Experimental study of excited states of ${}^{62}$Ni via one-neutron $(d,p)$ transfer up to the neutron-separation threshold and characteristics of the pygmy dipole resonance states

Abstract

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 $3\hslash$ $(l\le 2)$. To further investigate the microscopic structures causing the possible formation of a PDR beyond the $N=28$ neutron shell closure, we performed a ${}^{61}$Ni$(d,p){}^{62}$Ni experiment at the John D. Fox Superconducting Linear Accelerator Laboratory of Florida State University. To determine the angular momentum transfer populating possible $J^{\pi }=1^{-}$ states and other excited states of ${}^{62}$Ni, angular distributions and associated single-neutron transfer cross sections were measured with the Super-Enge Split-Pole Spectrograph. A number of $J^{\pi }=1^{-}$ states were observed below the neutron-separation threshold after being populated through $l=2$ angular momentum transfers. A comparison to available $(\gamma ,{\gamma }^{\prime })$ data for ${}^{58,60}$Ni 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 $(2p_{3/2}{)}^{-1}(3s_{1/2}{)}^{+1}$ one-particle-one-hole configuration plays only a minor role for $1^{-}$ states below the neutron-separation threshold in ${}^{62}$Ni.

arXiv comment15 pages, 8 figures, accepted for publication in Physical Review C

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• The CeBrA demonstrator for particle-$\gamma$ coincidence experiments at the FSU Super-Enge Split-Pole Spectrograph
• Nuclear structure and direct reaction studies in particle-$\gamma$ coincidence experiments at the FSU John D. Fox Superconducting Linear Accelerator Laboratory
• Experimental study of ${}^{53}$Cr via the $(d,p\gamma )$ reaction
• A detailed view at magnetic dipole strengths: The case of semi-magic ${}^{50}$Ti