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Background: The low-lying electric dipole strength provides insights into the parameters of the nuclear equation of state via its connection with the pygmy dipole resonance and nuclear neutron skin thickness.Purpose: The aim was to complement the systematic of the pygmy dipole resonance and first study its behavior across the $N=28$ neutron shell closure.Methods: Photon-scattering cross sections of states of $^{50,54}\mathrm{Cr}$ were measured up to an excitation energy of 9.7 MeV via the nuclear resonance fluorescence method using $\ensuremath{\gamma}$-ray beams from bremsstrahlung and Compton backscattering.Results: Transitions strengths, spin and parity quantum number, and average branching ratios for 55 excited states, 44 of which were observed for the first time, were determined. The comparison between the total observed strengths of the isotopes $^{50,52,54}\mathrm{Cr}$ shows a significant increase above the shell closure.Conclusions: The evolution of the pygmy dipole resonance is heavily influenced by the shell structure.

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.

The low-lying $M1$-strength of the open-shell nucleus $^{50}$Cr has been studied with the method of nuclear resonance fluorescence up to 9.7 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC and Compton backscattered photons at the High Intensity $\gamma$-ray Source (HI$\gamma$S) facility between 6 and 9.7 MeV of the initial photon energy. Fifteen $1^{+}$ states have been observed between 3.6 and 9.7 MeV. Following our analysis, the lowest $1^{+}$ state at 3.6 MeV can be considered as an isovector orbital mode with some spin admixture. The obtained results generally match the estimations and trends typical for the scissors-like mode. Detailed calculations within the Skyrme Quasiparticle Random-Phase-Approximation method and the Large-Scale Shell Model justify our conclusions. The calculated distributions of the orbital current for the lowest $1^{+}$-state suggest the schematic view of Lipparini and Stringari (isovector rotation-like oscillations inside the rigid surface) rather than the scissors-like picture of Lo Iudice and Palumbo. The spin M1 resonance is shown to be mainly generated by spin-flip transitions between the orbitals of the $fp$-shell.

Background: One-phonon mixed-symmetry quadrupole excitations are a well-known feature of near-spherical, vibrational nuclei. Their interpretation as a fundamental building block of vibrational structures is supported by the identification of multiphonon states resulting from a coupling of fully-symmetric and mixed-symmetric quadrupole phonons. In addition, the observation of strong M1 transitions between low-lying 3(-) and 4(+) states has been interpreted as an evidence for one-phonon mixed-symmetry excitations of octupole and hexadecapole character. Purpose: The aim of the present study is to identify collective one-and two-phonon excitations in the heaviest stable N = 52 isotone Ru-96 based on a measurement of absolute M1, E1, and E2 transition strengths. Methods: Inelastic proton-scattering experiments have been performed at the Wright Nuclear Structure Laboratory (WNSL), Yale University, and the Institute for Nuclear Physics (IKP), University of Cologne. From the acquired proton-gamma and gamma gamma coincidence data we deduced spins of excited states, gamma-decay branching ratios, and multipole mixing ratios, as well as lifetimes of excited states via the Doppler-shift attenuation method (DSAM). Results: Based on the new experimental data on absolute transition strengths, we identified the 2(+) and 3(+) members of the two-phonon mixed-symmetry quintuplet (2(1,ms)(+) circle times 2(1,s)(+)). Furthermore, we observed strong M1 transitions between low-lying 3(-) and 4(+) states suggesting one-phonon symmetric andmixed-symmetric octupole and hexadecapole components in their wave functions, respectively. The experimental results are compared to sdg-IBM-2 and shell-model calculations. Conclusions: Both the sdg-IBM-2 and the shell-model calculations are able to describe key features of mixed-symmetry excitations of Ru-96. Moreover, they support the one-phonon mixed-symmetry hexadecapole assignment of the experimental 4(2)(+) state.

The low-lying positive- and negative-parity states in $^{168}\mathrm{Yb}$ have been investigated by means of the $(\ensuremath{\alpha},2\mathrm{n}\ensuremath{\gamma})$ fusion evaporation reaction. Using the coincidence method, the level scheme was corrected and extended up to 3 MeV, for both the positive- and negative-parity states. Using the new branching ratios determined in the present experiment, the $K$ quantum number was proposed for two negative-parity bands by direct comparison with the Alaga rule. Like in some other nuclei, one negative-parity band was established, decaying predominantly to the $\ensuremath{\gamma}$-vibrational band. In a second experiment, the lifetimes of the low-lying excited states up to ${J}^{\ensuremath{\pi}}={6}^{+}$ in the ground-state band were measured by using the in-beam fast-timing method with the Bucharest mixed high-purity germanium (HPGe) and ${\mathrm{LaBr}}_{3}$:Ce detector array using the triple-$\ensuremath{\gamma}$ coincidence method. Reduced $E2$ transition probabilities were extracted from the measured lifetimes and compared with the corresponding observables in neighboring isotopes, showing a smooth behavior with increasing mass. The positive- and negative-parity states as well as $E1$ and $E2$ transition probability ratios revealed by these experiments are compared with the interacting boson model in the $sd$ and $spdf$ boson space, and with the confined beta-soft rotor model, and are found to be in good agreement.