Lifetimes and electromagnetic transition strengths in ${}^{157}\mathrm{Dy}$

Lifetimes of excited states of ${}^{157}\mathrm{Dy}$ were measured by recoil distance Doppler-shift (RDDS) and Doppler-shift attenuation (DSAM) methods. The $\gamma$-ray coincidence data were analyzed by the differential decay curve method. The level scheme and the newly deduced transition strengths were simultaneously described in the framework of the particle plus triaxial rotor model (PTRM) and the rigid-rotor model.

Figure 1

DSAM spectra of transitions in the yrast band of ${}^{157}\mathrm{Dy}$ obtained by setting a gate on the full $\gamma$ line shape of the transitions lying below the levels of interest. The gates $(\gamma$-ray energies in keV) and the detector rings of observation are also indicated.

Figure 2

Example of lifetime determination for the $21/2^{+}$ level in yrast band of ${}^{157}\mathrm{Dy}$ using gated spectra measured with the detectors at $34.6^{\circ }$. The gate is set on the shifted component of the 411-keV transition which directly feeds the level of interest. The left part of the figure presents fits of the line shape of the analyzed 311-keV $\gamma$-ray transition at different distances. The solid black line is the full fit. The fractions of the line shape related to the unshifted peak (dashed green line) and DSA effects (dotted red line) are shown. The right part of the figure illustrates the lifetime determination. On top, the $\tau$ curve derived is displayed together with a fit with a horizontal line within the region of sensitivity. The uncertainties of the latter are also shown. For details, see the text.

Figure 3

The same as in Fig. 2 but for the 411-keV transition, which depopulates the $25/2^{+}$ level. The gated spectra are measured with the detectors at $145.4^{\circ }$.

Figure 4

The line-shape analysis of the 762-keV transition from yrast band of ${}^{157}\mathrm{Dy}$ (Ii $\to$ If transition) and extraction of the lifetime of the $I^{\pi }=49/2^{+}$ level at 5004.1 keV [cf. Eq. (2)]. On the left-hand side are shown fits of the line shapes measured at the angles of ${35}^{\circ },{59}^{\circ },{121}^{\circ }$, and ${145}^{\circ }$ with respect to the beam axis. Dashed lines show the background as well as small contaminant peaks. The decay functions are presented in the middle with their statistical uncertainties. The decay function of the feeding transition of 803 keV is also shown as determined. It is determined independently and its area is normalized to that of the decay function of the 762-keV transition. The $\tau$ curves and their statistical uncertainties are presented on the right-hand side. The lifetimes ${\tau }_{\theta }$ derived at the four detector angles and their statistical errors are also displayed. The final adopted value of the lifetime is $\langle \tau \rangle =199\pm 7$ fs.

Figure 5

Comparison of the experimental and calculated within the PTRM level schemes of ${}^{157}\mathrm{Dy}$. The numbers on the arrows of the $\mathrm{\Delta }I=2$ transitions are the $B\left(E2\right)$ values in units ${\text{e}}^2{\text{b}}^2$. For the theoretical $\mathrm{\Delta }I=1$ transitions of mixed multipolarity, the $B\left(E2\right)$ values are given in ${\text{e}}^2{\text{b}}^2$ (top) and the $B\left(M1\right)$ values are in ${\mu }_N^2$ (bottom). See also text.

Figure 6

Reduced $B(E2,I\to I-2)$ transition strengths in the yrast band. The results of the RDDS (filled circles) and the DSAM measurements (open squares) are presented together and compared to the results from the rigid-rotor model with the indicated parameters.