Decay pattern of the pygmy dipole resonance in 60Ni
M. Scheck, V. Yu. Ponomarev, T. Aumann, J. Beller, M. Fritzsche, J. Isaak, J. H. Kelley, E. Kwan, N. Pietralla, R. Raut, C. Romig, G. Rusev, D. Savran, K. Sonnabend, A. P. Tonchev, W. Tornow, H. R. Weller, and M. Zweidinger
Accepted
Spin-1 states in 60Ni were excited with the ([(g)\vec],g) reaction exploiting the High Intensity [(g)\vec]-ray Source at TUNL. This facilty is capable to provide fully linearly-polarized, quasi-monochromatic Compton-backscattered photons in the entrance channel of the reaction. The depopulation of low-lying levels in an energy region far below the incident quasi-monochromatic photons allows to obtain average branching ratios of the excited spin-1 states. Levels within the energy region associated with the PDR showed a rather regular behaviour and 75% of their decays are direct ground-state decays. The levels in the energy region above the PDR exhibit a statistical decay behaviour to a large number of low-lying excited states and have only 50-60% branches to the ground state. Within the framework of the Quasiparticle Phonon Model this feature can be explained with the number of quasi-particles contributing to the wave functions of the excited spin-1 states. Quasi-monochromatic photon beams provide a new method to test the microscopic nature of 1- levels.