Abstract
States in the $N=28$ nucleus $^{46}$Ar have been studied by a two-neutron transfer reaction at REX-ISOLDE (CERN). A beam of radioactive $^{44}$ at an energy of 2.16~AMeV and a tritium loaded titanium target were used to populate $^{46}$ by the t($^{44}$,p) two-neutron transfer reaction. Protons emitted from the target were identified in the T-REX silicon detector array. The excitation energies of states in $^{46}$ have been reconstructed from the measured angles and energies of recoil protons. Angular distributions for three final states were measured and based on the shape of the differential cross section an excited state at 3695~keV has been identified as $J^\pi = 0^+$. The angular differential cross section for the population of different states are compared to calculations using a reaction model employing both sequential and direct transfer of two neutrons. Results are compared to shell model calculations using state-of-the-art effective interactions.
Highlights
Among the magic numbers which describe the shell structure of atomic nuclei, 28 is the first main shell gap created by the spin-orbit interaction
Spectroscopic factors extracted from the study of the N = 27 isotope 45Ar by a one-neutron (d, p) transfer reaction agree with shell-model results [20]. These results suggest that the N = 28 shell gap is still pronounced in 46Ar
The N = 28 nucleus 46Ar was studied by a (t, p) twoneutron transfer reaction at 2.16-AMeV beam energy using a radioactive tritium target
Summary
Among the magic numbers which describe the shell structure of atomic nuclei, 28 is the first main shell gap created by the spin-orbit interaction. The 1f7/2 orbital gets lowered in energy compared to the 1f5/2 orbital creating this gap within the N = 3 major oscillator shell. The evolution of the shell gap at 28 nucleons, both as a function of neutron and proton number, is influenced by the nature of the spin-orbit interaction. On the neutron-rich side of the valley of stability, it was shown that other terms in the nucleon interaction play a role in determining the size of the N = 28 shell gap [1]
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