Low-lying octupole isovector excitation in ¹⁴⁴Nd

The nature of low-lying 3⁻ levels in ¹⁴⁴Nd was investigated in the ¹⁴³Nd(n, γγ) cold neutron-capture reaction. The combination of the high neutron flux from the research reactor at the Institut Laue-Langevin and the high γ-ray detection efficiency of the EXILL setup allowed the recording of γγ 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→3⁻₁ decay strongly supports the assignment of the 3⁻₃ level at 2779 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.