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

TL;DR

This paper reports the experimental evidence for the collapse of the N=28 shell closure in very neutron-rich $^{42}$Si, showing it behaves as a deformed oblate rotor due to proton-neutron tensor forces.

Contribution

It provides new experimental data and shell model analysis demonstrating the disappearance of traditional shell closures in $^{42}$Si.

Findings

Low 2+ energy of 770 keV in $^{42}$Si indicating shell closure collapse.

Level schemes of $^{41,43}$P support shell closure disappearance.

Shell model suggests $^{42}$Si is a deformed oblate rotor.

Abstract

The energies of the excited states in very neutron-rich $^{42}$Si and $^{41,43}$P have been measured using in-beam $\gamma$-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.