Low-lying octupole isovector excitation in ${}^{144}\mathrm{Nd}$

The nature of low-lying $3^{-}$ levels in ${}^{144}\mathrm{Nd}$ was investigated in the ${}^{143}\mathrm{Nd}(n,\gamma \gamma )$ cold neutron-capture reaction. The combination of the high neutron flux from the research reactor at the Institut Laue-Langevin and the high $\gamma$-ray detection efficiency of the EXILL setup allowed the recording of $\gamma \gamma$ coincidences. From the coincidence data precise branching ratios were extracted. Furthermore, the octagonal symmetry of the setup allowed angular-distribution measurements to determine multipole-mixing ratios. Additionally, in a second measurement the ultra-high resolution spectrometer GAMS6 was employed to conduct lifetime measurements using the gamma-ray induced Doppler-shift technique (GRID). The confirmed strong $M1$ component in the $3_3^{-}\to 3_1^{-}$ decay strongly supports the assignment of the $3_3^{-}$ level at $2779\mathrm{keV}$ as low-lying isovector octupole excitation. Microscopic calculations within the quasiparticle phonon model confirm an isovector component in the wave function of the $3_3^{-}$ level, firmly establishing this fundamental mode of nuclear excitation in near-spherical nuclei.

Figure 1

Partial level scheme of low-lying levels in ${}^{144}\mathrm{Nd}$. The $3_i^{-}\to 3_1^{-}$ transitions are marked in blue ($i=2$) and red ($i=3$). In addition to the decay schemes of the $3_2^{-}$ and $3_3^{-}$ levels, the $2_i^{+}$ level, from which the second 1268-keV transition originates, is given. This transition obscured the crucial $3_3^{-}\to 3_1^{-}$ transition in Ref. [21].

Figure 2

Partial $\gamma$-ray spectrum obtained by gating on the 814-keV transition depopulating the $3_1^{-}$ level (for a partial level scheme see Fig. 1). The peaks relevant for this work are labeled.

Figure 3

Angular correlation of the 1268-keV $3_3^{-}\to 3_1^{-}$ transition when gated on the 814-keV $3_1^{-}\to 2_1^{+}$ transition.

Figure 4

Experimental line shape (red) of the 2082.52-keV $3_3^{-}\to 2_1^{+}$ transition observed using GAMS6. Additionally, the instrument response curve (yellow) is shown.