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E 2 decay strength of the M 1 scissors mode of The E2/M 1 multipole mixing ratio δ1→2 of the 1 + sc → 2 + 1 γ-ray decay in 156 Gd and hence the isovector E2 transition rate of the scissors mode of a well-deformed rotational nucleus has been measured for the first time. It has been obtained from the angular distribution of an artificial quasimonochromatic linearly polarized γ-ray beam of energy 3.07(6) MeV scattered inelastically off an isotopically highly-enriched 156 Gd target. The data yield first direct support for the deformation dependence of effective proton and neutron quadrupole boson charges in the framework of algebraic nuclear models. First evidence for a low-lying J π = 2 + member of the rotational band of states on top of the 1 + band head is obtained, too, indicating a significant signature splitting in the K = 1 scissors mode rotational band.Introduction. -Orbital out-of-phase oscillations of a coupled two-component many-body quantum system are generally called Scissors Modes (ScMs). A ScM has been discovered in deformed atomic nuclei [1]. It has later been identified in Bose-Einstein condensed gases [2,3] and is expected to occur in Fermi gases [4], in metallic clusters [5][6][7], and in deformed quantum dots [8]. ScMs are interesting quantum modes because their properties are sensitive to the restoring forces between the many-body subsystems. They inevitably break spherical symmetry and hence lead to a sequence of quantum states of the many-body system that form a rotational band.

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The low-lying M 1-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 γ-ray Source (HIγS) 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 f p-shell.

Background: The low-lying electric dipole strength provides insights on the parameters of the nuclear equation of state via its connection with the pygmy dipole resonance and nuclear neutron skin thickness.Purpose: 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 Cr were measured up to an excitation energy of 9.7 MeV via the nuclear resonance fluorescence method using γ-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 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: In the A ≈ 50 mass region M1 spin-flip transitions are prominent around 9 MeV. An accumulation of 1 − states between 5 and 8 MeV generating additional E1 strength, also denoted as Pygmy Dipole Resonance (PDR), has been established in many nuclei with neutron excess within the last decade. Purpose: The γ-decay behavior of J = 1 states has been investigated in an NRF experiment. M1 excitations have been compared to shell model calculations. Methods: J = 1 states were excited by quasi-monoenergetic, linearly polarized γ-ray beams generated by Laser-Compton backscattering at the HIγS facility, Durham, NC, USA. Depopulating γ-rays were detected with the multi-detector array γ 3 . Results: For eleven beam-energy settings the γ-decay behavior of dipole states was analyzed by a state-to-state analysis and average γ-decay branching ratios have been investigated. 34 parity quantum numbers were assigned to J = 1 states. Conclusions: Six 1 − states and two 1 + states have been investigated in NRF experiments for the first time. The M1 strength distribution is in good agreement with shell-model calculations.

The excitation of atomic nuclei via magnetic dipole transitions is closely related to the inelastic neutral-current neutrino-nucleus (NC-νA) scattering process due to the similarity of the transition operators. NC-νA-scattering serves for the detection of supernova neutrinos and poses a significant source of background in modern liquid-argon based high-energy neutrino detection experiments. In order to enable tests of the reliability of predictions for neutrino-nucleus scattering, the magnetic dipole response of 40 Ar below 7.7 MeV was characterized in a nuclear resonance fluorescence experiment using quasi-monoenergetic gamma-ray beams. The linear polarization of the beams allowed for assignments of electric or magnetic character to previously known dipole excitations. A total magnetic dipole strength of 0.36 +0.04 −0.05 µ 2 N was identified in the energy range of the present experiment. Combined with data from previous measurements, the full magnetic dipole strength of 40 Ar below the neutron separation threshold was investigated. Due to the low background in the energy range within the bandwidth of the gamma-ray beams, the previous sensitivity limit was improved. A large-scale nuclear shell model calculation in the sd-f p space satisfactorily agrees with the data in terms of excitation energies and strengths of the observed 1 + states. CONTENTS

Recent high-resolution Nuclear Resonance Fluorescence experiments performed on the even-even Chromium isotopes 50,52,54 Cr have lead to the identification (energy, spin, parity and transition strength) of altogether 108 nuclear levels of spin J=1 (70 levels with J π = 1 − and 38 with J π = 1 + ) at excitation energies Ex ranging roughly from 4.5 to 9.7 MeV. In this region just above the orbital magnetic dipole Scissors Mode sizable spin-flip magnetic dipole strength as well as electric dipole strength belonging to the Pygmy Dipole Resonance (PDR) is expected. Using statistical measures for short-and long-range correlations, we perform an analysis of the fluctuation properties in the subspectra of the energy levels and also of the distributions of their respective dipole transition strengths. We compare the results with those of a random matrix ensemble interpolating between Poisson statistics generally describing the fluctuation properties in the energy spectra of many-body systems with collective, i.e., regular motion of the particles and the Gaussian orthogonal ensemble (GOE) for complex, i.e., chaotic behavior. This comparison reveals that the spectral properties of the 1 + states are close to the GOE results while those of the 1 − states are closer to Poisson. This is confirmed by an analysis of the spectral fluctuations based on the method of Bayesian inference and corroborated by large-scale shell-model and quasiparticle-phonon model calculations, respectively. The nearly Poissonian behavior of the 1 − levels suggests a sizable collectivity of the PDR indeed.