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 $^{ 19}$B, $^{ 22}$C and $^{ 29}$F as well as that of $^{ 34}$Na. In addition, the most precise determinations to date for $^{ 23}$N and $^{ 31}$Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in $^{ 22}$C, with a dominant $\nu2s_{ 1/2}^2$ configuration, and a single-neutron halo in $^{ 31}$Ne with the valence neutron occupying predominantly the 2$p_{ 3/2}$ orbital. Despite a very low two-neutron separation energy the development of a halo in $^{ 19}$B is hindered by the 1$d_{ 5/2}^2$ character of the valence neutrons.
Categories: nucl-ex