Lifetime measurements in ${}^{162}\mathrm{Dy}$

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 $\left(n,\gamma \right)$, $\left(n,e^{-}\right)$, $\left(\alpha ,2n\right)$ reactions and most recently the $\left(p,t\right)$ 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 $K^{\pi }=2^{+}$ 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 $B\left(E2\right)$ values that are consistent with a single $\gamma$-vibrational phonon of several Weisskopf units. The first excited $K^{\pi }=4^{+}$ band, with a bandhead at 1535.66 keV, is strongly connected to the $K^{\pi }=2^{+}$ band with enhanced collective $B\left(E2\right)$ values and it is consistent with a double phonon vibrational $\left(\gamma \gamma \right)$ excitation. Lifetime of $K^{\pi }=0^{+}$ band members have also been measured, including the $4_{K^{\pi }=0_2^{+}}^{+}$ state at 1574.29 keV and the $2_{K^{\pi }=0_3^{+}}^{+}$ state at 1728.31 keV. This latter state also displays the characteristics of a double phonon excitation built on the $K^{\pi }=2^{+}$ 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 $\left(K^{\pi }=4^{+}\right)$ and the 1728.312-keV $2^{+}$ state of a $K^{\pi }=0^{+}$ band at 1666 keV connected with sizably collective $B\left(E2\right)$ values to the $\left(K^{\pi }=2^{+}\right)\gamma$ band at 888 keV.

Figure 1

Systematics of the first excited $K^{\pi }=2^{+}$ “$\gamma$” and $K^{\pi }=0^{+}$ bands in several isotopes of Sm, Gd, Dy, Er, Yb, and Hf as a function of neutron number “N” along with the observed $B(E2;2_{K=2^{+}}^{+}\to 0^{+})$ values for the $\gamma$ bands and the $B(E2;0_2^{+}\to 2_{\mathrm{g}.\mathrm{s}.}^{+})$ values for the first excited $K^{\pi }=0^{+}$ bands.

Figure 2

All levels of interest in ${}^{162}\mathrm{Dy}$ below 2 MeV. The 12 level lifetimes measured in this work are highlighted in red.

Figure 3

Partial decay scheme for the positive-parity $K^{\pi }$ levels in ${}^{162}\mathrm{Dy}$.

Figure 4

Partial decay scheme for the negative-parity $K^{\pi }$ levels in ${}^{162}\mathrm{Dy}$.