β-delayed γ spectroscopy of neutron rich ${}^{27,28,29}\mathrm{Na}$

The low-energy level structure of the exotic Na isotopes ${}^{27,28,29}\mathrm{Na}$ has been investigated through β-delayed γ spectroscopy. Detailed level structure of ${}^{28,29}\mathrm{Na}$ has been obtained through $\beta \gamma$ and $\beta \gamma \gamma$ coincidence measurements. The low-lying levels populated in ${}^{27}\mathrm{Na}$ by β decay were found to corroborate well with the in-beam data from the literature. Half-lives of the parent nuclides, ${}^{27,28,29}\mathrm{Ne}$, were measured using β fragment as well as fragment $\beta \gamma$ coincidences and compared to previous measurements. The β-delayed one- and two-neutron emission branching probabilities have been obtained from the γ activities of the grand daughter nuclei. A comparison of the level schemes and the β-decay branching ratios is made with shell-model predictions, both with and without intruder configurations, to understand the transition from normal-dominant to intruder-dominant excitations in these neutron-rich nuclei approaching the island of inversion.

Figure 1

Two-dimensional particle identification plot of time of flight $\mathrm{vs}$ energy loss for implants that have a correlated β decay, observed with the A1900 setting optimized for the transmission of ${}^{29}\mathrm{Ne}$ fragments. The other strongly transmitted nuclides are labeled, ${}^{31,30}\mathrm{Na}$ together form about 63% of the total transmitted nuclides, whereas ${}^{29}\mathrm{Ne}$ is about 23%. The yield for ${}^{29}\mathrm{Ne}$ was ∼1.5 pps for 10 pnA of primary beam current.

Figure 2

β-delayed γ-ray spectrum for events coming within the first 100 ms after a ${}^{29}\mathrm{Ne}$ implant. γ rays from ${}^{29}\mathrm{Ne}$ ${\beta }^{-},{}^{29}\mathrm{Ne}$ β-$n$, ${}^{29}\mathrm{Ne}$ β-$2n$ decay, and ${}^{29}\mathrm{Na}$ ${\beta }^{-},{}^{29}\mathrm{Na}$ β-$n$ decay are indicated.

Figure 3

Decay curve for ${}^{29}\mathrm{Ne}$ derived from β-decay-correlated implants for events collected for 1 s after the implant. Data were fitted with four exponential decays: an exponential decay for ${}^{29}\mathrm{Ne}$, exponential growth and decay for the daughter nuclei ${}^{29}\mathrm{Na}$ and ${}^{28}\mathrm{Na}$ formed by ${\beta }^{-}$ and $\beta \text{−}n$ decay, respectively, and an exponential background. The weights for the decay of the two daughter nuclides were taken from the calculated β-delayed neutron emission probability of 29(7)%. The decay constants for ${}^{29}\mathrm{Na}$ and ${}^{28}\mathrm{Na}$ were taken from the literature. The half-life obtained from the fit is 13.8(5) ms.

Figure 4

Decay curve for ${}^{29}\mathrm{Ne}$ derived from $\beta \gamma$-decay-correlated implants for events collected for 200 ms after the implant. Data were fitted with a single exponential for ${}^{29}\mathrm{Ne}$ decay and a constant background. The half-lives obtained in coincidence with the specified γ lines are consistent with the value from the fragment-β correlation only (see Fig. 3).

Figure 5

Proposed level scheme for ${}^{29}\mathrm{Na}$ populated following the β decay of ${}^{29}\mathrm{Ne}$. The absolute β-decay branching to each level per 100 decays is indicated along with the calculated log $\mathrm{ft}$ values.

Figure 6

Decay curve for ${}^{28}\mathrm{Ne}$ derived from β-decay-correlated implants for events collected for 1 s after the implant. A four-component fit was made to the data as for ${}^{29}\mathrm{Ne}$. The weights for the decay of the two daughter nuclei were taken from the calculated β-delayed neutron emission probability of 12(1)%. The decay constants for ${}^{28}\mathrm{Na}$ and ${}^{27}\mathrm{Na}$ were frozen to the values taken from the literature. The half-life obtained for the decay of ${}^{28}\mathrm{Ne}$ from the fit is 18.4(5) ms.

Figure 7

Decay curve for ${}^{28}\mathrm{Ne}$ derived from $\beta \gamma$-decay-correlated implants for events collected for 500 ms after the implant. Data were fitted with a single exponential to account for ${}^{28}\mathrm{Ne}$ decay and a constant background. The half-lives obtained in coincidence with the specified γ lines are consistent with the fragment-β correlation data.

Figure 8

The $\beta \gamma \gamma$ coincidences for ${}^{28}\mathrm{Ne}$ decay.

Figure 9

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

Figure 10

Decay curve for ${}^{27}\mathrm{Ne}$ derived from β-decay-correlated implants for events collected for 2 s after the implant. Data were fitted with four exponentials as before. The half-life obtained from the fit is 28.4(10) ms.

Figure 11

Decay curve for ${}^{27}\mathrm{Ne}$ derived from $\beta \gamma$-decay-correlated implants for events collected for 500 ms after the implant.

Figure 12

The $\beta \gamma \gamma$ coincidences for ${}^{27}\mathrm{Ne}$ decay.

Figure 13

Proposed level scheme for ${}^{27}\mathrm{Na}$ populated following the β decay of ${}^{27}\mathrm{Ne}$. (The calculated log $\mathrm{ft}$ values of only the strongly populated levels are shown.) The predictions of the shell model within the $\mathit{sd}$ shell [30] for the energy levels are also shown along with the predicted β-decay intensities for the allowed levels.

Figure 14

Proposed level scheme for ${}^{28,29}\mathrm{Na}$ compared with USD and SDPF-M shell-model calculation (see text for details).