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Nuclei near the drip line
| In general, the study of nuclei far
from the valley of beta stability has progressed in a way that calls in mind the
development of nuclear physics in its earliest days. In particular, during the last
decade the advent of radioactive beam facilities
enabled the systematic study of nuclei close to the drip lines.
For many of these nuclei, the weak binding of the last nucleon
or nucleons may lead to a wave function with an external tail
extending far outside of the nuclear core as a result of quantum
mechanical barrier penetration. This unusual structure known as
halo structure by far exceeds the well defined surface of a
liquid drop model nucleus (see Figure). One or more
neutrons walk beyond the drop surface and form a misty cloud- a
halo- with similarities to the electrons which form clouds
around nuclei and make the atoms. In that context, halo nuclei are fragile objects, they are larger than standard, they interact easier with other nuclei (present enhanced reaction cross sections) and they present collective states in very low energies (soft dipole resonances). Such features can dramatically change our views for both the nuclear structure and the interaction potential. Proton halos are less pronounced because of the Coulomb barrier. |
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| Another effect, which is similar to the halo, is the neutron skin. Neutron skin nuclei present a large difference in the Fermi proton and neutron energies . This results in a neutron radius much larger than the proton radius. The skin leads to a more diffuse nuclear surface with similar consequences, as in the halo case, to the spin-orbit interaction and the pairing of nucleons. Halo/or neutron skin typical light nuclei are 11Li, 8He, and 6He. Drip line nuclei like 6He belong also to another category, the Borromean nuclei according to the borromean rings. The Borromean rings are interlocked in such a way that if any one of them were removed, the other two would also fall apart. They are the heraldic symbol of the Prince of Borromeo, and are carved in the stone of their castle in Lake Maggiore in northern Italy. Our laboratory collaborates with the Saclay group in a long range plan for studying the halo nucleus 6He (http://www-dapnia.cea.fr/Sphn/Exotiques/index.shtml). Several experiments were devoted by the Saclay group to 6He. One of them, the elastic and inelastic proton scattering experiment in inverse kinematics was the subject for a PhD thesis of Anastasios Lagoyannis. |
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