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Register a new userThe decay of quadrupole-octupole $1^-$ states in $^{40}$Ca and $^{140}$Ce
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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: $(vec{gamma},gamma)$ 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.
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Low-spin states of 157Dy have been studied using the JUROGAM II array, following the 155Gd ({alpha}, 2n) reaction at a beam energy of 25 MeV. The level scheme of 157Dy has been expanded with four new bands. Rotational structures built on the [523]5/2- and [402]3/2+ neutron orbitals constitute new additions to the level scheme as do many of the inter- and intra-band transitions. This manuscript also reports the observation of cross I- to (I-1)- and I- to (I-1)+ E1 dipole transitions inter-linking structures built on the [523]5/2- (band 5) and [402]3/2+ (band 7) neutron orbitals. These interlacing band structures are interpreted as the bands of parity doublets with simplex quantum number s = -i related to possible octupole correlations.
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