The decay of quadrupole-octupole $1^{-}$ states in ${}^{40}$Ca and ${}^{140}$Ce

Abstract

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 $\gamma$-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: $(\vec{\gamma },{\gamma }^{\prime })$ experiments have been carried out at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility in combination with the high-efficiency $\gamma$-ray spectroscopy setup ${\gamma }^3$ consisting of HPGe and LaBr${}_3$ detectors. The setup enables the acquisition of $\gamma$-$\gamma$ 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.

Comments: 11 pages, 6 figures, as accepted in Phys. Rev. C