Decay pattern of the pygmy dipole resonance in60Ni

Abstract

Spin-1 states in ${}^{60}$Ni were excited with the $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction, exploiting the High Intensity $\stackrel{P\vec}{\ensuremath{\gamma}}$-ray Source at Triangle University Nuclear Laboratory. This facility is capable of providing fully linearly polarized, quasimonochromatic, Compton-backscattered photons in the entrance channel of the reaction. The depopulation of low-lying levels in an energy region far below the incident quasimonochromatic photons allows us to obtain average branching ratios of the excited spin-1 states. Levels within the energy region associated with the PDR showed regular behavior and $\ensuremath{\approx}$75$%$ of their decays are direct ground-state decays. The levels in the energy region above the PDR exhibit a statistical decay behavior to a large number of low-lying excited states and have only $\ensuremath{\approx}$50--60$%$ branches to the ground state. Within the framework of the quasiparticle phonon model this feature can be explained with the number of quasiparticles contributing to the wave functions of the excited spin-1 states. Quasimonochromatic photon beams provide a new method to test the microscopic nature of ${1}^{\ensuremath{-}}$ levels.