Collapse of the N=28 shell closure in ${}^{42}$Si

Abstract

The energies of the excited states in very neutron-rich ${}^{42}$Si and ${}^{41,43}$P have been measured using in-beam ${\hat{I}}^3$-ray spectroscopy from the fragmentation of secondary beams of ${}^{42,44}$S at 39 A.MeV. The low 2${}^{+}$ energy of ${}^{42}$Si, 770(19) keV, together with the level schemes of ${}^{41,43}$P provide evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that ${}^{42}$Si is best described as a well deformed oblate rotor.

4 pages, 3 figures, accepted for publication in Phys. Rev. lett