E1 and M1 excitations in 54Fe and low-energy M1 strength in open-shell Fe isotopes


E1 and M1 excitations in 54Fe and low-energy M1 strength in open-shell Fe isotopes

Schwengner, R.

Dipole excitations in the N = 28 nuclide 54Fe were studied in photon-scattering experiments using bremsstrahlung at the gELBE facility of Helmholtz-Zentrum Dresden-Rossendorf and using quasi-monoenergetic, polarized gamma beams at the HIgS facility of the Triangle Universities Nuclear Laboratory in Durham. We identified intense E1 as well as M1 transitions to spin-1 states up to about 10.6 MeV. In the second part, we present low-energy M1 strength functions of 60,64,68Fe determined on the basis of large-scale shell-model calculations with the goal to study their development from the bottom to the middle of the neutron shell. We find that the zero-energy spike develops toward the middle of the shell into a bimodal structure composed of a weaker zero-energy spike and a scissorslike resonance around 3 MeV, where the summed strengths of the two structures changes only slightly. The summed strength of the scissors region exceeds the total gammaabsorption strength from the ground state by a factor of about three, which explains the discrepancy between total strengths of the scissors resonance derived from (gamma,gamma') experiments and from experiments using light-ion induced reactions.

Keywords: Bremsstrahlung; polarized gamma beam; shell model; M1 gamma-strength function; scissors mode

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Publ.-Id: 25554