The decay of quadrupole-octupole $1^-$ states in $^{40}$Ca and $^{140}$Ce
V. Derya, N. Tsoneva, T. Aumann, M. Bhike, J. Endres, M. Gooden, A. Hennig, J. Isaak, H. Lenske, B. Löher, N. Pietralla, D. Savran, W. Tornow, V. Werner, A. Zilges
Background: Two-phonon excitations originating from the coupling of two collective one-phonon states are of great interest in nuclear structure physics. One possibility to generate low-lying $E1$ excitations is the coupling of quadrupole and octupole phonons.
Purpose: In this work, the $γ$-decay behavior of candidates for the $(2_1^+\otimes 3_1^-)_{1^-}$ state in the doubly-magic nucleus $^{40}$Ca and in the heavier and semi-magic nucleus $^{140}$Ce is investigated.
Methods: $(\vecγ,γ')$ experiments have been carried out at the High Intensity $γ$-ray Source (HI$γ$S) facility in combination with the high-efficiency $γ$-ray spectroscopy setup $γ^3$ consisting of HPGe and LaBr$_3$ detectors. The setup enables the acquisition of $γ$-$γ$ coincidence data and, hence, the detection of direct decay paths.
Results: In addition to the known ground-state decays, for $^{40}$Ca the decay into the $3^-_1$ state was observed, while for $^{140}$Ce the direct decays into the $2^+_1$ and the $0^+_2$ state were detected. The experimentally deduced transition strengths and excitation energies are compared to theoretical calculations in the framework of EDF theory plus QPM approach and systematically analyzed for $N=82$ isotones. In addition, negative parities for two $J=1$ states in $^{44}$Ca were deduced simultaneously.
Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the $1^-_1$ excitation in the light-to-medium-mass nucleus $^{40}$Ca as well as in the stable even-even $N=82$ nuclei.
Anharmonicity of multi-octupole-phonon excitations in $^{208}$Pb: analysis with multi-reference covariant density functional theory and subbarrier fusion of $^{16}$O+$^{208}$Pb
