Lifetime measurements in the chiral-candidate doublet bands of ${}^{130}\mathrm{La}$

The lifetimes of the excited states belonging to the chiral-candidate doublet bands built on the $\pi h_{11/2}\otimes \nu h_{11/2}^{-1}$ configuration in ${}^{130}\mathrm{La}$ have been measured by applying the Doppler-shift attenuation method and the in-beam fast timing technique. Excited states were populated in the ${}^{121}\mathrm{Sb}({}^{12}\mathrm{C},3n)$ reaction at a bombarding energy of 54 MeV. The $7^{+}$ bandhead of the yrast band was identified as an isomeric state with a lifetime of $\tau \left(7^{+}\right)=0.38\left(7\right)\mathrm{ns}$. Similar reduced transition probabilities $B\left(M1\right)$ and $B\left(E2\right)$ were found for the states with the same spin in the partner bands. The results revealed an even-odd spin dependence in the $B\left(E2\right)$ values, whereas the $B\left(M1\right)$ values vary nearly monotonically with increasing spin. The experimental properties of the doublet bands in ${}^{130}\mathrm{La}$ are compared with theoretical calculations done in the frame of the two-quasiparticles-plus-triaxial-rotor model. The static chirality is not supported by the obtained data; however, chiral vibrations cannot be excluded.

Figure 1

Left: Partial level scheme of ${}^{130}\mathrm{La}$ showing the yrast and the side bands of the $\pi h_{11/2}\otimes \nu h_{11/2}^{-1}$ configuration, as well as the decay out of the yrast band, from Refs. [4, 30, 31, 32] and present work. New transitions observed in the present work are marked with a star. The widths of the arrows represent the relative intensity of the $\gamma$-ray transitions (from Ref. [32] and present experiment), with the internal conversion contribution indicated by the white component of the arrow. Energies are given in keV. Right: Expanded view of the lower part of the yrast band and the most intense transitions of its decay.

Figure 2

Experimental and calculated lineshapes for transitions deexciting the states in the yrast band of ${}^{130}\mathrm{La}$ with $J^{\pi }={14}^{+}$ (a), ${15}^{+}$ (b), and ${16}^{+}$ (c). The coincidence spectra were created with narrow gates on lower-lying transitions emitted from stopped nuclei. The fitted DSAM spectra are shown in red dashed lines.

Figure 3

Experimental and calculated lineshapes for transitions deexciting the states in the side band of ${}^{130}\mathrm{La}$ with $J^{\pi }={14}^{+}$ (a), ${16}^{+}$ (b), and ${18}^{+}$ (c). The coincidence spectra were created with narrow gates on lower-lying transitions emitted from stopped nuclei. Contaminant peaks are shown in dotted lines and the fitted DSAM spectra in red dashed lines.

Figure 4

(a) Energy projection of the $\gamma -\gamma$ coincidence HPGe matrix in the energy range from 100 to 400 keV. The transitions belonging to the yrast band and its decay are labeled by energy. (b)–(d) $\gamma$-ray spectra from the ${\mathrm{LaBr}}_3$(Ce) detectors with gates on selected transitions in the HPGe detectors.

Figure 5

Time difference spectra obtained from the ${\mathrm{LaBr}}_3$(Ce) $E_{\gamma 1}-E_{\gamma 2}-\mathrm{\Delta }T$ cube for selected pair of transitions in ${}^{130}\mathrm{La}$. Additional gates were put on appropriate $\gamma$ rays in the HPGe detectors.

Figure 6

Values of the energy separation $\mathrm{\Delta }E\left(J\right)$ (a), staggering $S\left(J\right)$ (b), $B\left(M1\right)/B\left(E2\right)$ ratio (c), reduced transition probability $B\left(M1\right)$ (d), and reduced transition probability $B\left(E2\right)$ (e), as a function of spin for the chiral candidate doublet bands in ${}^{130}\mathrm{La}$.

Figure 7

Comparison of the experimental energy separation with calculated values at different $\gamma$ deformations.

Figure 8

Experimental signature splitting compared with calculation using the two-quasiparticle-plus-triaxial-rotor model, without and with the residual proton-neutron interaction.

Figure 9

Comparison of the experimental $B\left(M1\right)$ and $B\left(E2\right)$ reduced transition strengths for in-band and interband transitions (black) compared with the calculated values using the TQRM for deformation parameters ${\epsilon }_2=0.2$ and $\gamma ={25}^{\circ }$ (red).