7.2 Halo Nuclei
317
the vicinity of the 4n-emission threshold as a result of the collective coupling of
many-body resonances via the 4n-emission channel. Tetraneutron correlations could
exist, for example, in the ground states of 10 He, 16 Be, 19 B, and in the low-energy
continuum of 8 He, 14 Be, 22 C. Manifestation of tetraneutron correlations in heavier
nuclei is plausible in relatively high partial waves ( ≤ 3) [38].
7.2
Halo Nuclei
As one approaches the particle driplines, the amount of spectroscopic information
concerning bound states becomes scarce, and one relies on the analysis of decaying
states and reaction dynamics to extract nuclear structure data. A halo in the spatial
distribution of loosely bound neutron-rich nuclei appears [39, 40], and its radial
extension for neutrons in low- orbits is limited only by the two-body correlations
which cause an anti-halo effect [41, 42]. Similar phenomena related to the opening
of quantum system appear, e.g. in 4 He N atomic clusters where giant loosely bound
clusters have been observed [43]. Theoretically, a proper microscopic description
of both the many-body correlations and the continuum of positive energy states and
decay channels is mandatory for these systems. Experimentally, a halo structure is
noticed by an increase of the matter radius, larger by a sizable factor than the typical
value of 1.25 A 1/3 fm encountered in well-bound nuclei [44]. The first halo nucleus
to have been discovered is that of the 11 Li ground state. 11 Li has been studied
at Lawrence Berkeley Laboratory by Tanihata et al. [39, 45], where a very large
cross section involving neutron-rich helium and lithium isotopes was measured.
This pointed out to the halo structure of the 11 Li ground state, which, however,
demanded another 20 years to be confirmed [46]. Interestingly, first halo nucleus
6 He had been produced already in 1936, at the early age of quantum mechanics,
using neutron beams on a 9 Be target [47].
Halo states can exist only because of their quantum nature. Indeed, they consist
of a few loosely bound nucleons, in most cases one or two, moving outside a wellbound core, and whose wave function extends in the classically forbidden region.
The simplest example of a halo nucleus is that of a weakly bound neutron in = 0
state moving in a spherical potential generated by the core. Due to the very slow
exponential decay of the neutron wave function, the probability to find a neutron in
the classically forbidden region will be large.
Several factors have to be gathered in order for halo to develop. Firstly, the
considered nuclear state must be loosely bound, as otherwise all nucleons remain
close to the center of the nucleus. Hence, halo is a phenomenon in the vicinity of
particle-emission threshold. Moreover, the wave function of nucleons in the halo has
to be formed mainly by partial waves of small orbital angular momentum. Indeed,
partial waves of large orbital angular momentum have an important confining effect
due to the centrifugal barrier, so that the least bound nucleons remain trapped,
preventing the halo from appearing. Additionally, two-neutron halos have been
noticed to arise in the so-called Borromean nuclei where all A − 1 subsystems
are unbound but A and A − 2 systems are bound because of the residual nucleon–
317
the vicinity of the 4n-emission threshold as a result of the collective coupling of
many-body resonances via the 4n-emission channel. Tetraneutron correlations could
exist, for example, in the ground states of 10 He, 16 Be, 19 B, and in the low-energy
continuum of 8 He, 14 Be, 22 C. Manifestation of tetraneutron correlations in heavier
nuclei is plausible in relatively high partial waves ( ≤ 3) [38].
7.2
Halo Nuclei
As one approaches the particle driplines, the amount of spectroscopic information
concerning bound states becomes scarce, and one relies on the analysis of decaying
states and reaction dynamics to extract nuclear structure data. A halo in the spatial
distribution of loosely bound neutron-rich nuclei appears [39, 40], and its radial
extension for neutrons in low- orbits is limited only by the two-body correlations
which cause an anti-halo effect [41, 42]. Similar phenomena related to the opening
of quantum system appear, e.g. in 4 He N atomic clusters where giant loosely bound
clusters have been observed [43]. Theoretically, a proper microscopic description
of both the many-body correlations and the continuum of positive energy states and
decay channels is mandatory for these systems. Experimentally, a halo structure is
noticed by an increase of the matter radius, larger by a sizable factor than the typical
value of 1.25 A 1/3 fm encountered in well-bound nuclei [44]. The first halo nucleus
to have been discovered is that of the 11 Li ground state. 11 Li has been studied
at Lawrence Berkeley Laboratory by Tanihata et al. [39, 45], where a very large
cross section involving neutron-rich helium and lithium isotopes was measured.
This pointed out to the halo structure of the 11 Li ground state, which, however,
demanded another 20 years to be confirmed [46]. Interestingly, first halo nucleus
6 He had been produced already in 1936, at the early age of quantum mechanics,
using neutron beams on a 9 Be target [47].
Halo states can exist only because of their quantum nature. Indeed, they consist
of a few loosely bound nucleons, in most cases one or two, moving outside a wellbound core, and whose wave function extends in the classically forbidden region.
The simplest example of a halo nucleus is that of a weakly bound neutron in = 0
state moving in a spherical potential generated by the core. Due to the very slow
exponential decay of the neutron wave function, the probability to find a neutron in
the classically forbidden region will be large.
Several factors have to be gathered in order for halo to develop. Firstly, the
considered nuclear state must be loosely bound, as otherwise all nucleons remain
close to the center of the nucleus. Hence, halo is a phenomenon in the vicinity of
particle-emission threshold. Moreover, the wave function of nucleons in the halo has
to be formed mainly by partial waves of small orbital angular momentum. Indeed,
partial waves of large orbital angular momentum have an important confining effect
due to the centrifugal barrier, so that the least bound nucleons remain trapped,
preventing the halo from appearing. Additionally, two-neutron halos have been
noticed to arise in the so-called Borromean nuclei where all A − 1 subsystems
are unbound but A and A − 2 systems are bound because of the residual nucleon–