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 ${}^{60}$Ni 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 $(\gamma ,{\gamma }^{\prime })$ 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 $B\left(E1\right)$ or $B\left(M1\right)$ 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 $[\sum B(E1)\uparrow =(153.8\pm 9.5)$ $e^2$(fm)${}^2]$ exhausts 0.5% of the Thomas-Reiche-Kuhn sum rule. This value increases to 0.8% of the sum rule $[\sum B(E1)\uparrow =(250.9\pm 31.1)$ $e^2$(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 $f_{7/2}\to f_{5/2}$ spin-flip excitations. The $B\left(M1\right)\uparrow$ strength of these structures accumulates to 3.94(27)${\mu }_N^2$.

• Received 9 July 2013

DOI:https://doi.org/10.1103/PhysRevC.88.044304