Competition between normal and intruder states inside the “island of inversion”

The ${\beta }^{-}$ decay of the exotic ${}^{30}\mathrm{Ne}$ ($N=20$) is reported. For the first time, the low-energy level structure of the $N=19$, ${}^{30}\mathrm{Na}$ ($T_Z=4$), is obtained from β-delayed γ spectroscopy using fragment-$\beta \text{−}\gamma \text{−}\gamma$ coincidences. The level structure clearly displays “inversion,” i.e., intruder states with mainly $2p2h$ configurations displacing the normal states to higher excitation energies. The good agreement in excitation energies and the weak and electromagnetic decay patterns with Monte Carlo shell model calculations with the SDPF-M interaction in the $\mathit{sdpf}$ valence space confirms the small $d_{3/2}\text{−}{f}_{7/2}$ shell gap. The relative position of the normal dominant and intruder dominant excited states provides valuable information to understand better the $N=20$ shell gap.

Figure 1

γ spectrum for events within the first 50 ms after a ${\beta }^{-}$ correlated ${}^{30}\mathrm{Ne}$ implant. The γ rays assigned to ${}^{30}\mathrm{Na}$ (asterisk) and transitions in the β-n decay daughter, ${}^{29}\mathrm{Na}$, are indicated. Other transitions originate from daughter and grand daughter activity, 1: ${}^{30}\mathrm{Mg}$; 2: ${}^{29}\mathrm{Mg}$; 3: ${}^{30}\mathrm{Al}$.

Figure 2

${}^{30}\mathrm{Ne}\text{−}\beta \text{−}\gamma \text{−}\gamma$ coincidences, gating transition indicated.

Figure 3

Experimental and theoretical level schemes for ${}^{30}\mathrm{Na}$ (For MCSM, levels from $0^{+}-4^{+}$ only are shown). Energies are in keV. Spin assignments for the observed states and the absolute intensities for the γ transitions (%) are indicated. The ground state of ${}^{30}\mathrm{Na}$ is experimentally known to be $2^{+}$ [24] consistent with negligible beta branching.

Figure 4

Time spectra for ${}^{30}\mathrm{Ne}$ decay in coincidence with the 151 keV γ, fitted with a decaying exponential and a constant background to extract the half-life of ${}^{30}\mathrm{Ne}$.

Figure 5

Partial level scheme of ${}^{29}\mathrm{Na}$ from Ref. [11]. The $J^{\pi }$ assignments to the states at $~1.5$ MeV are from the present work. Configuration of the levels ($2p2h$ and $0p0h$) obtained with the SDPF-M interaction are listed (in %).