Collapse of the $N=28$ Shell Closure in ${}^{42}\mathrm{S}\mathrm{i}$

The energies of the excited states in very neutron-rich ${}^{42}\mathrm{S}\mathrm{i}$ and ${}^{41,43}\mathrm{P}$ have been measured using in-beam $\gamma$-ray spectroscopy from the fragmentation of secondary beams of ${}^{42,44}\mathrm{S}$ at $39A\mathrm{MeV}$. The low $2^{+}$ energy of ${}^{42}\mathrm{S}\mathrm{i}$, 770(19) keV, together with the level schemes of ${}^{41,43}\mathrm{P}$, provides 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 ${}^{42}\mathrm{S}\mathrm{i}$ is best described as a well-deformed oblate rotor.

Figure 1

$\gamma$-ray spectra observed in coincidence with the ${}^{42}\mathrm{S}\mathrm{i}$ (upper), ${}^{43}\mathrm{P}$ (middle), and ${}^{41}\mathrm{P}$ (bottom) nuclei. In the insets $\gamma \mathrm{\text{−}}\gamma$ coincidence spectra are presented, gated with transitions belonging to the ${}^{41,43}\mathrm{P}$ nuclei.

Figure 2

Level schemes for ${}^{41,43}\mathrm{P}$ and the corresponding shell model calculations (SM) performed in the present work. Positions of the $2^{+}$ states in ${}^{40,42}\mathrm{S}\mathrm{i}$ are shown by dashed lines.

Figure 3

Energies of the $2^{+}$ states measured in the Ca and Si isotopes. Present result for ${}^{40,42}\mathrm{S}\mathrm{i}$—$770(19)\mathrm{keV}$—brings evidence for the collapse of the $N=28$ shell closure at $Z=14$.