We consider the evolution of the neutron-nucleus scattering length
for the lightest nuclei. We show that, when increasing the number of
neutrons in the target nucleus, the strong Pauli repulsion is weakened
and the balance with the attractive nucleon-nucleon interaction results
into a resonant virtual state in ^{18}18B.
We describe ^{19}19B
in terms of a ^{17}17B-nn-nn
three-body system where the two-body subsystems
^{17}17B-nn
and nn-nn
are unbound (virtual) states close to the unitary limit. The energy of
^{19}19B
ground state is well reproduced and two low-lying resonances are
predicted. Their eventual link with the Efimov physics is discussed.
This model can be extended to describe the recently discovered resonant
states in ^{20,21}20,21B.
THE THREE-BODY SYSTEM δ CIRCINI