Neutron-neutron correlation in the halo dissociation of light exotic nuclei

Abstract

We present model results for the two-halo-neutron correlation functions, C nn , for the dissociation process of light exotic nuclei modelled as two neutrons and a core. A minimum is predicted for C nn as a function of the relative momentum of the two neutrons, p nn , due to the coherence of the neutrons in the halo and final state interaction. Studying the systems 14 Be, 11 Li,and 6 He within this model, we show that the numerical asymptotic limit, C nn → 1, occurs only for p nn > ∼ 400 MeV/c, while such limit is reached for much lower values of p nn in an independent particle model as the one used in the analysis of recent experimental data. Our model is consistent with data once the experimental correlation function is appropriately normalized. The discovery of radioactive exotic weakly bound nuclei, rich in neutrons or protons, beyond the drip line have brought a lot of interest in the nuclear structure and reactions of these unstable nuclei. The traditional nuclear models are unable to describe the long-range correlations between the nucleons of the halo. The large size of the halo has stimulated several experimental and theoretical studies devoted to clarify the new aspects of the structure, stability, and reaction of these nuclei, including astrophysical applications (see, for example, [1][2][3]).The effect of the large spatial extension of the neutron halo, was probed in a recent fusion experiment of 6 He with 238 U target [4]. It was observed a large reaction cross section due to a direct 2n (n represents a neutron) transfer rather than an enhancement of the complete fusion cross section. In view of the large size of the halo (r rms nn = 5.9 ± 1.2 fm [5], r rms nn is the nn root-mean-square radius) which is comparable to the size of 238 U itself, it is most likely that a correlated neutron-neutron (nn) is transferred to 238 U while the 4 He is still far from the fusion barrier, favoring the 2n transfer process.A novel view of the complete fusion reaction for large halo nuclei also emerges [6]: It is most probable that the core approaches the fusion barrier of an excited target nucleus that has already absorbed the halo. This interpretation is in line with the concept of an absorptive many-body potential written in the relative coordinates of the core, two neutrons, and the target [7].The need for a deeper insight into the key aspects of the structure and dynamics of the halo in Borromean three-body systems (where all the subsystems are unbound), like 6 He is evident. In this respect, using intensity interferometry applied with a new iterative technique, Marqués et al. [5,8] recently probed the spatial configuration of two-neutron halo systems and estimated the mean-square nn distances as well, considering the dissociation of 6 He, 11 Li, and 14 Be in the field of a heavy nucleus target. The spatial configuration of 11 Li was also studied by Petrascu et al. [9,10]. The nn correlation function, C nn , is extracted as a function of the relative momentum between the neutrons, p nn . As the absolute normaliza...

Spatial characteristics of borromean, tango, samba and all-bound halo nuclei

“…As detailed in ref. [6], our results for C nn calls for a different asymptotic normalization of the experimental data. We believe that our qualitative picture, evidencing a minimum of C nn , will survive in a more realistic three-body approach.…”

“…The interference effect produced by the inclusion of FSI originates a minimum for C nn , pushing the asymptotic limit to much larger values of p A than the ones considered in the asymptotic normalization of the experimental data (see Ref. [6]). …”

“…In this case, we shown in Table 1 the results of our calculations of the root-meansquare distances for the two neutrons of the core, for the halo-nuclei 6 He, 11 Li, 14 Be and 20 C (for an extended version of this table, see Ref. [5]).…”

“…If we consider the same three-body energy for all the configurations, the following sequence is applied for the mean-square radii (msr): msr borromean <msr tango <msr samba <msr all−bound . With the available low-energy twobody observables, in our scaling limit formalism, we have estimated the mean-square distances for the nuclei 6 He, 11 Li, 14 Be and 20 C, in a model where they are described as inert cores with two halo neutrons. Finally, we presented our model approach for the two-neutron correlation function C nn .…”

“…For the nnC three-body system, C nn is given by [6] TABLE 1. Neutron-neutron root-mean-square distances in halo nuclei.…”