Photoresponse of 60Ni below 10-MeV excitation energy:evolution of dipole resonances in fp-shell nuclei near N=Z

Background: Within the last decade, below the giant dipole resonance the existence of a concentration of additional electric dipole strength has been established. This accumulation of low-lying E1 strength is commonly referred to as pygmy dipole resonance (PDR). Purpose: The photoresponse of Ni-60 has been investigated experimentally and theoretically to test the evolution of the PDR in a nucleus with only a small neutron excess. Furthermore, the isoscalar and isovector M1 resonances were investigated. Method: Spin-1 states were excited by exploiting the (gamma, gamma') nuclear resonance fluorescence technique with unpolarized continuous bremsstrahlung as well as with fully linearly polarized, quasimonochromatic, Compton-backscattered laser photons in the entrance channel of the reaction. Results: Up to 10 MeV a detailed picture of J = 1 levels was obtained. For the preponderant number of the individual levels spin and parity were firmly assigned. Furthermore, branching ratios, transition widths, and reduced B(E1) or B(M1) excitation probability were calculated from the measured scattering cross sections. A comparison with theoretical results obtained within the quasiparticle phonon model allows an insight into the microscopic structure of the observed states. Conclusions: Below 10 MeV the directly observed E1 strength [Sigma B(E1) up arrow = (153.8 +/- 9.5) e(2)(fm)(2)] exhausts 0.5% of the Thomas-Reiche-Kuhn sum rule. This value increases to 0.8% of the sum rule [Sigma B(E1) up arrow = (250.9 +/- 31.1) e(2)(fm)(2)] when indirectly observed branches to lower-lying levels are considered. Two accumulations of M1 excited spin-1 states near 8 and 9 MeV excitation energy are identified as isoscalar and isovector M1 resonances dominated by proton and neutron f(7/2) -> f(5/2) spin-flip excitations. The B(M1) up arrow strength of these structures accumulates to 3.94(27)mu(2)(N)