Discovery of ${}^{39}\mathrm{Na}$

The new isotope ${}^{39}\mathrm{Na}$, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ${}^{48}\mathrm{Ca}$ beam at $345\mathrm{MeV}/\text{nucleon}$ on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ${}^{39}\mathrm{Na}$ events have been unambiguously observed in this work and clearly establish the particle stability of ${}^{39}\mathrm{Na}$. Furthermore, the lack of observation of ${}^{35,36}\mathrm{Ne}$ isotopes in this experiment significantly improves the overall confidence that ${}^{34}\mathrm{Ne}$ is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ${}^{39}\mathrm{Na}$ has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ${}^{39}\mathrm{Na}$ at the neutron number $N=28$, which is normally a magic number.

Figure 1

$Z$ versus $A/Z$ particle identification plots for projectile fragments produced in the ${}^{48}\mathrm{Ca}+\mathrm{Be}$ reaction at $345\mathrm{MeV}/\text{nucleon}$ are shown for the (a) ${}^{39}\mathrm{Na}$ and (b) ${}^{36}\mathrm{Ne}$ settings. Nine events were observed for ${}^{39}\mathrm{Na}$ in the ${}^{39}\mathrm{Na}$ setting. No events were observed for ${}^{35,36}\mathrm{Ne}$ in the ${}^{36}\mathrm{Ne}$ setting.

Figure 2

Measured production cross sections are shown as a function of mass number for neutron-rich (a) neon, (b) sodium, and (c) magnesium isotopes produced in the projectile fragmentation of a $345\mathrm{MeV}/\text{nucleon}$ ${}^{48}\mathrm{Ca}$ beam on a beryllium target, along with our previous data [1, 48]. Predictions from the epax2.15 systematics and the $Q_g$ systematics are shown by solid and dashed curves, respectively.