Do Halo Nuclei Follow Rutherford Elastic Scattering at Energies Below the Barrier? The Case ofLi11

Abstract

The first measurement of the elastic scattering of the halo nucleus 11 Li and its core 9 Li on 208 Pb at energies near the Coulomb barrier is presented. The 11 Li þ 208 Pb elastic scattering shows a strong reduction with respect to the Rutherford cross section, even at energies well below the barrier and down to very small scattering angles. This drastic change of the elastic differential cross section observed in 11 Li þ 208 Pb is the consequence of the halo structure of 11 Li, as it is not observed in the elastic scattering of its core 9 Li at the same energies. Four-body continuum-discretized coupled-channels calculations, based on a three-body model of the 11 Li projectile, are found to explain the measured angular distributions and confirm that the observed reduction is mainly due to the strong Coulomb coupling to the dipole states in the low-lying continuum of 11 Li. These calculations suggest the presence of a low-lying dipole resonance in 11 Li close to the breakup threshold. One century ago, Rutherford [1] inferred the structure of the atom from the reaction data measured by Geiger and Marsden [2]. Since then, nuclear structure properties have often been deduced from nuclear reaction studies. With the advent of the first accelerated radioactive beams, new nuclear structures were discovered, such as the existence of a halo in some very loosely bound nuclei. In fact, 25 years ago, Hansen and Jonson [3] interpreted the large interaction cross section observed in 11 Li with light targets by Tanihata et al.[4] as due to the high probability of the outermost nucleons to be at large distances from the central core, which they referred to as a halo structure. The halo structure is a threshold phenomenon due to the low binding energy of the last nucleons. Halo nuclei have several features in common, such as a rather compact core, an extended neutron distribution, and very few, if any, excited states.The discovery of halo nuclei brought renewed interest in the modeling of nuclear reactions. This peculiar structure should affect the reaction properties at near-Coulomb barrier energies. Due to their low binding energy, the description of reactions involving halo nuclei should incorporate the coupling into the continuum. Current approaches to reaction theory involve different approximations whose validity needs to be checked when applied to halo nuclei.The most neutron-rich bound lithium isotope, 11 Li, is a fascinating case. Predicted to be unbound, it was identified in 1966 [5] and it is the archetype of a Borromean halo nucleus; i.e., the two different binary subsystems, 9 Li-n and n-n, are unbound, whereas the three-body system is bound by S 2n ¼ 369:15 AE 0:65 keV [6]. The ground state density distribution of 11 Li extends well beyond its core; i.e., the rms matter radius for the 9 Li isotope is 2:44 AE 0:06 fm Due to the loosely bound structure, the neutron halo is easily polarizable in the strong electric field of a heavy PRL 109, 262701 (2012) P H Y S I C A L

Li11Breakup onPb208at Energies Around the Coulomb Barrier

“…[15]. Here, we present the breakup data and the corresponding theoretical analysis in terms of semiclassical and coupled-channels calculations.…”

Simultaneous analysis of the elastic scattering and breakup channel for the reactionLi11+Pb208at energies near the Coulomb barrier

“…[24] and references therein). This was the aim of the experiment performed at the TRI-University Meson Facility (TRIUMF) for the 11 Li+ 208 Pb reaction at energies around (29.8 MeV) and below (24.3 MeV) the Coulomb barrier (V B ≈ 28 MeV) [22,23]. As expected, the experiment showed a strong reduction of the elastic cross section with respect to the Rutherford formula and a large yield of 9 Li fragments.…”

“…III. They show a remarkable suppression of the 11 Li elastic cross section with respect to the Rutherford formula [22], even at very forward angles. Note that data oscillations around 75…”

“…Recent advances in low-energy beam intensities in radioactive ion beam (RIB) facilities have allowed us to study these phenomena for several exotic nuclei, such as 6 He [14][15][16][17][18][19], 11 Be [20,21], and 11 Li [22,23] (see also Ref. [24] and references therein).…”

“…[22,23], the elastic and breakup data were compared with four-body continuum-discretized coupled-channel (4b-CDCC) calculations, using a three-body model of the 11 Li nucleus. These calculations provided a good agreement with both observables and confirmed that the large yield of 9 Li could be accounted for by considering a direct breakup mechanism, arising mainly from the strong dipole Coulomb couplings to the continuum states of 11 Li.…”