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Evolution of the shell structure in medium-mass nuclei : Search for the 2d5/2 neutron orbital in 69Ni

Abstract : The harmonic oscillator shell closure at N=40 in 68Ni is weak and loses its strength when removing (or adding) pair of protons. Calculations performed in this mass region predict a new island of inversion at N=40 similar to the one at N=20. Using a large valence space, the neutron orbital 2d5/2 is shown to be a crucial ingredient for the interpretation of the nuclear structure at N≈40. The neutron 1g9/2-2d5/2 energy difference has been determined in 69Ni using the d(68Ni,p) transfer reaction in inverse kinematics. The experiment performed at GANIL used a 68Ni beam at 25.14 MeV/u seperated by the LISE3 spectrometer was impinging a CD2 target of 2.6 mg/cm2 thickness. The experimental setup consisted of CATS/MUST2-S1/EXOGAM detectors coupled to an ionization chamber and a plastic scintillator. The angular momenta and spectroscopic factors of the ground state (Jπ = 9/2+) and a doublet of states ( Jπ = 5/2+) around 2.48 MeV corresponding to the population of the 1g9/2 and the 2d5/2 orbitals, were obtained from the comparison between the experimental cross-sections as a function of the proton detection angle and ADWA calculations. The spins of the observed states were assigned by comparaison to large scale Shell-Model calculations. The position of the 2d5/2 orbital in 69Ni has been established for the first time. Our measurements support the hypothesis of a low-lying 2d5/2 orbital (≈2.5 MeV) with respect to the 1g9/2 neutron orbital and thus its major role in the structure of the nuclei around N=40.
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Submitted on : Monday, October 29, 2012 - 4:58:51 PM
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Mohamad Moukaddam. Evolution of the shell structure in medium-mass nuclei : Search for the 2d5/2 neutron orbital in 69Ni. Nuclear Experiment [nucl-ex]. Université de Strasbourg, 2012. English. ⟨tel-00746802⟩

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