Experimental study of excited states of 62Ni via one-neutron
(d,p) transfer up to the neutron-separation threshold and characteristics
of the pygmy dipole resonance states
The degree of collectivity of the Pygmy Dipole Resonance (PDR) is an open
question. Recently, Ries {\it et al.} have suggested the onset of the PDR
beyond N=28 based on the observation of a significant E1 strength increase
in the Cr isotopes and proposed that the PDR has its origin in a few-nucleon
effect. Earlier, Inakura {\it et al.} had predicted by performing systematic
calculations using the random-phase approximation (RPA) with the Skyrme
functional SkM* that the E1 strength of the PDR strongly depends on the
position of the Fermi level and that it displays a clear correlation with the
occupation of orbits with orbital angular momenta less than 3ℏ(l≤2). To further investigate the microscopic structures causing the possible
formation of a PDR beyond the N=28 neutron shell closure, we performed a
61Ni(d,p)62Ni experiment at the John D. Fox Superconducting Linear
Accelerator Laboratory of Florida State University. To determine the angular
momentum transfer populating possible Jπ=1− states and other excited
states of 62Ni, angular distributions and associated single-neutron
transfer cross sections were measured with the Super-Enge Split-Pole
Spectrograph. A number of Jπ=1− states were observed below the
neutron-separation threshold after being populated through l=2 angular
momentum transfers. A comparison to available (γ,γ′) data for
58,60Ni provides evidence that the B(E1) strength shifts further down
in energy. The (d,p) data clearly prove that l=0 strength, i.e., the
neutron (2p3/2)−1(3s1/2)+1 one-particle-one-hole configuration
plays only a minor role for 1− states below the neutron-separation threshold
in 62Ni.Comment: 15 pages, 8 figures, accepted for publication in Physical Review