Shape transition in the neutron rich sodium isotopes

Mass spectrometer measurements of the neutron rich sodium isotopes show a sudden increase at ³¹Na in the values of the two-neutron separation energies. The spherical shell model naturally predicts a sudden decrease at ³²Na after the N = 20 shell closure. We propose that the explanation for this disagreement lies in the fact that sodium isotopes in this mass region are strongly deformed due to the filling of negative parity orbitais from the 1f_{$\text{7}\text{2}$} shell. Hartree-Fock calculations are presented in support of this conjecture.