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5 Halo Nuclei: Properties and Experimental Techniques
Fig. 5.3 Charge exchange
and interaction cross sections
for three Li isotopes
quadrupole moments of
8,9,11 Li [17]. It was observed that unlike the total interactions
the charge-changing cross sections remain same for
8,9,11 Li (Fig. 5.3). In addition,
experiments at the ISOLDE in CERN [17] showed the quadrupole moments of
9 Li
and
11 Li to be nearly equal. Putting both the results together the obvious conclusion
that emerges about the structure of
11 Li nucleus is a compact core of
9 Li surrounded
by a loosely bound veil of two valence neutrons forming a nuclear halo. The term
halo was coined in a paper by Hansen and Jonson in 1987 [43]. One can justifiably
say the discovery of
11 Li and its exotic structure was epochal and ushered in a new
era of nuclear physics.
5.2 Salient Structural Features of Halo Nuclei
More than three decades after Tanihata’s experiment, the concerted efforts of the
nuclear physicists, both theorists and experimentalists, coupled with phenomenal
progress in RIB technologies, have resulted in the synthesis of a large number of
neutron-rich halo nuclei with structural properties very similar to the
11 Li nucleus.
We summarize below the striking features of halo nuclei that set them apart from
more stable nuclei away from the drip lines. They are,
1. Unusually small separation energy for the last neutron (S n ) or last two neutrons
(S 2n )
2. Very large matter radius
3. Narrow momentum distribution of the fragments of halo nuclei
4. Borromean property of many two-neutron halo nuclei.