Dipole transitions to bound states in Fe-56 and Ni-58 - art. no. 024302

A high-resolution study of dipole transitions to bound states up to 10 MeV in the fp-shell nuclei Fe-56 and Ni-58 was performed by means of the nuclea r resonance fluorescence method. Unpolarized and partially linearly polariz ed bremsstrahlung photons with different end point energies (6.5, 10, and 1 2 MeV) were used to determine excitation energies, spins, parities, and tra nsition widths of more than 60 levels observed in each nucleus between 2 an d 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 connecte d with the E1 pygmy resonance in heavier nuclei. The M1 spin-flip strength shows a concentration around 8.7 MeV in Ni-58. On the contrary, in Fe-56 it s distribution is rather flat. The observed M1 strength is weak in both nuc lei. A study was made of the feeding and branching of the observed levels. Calculations were performed for these nuclei in the framework of the quasip article phonon model and a comparison with experimental results is made. Ap art from the strength, experimental and calculated results agree quite well and information about a scissorslike mode in Fe-56 and a 1(-) two-phonon s tate in Ni-58 could be extracted.