Shell Erosion and Shape Coexistence in ${}_{16}{}^{43}\mathrm{S}_{27}$

We report on the $g$-factor measurement of the first isomeric state in ${}_{16}{}^{43}\mathrm{S}_{27}$ [$E_x=320.5(5)\mathrm{keV}$, $T_{1/2}=415(5)\mathrm{ns}$, and $g=0.317(4)$]. The $7/2^{-}$ spin-parity of the isomer and the intruder nature of the ground state of the nucleus are experimentally established for the first time, providing direct and unambiguous evidence of the collapse of the $N=28$ shell closure in neutron-rich nuclei. The shell model, beyond the mean-field and semiempirical calculations, provides a very consistent description of this nucleus showing that a well deformed prolate and quasispherical states coexist at low energy.

Figure 1

Left: Outline of the experimental setup. The dashed-dotted line shows the orientation of the spin of the ${}^{43m}\mathrm{S}$ state at $t=0$. $\alpha$ and $\theta$ are defined in the text. Right: Full statistics $\gamma$-ray spectrum obtained in the present work.

Figure 2

$R(t)$ functions associated with the 320.5 keV $\gamma$-ray for (a) the magnetic field $B_1$ and (b) $B_2$. Solid curves display the result of the fit using the $R(t)$ function from Eq. (1).

Figure 3

Experimental (expt) level scheme of ${}^{43}\mathrm{S}$, compared to SM and PR models. The states, of negative parity, are labeled by twice their spin value. Transition probabilities [in $e^2·{\mathrm{fm}}^4$ (${\mu }_N$) for $E2$ ($M1$) transitions] are reported along the arrows. Arrow width is proportional to the strength of the transition. The insert compares PR and SM predictions for the magnetic moments of the states of the rotational band.

Figure 4

HFB potential energy curves with zero-point energy corrections for the first blocked states in ${}^{43}\mathrm{S}$. States are labeled by their $K$ values (see text for details).