Experimental study of excited states of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Ni</mml:mi><mml:mprescripts/><mml:none/><mml:mn>62</mml:mn></mml:mmultiscripts></mml:math> via one-neutron <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>(</mml:mo><mml:mi>d</mml:mi><mml:mo>,</mml:mo><mml:mi>p</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> 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 et al. 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 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 $3\ensuremath{\hbar}\phantom{\rule{4pt}{0ex}}(l\ensuremath{\le}2)$. To further investigate the microscopic structures causing the possible formation of a PDR beyond the $N=28$ neutron shell closure, we performed a $^{61}\mathrm{Ni}(d,p)^{62}\mathrm{Ni}$ experiment at the John D. Fox Superconducting Linear Accelerator Laboratory of Florida State University. To determine the angular momentum transfer populating possible ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ states and other excited states of $^{62}\mathrm{Ni}$, angular distributions and associated single-neutron transfer cross sections were measured with the Super-Enge Split-Pole Spectrograph. A number of ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ states were observed below the neutron-separation threshold after being populated through $l=2$ angular momentum transfers. A comparison to available $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ data for $^{58,60}\mathrm{Ni}$ 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 ${(2{p}_{3/2})}^{\ensuremath{-}1}{(3{s}_{1/2})}^{+1}$ one-particle--one-hole configuration, plays only a minor role for ${1}^{\ensuremath{-}}$ states below the neutron-separation threshold in $^{62}\mathrm{Ni}$.

Topics & Concepts