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We report on our recent nuclear resonance fluorescence experiments on ¹⁵²,¹⁵⁴,¹⁵⁶Gd. Decay branches of the scissors mode to intrinsic excitations are observed. They are interpreted as a new signature for a spherical-to-deformed nuclear shape phase transition.

Photon-scattering experiments off the nuclei ⁹⁴Zr and ⁹⁶Zr were performed at the 10-MeV bremsstrahlung facility DHIPS at the super-conducting electron linear accelerator S-DALINAC at Darmstadt. The dipole excitation-strength distributions were determined in an energy range between 2 and 8 MeV, including the 1⁻ quadrupole-octupole coupled Two-Phonon-State and parts of the Pygmy Dipole Resonance. In total 74 transitions in ⁹⁶ Zr were observed and for most of the excited states the spin quantum number could be firmly assigned to J = 1. Calculations within the Quasiparticle Phonon Model show a good agreement.

Background: Numerous studies of the ground-state decay of the pygmy dipole resonance (PDR) have been carried out in the past. However, data on the decay of the PDR to low-lying excited states is still very scarce due to limitations of the sensitivity to weak branching transitions of experimental setups. Purpose: We present a detailed examination of the low-energy dipole response of Te-128 and Te-130 below their neutron separation thresholds of 8.8 and 8.5 MeV, respectively. Methods: Photonuclear reactions with the subsequent gamma-ray spectroscopy of the decay channel with continuous-energy bremsstrahlung at varying endpoint energies and linearly polarized quasimonochromatic gamma-ray beams with energies ranging from 2.7 to 8.9 MeV in steps of roughly 250 keV were used for probing the decay behavior of the low-energy dipole response in Te-128 and Te-130. In addition, ((gamma) over right arrow, gamma ' gamma '') reactions were used to study the population of low-lying states of Te-128. Results: Spin-parity quantum numbers and reduced transition probabilities are determined for individual photo-excited states. The analysis of average decay properties for nuclear levels in narrow excitation-energy bins enable the extraction of photoabsorption cross sections, average branching ratios to the 2(1)(+) state, and the distinction between E1 and M1 transitions to the ground state and to the 2(1)(+) state accounting for resolved and unresolved transitions. Conclusions: Above 5 MeV, the experimental data are in reasonable agreement to calculations within the quasiparticle phonon model. The major fraction of the ground-state decay channel is due to E1 transitions, while less than 5-10% stem from M1 transitions. Furthermore, first direct experimental evidence is provided that the population of the 2(1)(+) state of Te-128 via primary gamma-ray transitions from excited states in the PDR region from 5 to 9 MeV is dominated by E1 transitions of 1(-) states.

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink-Axel hypothesis and the related concept of so-called photon strength functions to describe gamma-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasimonochromatic photon beams and a newly implemented gamma-gamma coincidence setup. This technique does not assume a priori the validity of the Brink-Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus Te-128 were obtained for y-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink-Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink-Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold S-n= 8.78 MeV) for the studied case of Te-128. (C) 2018 The Authors. Published by Elsevier B.V.

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 Cr-50,Cr-54 were measured up to an excitation energy of 9.7 MeV via the nuclear resonance fluorescence method using 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 Cr-50,Cr-52,Cr-54 shows a significant increase above the shell closure. Conclusions: The evolution of the pygmy dipole resonance is heavily influenced by the shell structure.

The low-lying M1 strength of the open-shell nucleus Cr-50 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: 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 Ni-60 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 (gamma, gamma') nuclear resonance fluorescence technique with unpolarized continuous bremsstrahlung as well as with fully linearly polarized, quasimonochromatic, Compton-backscattered 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 [Sigma B(E1) up arrow = (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 [Sigma B(E1) up arrow = (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) up arrow strength of these structures accumulates to 3.94(27)mu(2)(N).

The low-lying dipole strength of the open-shell nucleus Mo-94 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. gamma-ray Source (HI gamma S) facility. In total, 83 excited states were identified. Exploiting polarized quasi-monoenergetic photons at HI gamma 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 HI gamma 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. gamma-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.

The dipole strength distribution of Se-76 has been investigated via photon scattering in the energy region below 9 MeV utilizing bremsstrahlung produced at the S-DALINAC facility at the TU Darmstadt. About 0.20(1)% of the classical E1 sum rule is exhausted by observed J = 1 states of justifiably assumed negative parity. An extrapolation of the GDR below 9 MeV suggests that considerable strength may remain unobserved due to background, finite detector resolution, and fragmentation. The observed strength thus represents a lower limit. Candidates for the 2(1,ms)(+) state and a fragment of the 1(sc)(+) mixed-symmetry states are presented.