Walking at the drip line

Among exotic nuclei those at the drip line which are unstable against neutron emission are particularly interesting because they convey information on the nuclear force in the most extreme situations. Strictly speaking they are not "nuclei" but they exist thanks to long living resonances between a neutron and a bound "core" nucleus. Adding one more neutron they become bound and are called "borromean". Being particularly exotic they have attracted much attention in past years, see for example Refs.[1, 2, 3]. One very challenging example is Be-13 whose level ordering has been discussed in a large number of papers in which it has been studied by transfer [4] and fragmentation experiments [5]-[11], or it has been discussed theoretically[12]-[19]. Although projectile fragmentation spectra show evident similarities, the interpretations of data all differ from each other. In this paper we argue that a way trough the problem could be to try to establish first, or at the same time, the quite elusive "nature" of the second s-state in the Beryllium isotopes with A=9-14. On the other hand there are other recent neutron removal experiments leading to nuclei unstable by one or more proton emissions [20], and thus somewhat mirror to borromean nuclei, performed with nuclei close to the proton drip line. It has been shown that by taking in coincidence all (charged) particles but the removed neutron, reconstructing the invariant mass and gating on the ground state peak, it is possible to obtain the longitudinal momentum distribution of the unbound "core". One can link it to the original wave function of the bound orbital and thus determine the initial neutron angular momentum from the shape of the distribution and the initial occupation probability from the absolute removal cross section. Then it is clear that modern experiments and theories are able to study unstable nuclei with the same degree of accuracy as stable nuclei. Such a line of research offers a great potential for numerous further studies beyond the drip line.