Direct Mass Measurements of ${}^{19}\mathrm{B}$, ${}^{22}\mathrm{C}$, ${}^{29}\mathrm{F}$, ${}^{31}\mathrm{Ne}$, ${}^{34}\mathrm{Na}$ and Other Light Exotic Nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei ${}^{19}\mathrm{B}$, ${}^{22}\mathrm{C}$, and ${}^{29}\mathrm{F}$ as well as that of ${}^{34}\mathrm{Na}$. In addition, the most precise determinations to date for ${}^{23}\mathrm{N}$ and ${}^{31}\mathrm{Ne}$ are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in ${}^{22}\mathrm{C}$, with a dominant $\nu 2s_{1/2}^2$ configuration, and a single-neutron halo in ${}^{31}\mathrm{Ne}$ with the valence neutron occupying predominantly the $2p_{3/2}$ orbital. Despite a very low two-neutron separation energy the development of a halo in ${}^{19}\mathrm{B}$ is hindered by the $1d_{5/2}^2$ character of the valence neutrons.

Figure 1

Two-neutron separation energies ($S_{2n}$) as a function of neutron number. Red symbols correspond to masses measured here for the first time and gray points to masses estimated on the basis of extrapolations [26]. The horizontal line delineates the limit of binding ($S_{2n}=0\mathrm{MeV}$), while vertical lines indicate known (sub-) shell closures. The lines connecting the points for each $Z$ are to guide the eye.

Figure 2

Matter radii, $R_m$, for (a) ${}^{19}\mathrm{B}$, (b) ${}^{22}\mathrm{C}$, and (c) ${}^{31}\mathrm{Ne}$ as a function of neutron separation energy [$S_{(2)n}$]. Curves marked with closed circle, open circle, and upward triangle correspond to calculations assuming $1d$, $2s$, and $2p$ valence neutron configurations, respectively. The experimentally deduced $R_m$ and present mass determinations are shown (shaded rectangles). The insets indicate the principal valence neutron configuration deduced for each nucleus. Panel (d): $R_m$ for ${}^{22–31}\mathrm{Na}$ as a function of interaction radius ($R_I$) where the solid line represents a linear adjustment. The filled square corresponds to the matter radius for ${}^{31}\mathrm{Ne}$ deduced from its interaction radius (see text).