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.
Gamma-ray and neutron spectra from the 18 O + 76 Se reaction at 15.3 MeV/u were measured with the EDEN array of liquid scintillators at the LNS. The results were compared to GEANT Hadrontherapy physics list simulations in order to assess the reliability of this model for the development of the NUMEN project. A good agreement with the shape of the experimental gamma-ray spectra and a reasonable agreement with the total count rates were obtained. The gamma spectra originated from the nuclear reactions were selected by time coincidence with the Superconducting Cyclotron radio-frequency reference signal. The random coincidence background rate was appropriately described
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