${}^{29}\mathrm{Na}$: Defining the Edge of the Island of Inversion for $Z=11$

The low-energy level structure of the exotic Na isotopes ${}^{28,29}\mathrm{Na}$ has been investigated through $\beta$-delayed $\gamma$ spectroscopy. The $N=20$ isotones for $Z=10–12$ are considered to belong to the “island of inversion” where intruder configurations dominate the ground state wave function. However, it is an open question as to where and how the transition from normal to intruder dominated configurations happens in an isotopic chain. The present work, which presents the first detailed spectroscopy of ${}^{28,29}\mathrm{Na}$, clearly demonstrates that such a transition in the Na isotopes occurs between ${}^{28}\mathrm{Na}$ ($N=17$) and ${}^{29}\mathrm{Na}$ ($N=18$), supporting the smaller $N=20$ shell gap in neutron-rich $sd$ shell nuclei. The evidence for inverted shell structure is found in $\beta$-decay branching ratios, intruder dominated spectroscopy of low-lying states, and shell model analysis.

Figure 1

${\beta }^{-}$-delayed $\gamma$-ray spectrum in the range 1.0–3.0 and 0–200 keV for events coming within the first 100 ms after a ${}^{29}\mathrm{Ne}$ implant. Gamma rays from ${}^{29}\mathrm{Ne}$ ${\beta }^{-}$, $\beta \mathrm{\text{−}}n$ decay and ${}^{29}\mathrm{Na}$ ${\beta }^{-}$, $\beta \mathrm{\text{−}}n$ decay are indicated. The inset shows the $\beta \mathrm{\text{−}}\gamma \mathrm{\text{−}}\gamma$ coincidences gated by the 72 keV transition.

Figure 2

Proposed level scheme for ${}^{29}\mathrm{Na}$ populated following the ${\beta }^{-}$ decay of ${}^{29}\mathrm{Ne}$. The absolute $\beta$-decay branching to each level per 100 decays is indicated along with the calculated $\log ft$ values. Shown on the right are shell model calculations with the USD and SDPF-M interactions.

Figure 3

The fragment $-\beta \mathrm{\text{−}}\gamma \mathrm{\text{−}}\gamma$ coincidences for ${}^{28}\mathrm{Ne}$ decay.

Figure 4

Proposed level scheme for ${}^{28}\mathrm{Na}$ populated following the ${\beta }^{-}$ decay of ${}^{28}\mathrm{Ne}$.