Decay of quadrupole-octupole1−states inCa40andCe140

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 $\ensuremath{\gamma}$-decay behavior of candidates for the ${({2}_{1}^{+}\ensuremath{\bigotimes}{3}_{1}^{\ensuremath{-}})}_{{1}^{\ensuremath{-}}}$ state in the doubly magic nucleus $^{40}\mathrm{Ca}$ and in the heavier and semimagic nucleus $^{140}\mathrm{Ce}$ is investigated.Methods: $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}})$ experiments have been carried out at the High Intensity $\ensuremath{\gamma}$-ray Source $(\mathrm{HI}\ensuremath{\gamma}\mathrm{S})$ facility in combination with the high-efficiency $\ensuremath{\gamma}$-ray spectroscopy setup ${\ensuremath{\gamma}}^{3}$ consisting of HPGe and ${\mathrm{LaBr}}_{3}$ detectors. The setup enables the acquisition of $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence data and, hence, the detection of direct decay paths.Results: In addition to the known ground-state decays, for $^{40}\mathrm{Ca}$ the decay into the ${3}_{1}^{\ensuremath{-}}$ state was observed, while for $^{140}\mathrm{Ce}$ 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 $^{44}\mathrm{Ca}$ were deduced simultaneously.Conclusions: The experimental findings together with the theoretical calculations support the two-phonon character of the ${1}_{1}^{\ensuremath{-}}$ excitation in the light-to-medium-mass nucleus $^{40}\mathrm{Ca}$ as well as in the stable even-even $N=82$ nuclei.