Low-lying2+states generated bypn-quadrupole correlation andN=28shell quenching

Abstract

The quadrupole vibrational modes of neutron-rich N = 28 isotones ( 48 Ca, 46 Ar, 44 S, and 42 Si) are investigated by using the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory with several choice of energy density functionals, including nuclear pairing correlation. It is found that the quenching of the N = 28 shell gap and the proton holes in the sd shell trigger quadrupole correlation and increase the collectivity of the low-lying 2 + state in 46 Ar. It is also found that the pairing correlation plays an important role to increase the collectivity. We also demonstrate that the same mechanism to enhance the low-lying collectivity applies to other N = 28 isotones 44 S and 42 Si, and it generates a couple of low-lying 2 + states which can be associated with the observed 2 + states.

Spectroscopy ofAr46by the(t,p)two-neutron transfer reaction

“…The value determined in the Coulomb excitation experiments (B(E2; 2 + 1 → 0 + gs ) = 39(8) e 2 fm 4 [4], 44(6) e 2 fm 4 [5], and 54(5) e 2 fm 4 [6]) points to a moderate deformation and collectivity in 46 Ar consistent with a the expectation for a semi-magic nucleus. This is supported by timedependent Hartree-Fock-Bogoliubov calculations [8] that link the increase in collectivity with respect to 48 Ca to a quenching of the N = 28 shell gap. Shell model calculations on the other hand favor the result of a larger B(E2) value as determined by the lifetime measurement (B(E2; 2 + 1 → 0 + gs ) = 114 +67 −32 e 2 fm 4 [7]).…”

Coulomb excitation ofCa44andAr46

“…The 46 Ar nuclei were produced at the GANIL facility in the interactions of a 60 AMeV 48 Ca 19+ beam, at an average intensity of ∼ 4 μA, in a 145-μm 9 Be target. An additional setting of the spectrometer was required to select the stable 44 Ca nucleus that was used for calibration purpose.…”

“…In particular, the gradual decrease of the 2 + 1 energies in the N = 28 isotones from 3831 keV in 48 Ca, 1577 keV in 46 Ar [2], 1329 keV in 44 S [3], and 770 keV in 42 Si [4,5] indicates that a large deformation is steadily establishing toward 42 Si. Mean field approaches [6][7][8][9] as well as shell model calculations [10][11][12][13][14] predict a close-to-spherical 46 Ar, a shape mixing or shape coexistence in 44 S, and a large oblate deformation in 42 Si. The properties of the recently discovered low-lying isomeric 0 + 2 state in 44 S [15] supports a shape mixing between the 0 + 1 and 0 + 2 states.…”

Microscopic study of the Shell Structure evolution in isotopes of light to middle mass range Nuclides

“…In ref. [6,7], the weakening of the shell closure for N = 28 has been reported by investigating the experimental 2 + excitation energies of Si isotopes. Similarily, the conventional shell structure picture of magic numbers of nucleons in stable nuclides is no longer applicable to the many exotic nuclei both in theoretical and experimental extractions as reported in ref.…”