Mass and Beta Decay ofNa20

The $\beta$ and $\gamma$ spectra of ${\mathrm{Na}}^{20}$ have been measured with a wedge-gap magnetic spectrometer and a 3-${\mathrm{cm}}^3$ lithium-drifted germanium detector, respectively. ${\mathrm{Na}}^{20}$ was produced by a ($p,\alpha n$) reaction on a natural magnesium target in the UCLA cyclotron. The mass excess of ${\mathrm{Na}}^{20}$ is measured to be 6863±40 keV. The half-life of ${\mathrm{Na}}^{20}$ is 0.408±0.006 sec. Two $\beta$ branches of 11.25±0.04 and 5.55±0.15 MeV were observed, and their relative intensities measured to be 100±2 and 9±1, respectively. The more energetic $\beta$ branch feeds the first excited state of ${\mathrm{Ne}}^{20}$; the excitation energy is determined to be 1633.2±1.0 keV from a study of its $\gamma$ transition. The 5.55-MeV $\beta$ branch establishes a link with the known delayed-$\alpha$ spectra from the highly excited states of ${\mathrm{Ne}}^{20}$, and allows us to propose a decay scheme for ${\mathrm{Na}}^{20}$. Limits on the intensities of possible unobserved $\beta$ transitions are discussed.