Dipole transitions to bound states in56Feand58Ni

Abstract

A high-resolution study of dipole transitions to bound states up to 10 MeV in the f p-shell nuclei 56 Fe and 58 Ni was performed by means of the nuclear resonance fluorescence method. Unpolarized and partially linearly polarized bremsstrahlung photons with different end point energies ͑6.5, 10, and 12 MeV͒ were used to determine excitation energies, spins, parities, and transition widths of more than 60 levels observed in each nucleus between 2 and 10 MeV. The vast majority of the observed transitions are dipole ones and to the strongest of them an E1 assignment could be given. The E1 strength distribution shows a high concentration around 8.2 MeV that may be connected with the E1 pygmy resonance in heavier nuclei. The M 1 spin-flip strength shows a concentration around 8.7 MeV in 58 Ni. On the contrary, in 56 Fe its distribution is rather flat. The observed M 1 strength is weak in both nuclei. A study was made of the feeding and branching of the observed levels. Calculations were performed for these nuclei in the framework of the quasiparticle phonon model and a comparison with experimental results is made. Apart from the strength, experimental and calculated results agree quite well and information about a scissorslike mode in 56 Fe and a 1 Ϫ two-phonon state in 58 Ni could be extracted.

Low-energy magnetic dipole response in 56Fe from high-resolution electron scattering

“…A study of the possible mixed-symmetry character (within the interacting boson model-2) of low-lying 1 + states predicts a splitting in two levels at E x ≈ 3.5 MeV [45] which is not observed in the present experiment, in line with the experimental findings of [28]. However, the total g.s.…”

“…Emphasis is put on the energy region below the spin-flip resonance where the scissors mode is expected. Some information on M1 transitions in 56 Fe exists from (γ , γ ) studies [26][27][28]. However, investigation of their momentum transfer dependence provides a deeper insight, in particular into the role of spin-orbit interference effects.…”

Resonant absorption troughs in the gamma-ray spectra of QSO

“…Figure 4 illustrates the strength distribution of the "pygmy" dipole resonances (E1) for heavy nuclei, like 138 Ba, 140 Ce, and 144 Sm (Zilges et al 2002). Lighter nuclei like 51 V, 52 Cr, 40 Ca, 48 Ca, and 56 Fe also show this type of concentration of electric dipole strength in the region of 6-7 MeV of photon energy (Enders et al 1998;Bauwens et al 2000;Kaiser et al 2000;Andrejscheff et al 2001;Babilon et al 2002;Hartmann et al 2002;Pietralla et al 2002).…”

“…It is stated that this effect can be best understood in the boundaries of a nuclei "Dipole Core Polarization" suggested by Bohr & Mottelson (1975). The early conclusion that the levels in the peak of the cross section connect unusually strongly to the ground states in a number of nuclei remains valid until the present (see Axel et al 1963;Berg et al 1975;Moreh et al 1988;Chambers et al 1994;Oros et al 1998;Bauwens et al 2000;Andrejscheff et al 2001;Babilon et al 2002;Hartmann et al 2002). However, systematic study of nuclides in this energy range up to the particle threshold has not yet been performed Zilges et al 2002).…”

“…the photoproduction threshold can be produced: first, via photon absorption to the excitation level of the nuclei (Hanna & Meyer-Schützmeister 1959;Axel 1962;Hayward 1977) and, second, via the photon capture into the so-called "pygmy" dipole resonance (PDR) (Bohr & Mottelson 1975;Suzuki et al 1990;Van Isacker et al 1992;Enders et al 1998;Bauwens et al 2000;Kaiser et al 2000;Andrejscheff et al 2001;Babilon et al 2002;Hartmann et al 2002;Pietralla et al 2002;Zilges et al 2002). The integrated cross section for the level excited by a γ-ray with energy E = hc/λ is usually expressed in terms of the ground state width Γ 0 :…”