Direct observation of the exotic $\beta -\gamma -\alpha$ decay mode in the $T_z=-1$ nucleus ${}^{20}\mathrm{Na}$

The exotic $\beta -\gamma -\alpha$ decay mode of ${}^{20}\mathrm{Na}$ has been directly observed for the first time in the Day-one experiment at the Beijing Radioactive Ion-beam Facility. The ${}^{20}\mathrm{Na}$ source was produced by using a 100-MeV proton beam bombarding a stack of microporous MgO thick target and delivered as an intense mass separated beam after online ionization. A high-efficiency simultaneous measurement of $\beta , \gamma$, and $\alpha$ transitions enables the $\beta$-delayed $\gamma -\gamma$ and $\alpha -\gamma$ coincidence spectroscopy. Three $\beta -\gamma -\alpha$ exotic decay sequences in ${}^{20}\mathrm{Na}$ are discovered, which expands the rare decay modes observed in $\beta$ decay. Moreover, a $\beta -\alpha$–decay sequence to the 6130-keV $3^{-}$ state of ${}^{16}\mathrm{O}$ is observed, which is likely through the $12367$-keV $1^{+}$ state in ${}^{20}\mathrm{Ne}$. The experimentally deduced $B\left(F\right)$ and $B\left(GT\right)$ are compared to the shell-model calculation, the allowed $\beta$ transition strengths can be well accounted for by using $sd$ shell-model space interactions.

Figure 1

Experimental setup for the $\beta$ decay of ${}^{20}\mathrm{Na}$. The simultaneous measurement of $\beta , \alpha$, and $\gamma$ transitions from the ${}^{20}\mathrm{Na}$ decay is realized by using five pieces of MSQ, two pieces of DSSD and two HPGe detectors, respectively. The top MSQ is removed to show the collection foil.

Figure 2

$\alpha$ and $\gamma$ singles spectrum of ${}^{20}\mathrm{Na}$ decay. The peak in (a) labeled by an asterisk is dominated by ${}^{16}\mathrm{O}$ recoils.

Figure 3

Two-dimensional $\alpha -\gamma$ coincidence spectrum of ${}^{20}\mathrm{Na}$ decay. The gates used for the projection on $E_{\gamma }$ are indicated by rectangles.

Figure 4

$\gamma$ spectra projected from the $\alpha -\gamma$ matrix with different $\alpha$ particle energy gates. See text for details.

Figure 5

Decay scheme of ${}^{20}\mathrm{Na}$. The newly discovered exotic decay sequences are shown with red arrows. The ${}^{20}\mathrm{Ne}$ states are labeled by excitation energy in keV, $\beta$ branching ratio, and $\log ft$ value on the right side.