Search for octupole correlations in ${}^{147}\mathrm{Nd}$

Properties of excited states in ${}^{147}\mathrm{Nd}$ have been studied with the multispectra and $\gamma \gamma$ coincidence measurements. Twenty-four new $\gamma$ lines and three new levels have been introduced into the level scheme of ${}^{147}\mathrm{Nd}$. Using the advanced time-delayed $\beta \gamma \gamma$($t$) method, we measured lifetimes of eight excited levels in ${}^{147}\mathrm{Nd}$, populated via the $\beta$ decay of ${}^{147}\mathrm{Pr}$. We have determined reduced transition probabilities for 30 $\gamma$ transitions. Multidimensional potential energy surface calculations performed for ${}^{147}\mathrm{Nd}$ suggest two single-quasiparticle configurations with nonzero octupole deformation, with $K=1/2$ and $K=5/2$. Our calculations also predict a sizable value of the electric dipole moment $|D_0|=0.26e$ fm for this nucleus, while experimentally, a lower limit of $|D_0|\ge 0.02e$ fm has been evaluated for the supposed $K=1/2$ parity doublet. In contrast to the theoretical results, we do not observe the parity doublet bands with $K=5/2$. This, and the lack of theoretically expected $E1$ strength in ${}^{147,149}\mathrm{Nd}$ may signal some poorly understood structural effect in the odd-$N$ lanthanides.

Figure 1

Examples of the decay curves of $\gamma$ rays from the $\beta$ decay of ${}^{147}\mathrm{Pr}$ obtained in the MSS measurements.

Figure 2

Coincident $\gamma$-ray spectra from the Ge and ${\mathrm{BaF}}_2$ detectors. Left top panel shows spectrum from the 80% HPGe detector gated by the 86.7 keV line in the LEP detector. Right top panel shows spectrum from the 30% HPGe detector gated by the 249.0 keV line in the LEP detector. Panels at the bottom show the corresponding coincident spectra sorted onto the ${\mathrm{BaF}}_2$ detector from the $\beta$-Ge-${\mathrm{BaF}}_2$ data.

Figure 3

Coincident $\gamma$-ray spectra from the Ge and ${\mathrm{BaF}}_2$ detectors. Left top panel shows spectrum from the LEP detector gated by the 335.7 keV line in the 30% HPGe detector. Right top panel shows spectrum from the LEP detector gated by the 578.0 keV line in the 80% HPGe detector. Panels at the bottom show the corresponding coincident spectra from the ${\mathrm{BaF}}_2$ detector.

Figure 4

Part I of the level scheme of ${}^{147}\mathrm{Nd}$ from the $\beta$ decay of ${}^{147}\mathrm{Pr}$. Full and empty circles indicate strong and weak $\gamma \gamma$ coincidence relations, respectively. Dotted lines mark transitions reported in Ref. [14] but not observed in our data due to limited statistics. Asterisks indicate new levels and transitions observed in this work in the $\beta$ decay of ${}^{147}\mathrm{Pr}$.

Figure 5

Part II of the level scheme of ${}^{147}\mathrm{Nd}$ from the $\beta$ decay of ${}^{147}\mathrm{Pr}$.

Figure 6

Part III of the level scheme of ${}^{147}\mathrm{Nd}$ from the $\beta$ decay of ${}^{147}\mathrm{Pr}$.

Figure 7

Time-delayed $\beta \gamma \gamma$($t$) spectra showing slopes due to the lifetimes of the 127.9, 214.6, and 314.7 keV levels in ${}^{147}\mathrm{Nd}$. Each figure shows experimental points, prompt Gaussian spectrum and slope curve, which was fitted in the deconvolution process.

Figure 8

Determination of the half-life for the 463.6 keV level in ${}^{147}\mathrm{Nd}$ by centroid shift technique. The prompt time reference curve (solid circles) made for the ${}^{138}\mathrm{Cs}$ calibration source is shown together with the centroid positions of two time-delayed spectra marked by the energies (in keV) of $\gamma$-ray gates in the Ge and ${\mathrm{BaF}}_2$ detectors which were used to generate them: 329–249 (empty squares) and 329–414 (full triangles). The shift of the time-delayed centroid from the prompt reference curve gives the mean-life of the level (calibration 12.85 ps/ch); see text for details.

Figure 9

Interpretation of the band structure in ${}^{147}\mathrm{Nd}$.

Figure 10

Potential energy surfaces calculated for the lowest single quasiparticle configurations in ${}^{147}\mathrm{Nd}$ with $K=1/2$, 3/2, and 5/2 and single particle state numbers $Nst=42$, 43, 44, and 45. The energy distance between the contour lines is equal to 0.5 MeV.