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Background: Within the last decade, below the giant dipole resonance the existence of a concentration of additional electric dipole strength has been established. This accumulation of low-lying E1 strength is commonly referred to as pygmy dipole resonance (PDR). Purpose: The photoresponse of 60 Ni has been investigated experimentally and theoretically to test the evolution of the PDR in a nucleus with only a small neutron excess. Furthermore, the isoscalar and isovector M1 resonances were investigated. Method: Spin-1 states were excited by exploiting the (γ, γ ) nuclear resonance fluorescence technique with unpolarized continuous bremsstrahlung as well as with fully linearly polarized, quasimonochromatic, Comptonbackscattered laser photons in the entrance channel of the reaction. Results: Up to 10 MeV a detailed picture of J = 1 levels was obtained. For the preponderant number of the individual levels spin and parity were firmly assigned. Furthermore, branching ratios, transition widths, and reduced B(E1) or B(M1) excitation probability were calculated from the measured scattering cross sections. A comparison with theoretical results obtained within the quasiparticle phonon model allows an insight into the microscopic structure of the observed states. Conclusions: Below 10 MeV the directly observed E1 strength [ B(E1) ↑= (153.8 ± 9.5) e 2 (fm) 2 ] exhausts 0.5% of the Thomas-Reiche-Kuhn sum rule. This value increases to 0.8% of the sum rule [ B(E1) ↑= (250.9 ± 31.1) e 2 (fm) 2 ] when indirectly observed branches to lower-lying levels are considered. Two accumulations of M1 excited spin-1 states near 8 and 9 MeV excitation energy are identified as isoscalar and isovector M1 resonances dominated by proton and neutron f 7/2 → f 5/2 spin-flip excitations. The B(M1) ↑ strength of these structures accumulates to 3.94(27)μ 2 N .

The low-lying dipole strength of the open-shell nucleus 94 Mo was studied via the nuclear resonance fluorescence technique up to 8.7 MeV excitation energy at the bremsstrahlung facility at the Superconducting Darmstadt Electron Linear Accelerator (S-DALINAC), and with Compton backscattered photons at the High Intensity γ-ray Source (HIγ S) facility. In total, 83 excited states were identified. Exploiting polarized quasi-monoenergetic photons at HIγ S, parity quantum numbers were assigned to 41 states excited by dipole transitions. The electric dipole-strength distribution was determined up to 8.7 MeV and compared to microscopic calculations within the quasiparticle phonon model. Calculations and experimental data are in good agreement for the fragmentation, as well as for the integrated strength. The average decay pattern of the excited states was investigated exploiting the HIγ S measurements at five energy settings. Mean branching ratios to the ground state and first excited 2 + 1 state were extracted from the measurements with quasi-monoenergetic photons and compared to γ-cascade simulations within the statistical model. The experimentally deduced mean branching ratios exhibit a resonance-like maximum at 6.4 MeV which cannot be reproduced within the statistical model. This indicates a nonstatistical structure in the energy range between 5.5 and 7.5 MeV.

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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.