Dipole transitions to bound states in56Feand58Ni

Abstract

A high-resolution study of dipole transitions to bound states up to 10 MeV in the $\mathrm{fp}$-shell nuclei ${}^{56}\mathrm{Fe}$ and ${}^{58}\mathrm{Ni}$ 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 MeV) 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 $M1$ spin-flip strength shows a concentration around 8.7 MeV in ${}^{58}\mathrm{Ni}.$ On the contrary, in ${}^{56}\mathrm{Fe}$ its distribution is rather flat. The observed $M1$ 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 ${}^{56}\mathrm{Fe}$ and a ${1}^{\ensuremath{-}}$ two-phonon state in ${}^{58}\mathrm{Ni}$ could be extracted.