at 1666 keV connected with sizably collective $B(E2)$ values to the $({K}^{\ensuremath{\pi}}={2}^{+})\phantom{\rule{0.16em}{0ex}}\ensuremath{\gamma}$ band at 888 keV.