Neutron halo ofHe6in a microscopic model

Abstract

The two-neutron separation energy of 6 He has been reproduced for the first time in a realistic parameter-free microscopic multicluster model comprising the α + n + n and t + t clusterizations, with α cluster breathing excitations included. The contribution of the t + t channel is substantial. A very thick (0.85 fm) neutron halo has been found in full agreement with the results of the latest phenomenological analysis.Recently nuclei far from stability attract much interest in nuclear physics. Prominent representatives of these nuclei are e.g. 6 He, 8 He, 8 Li, 8 B, 11 Li. There are several calculations for their description in macroscopic [1,2,3,4], semimicroscopic [5] and microscopic [6] models. In this paper we use our dynamical microscopic multiconfiguration multicluster model, developed and applied recently to the ground state of 6 Li [7], to study the neutron halo structure of the ground state of 6 He. All realistic macroscopic three-body models underbind 6 He by about 0.6-0.3 MeV. The situation is similar for 11 Li, which is predicted to be unbound by the most realistic parameter-free variational calculation [2]. Our first aim is to check the validity of these models by comparing them with our microscopic model, and to understand the physics of this underbinding. Secondly, we calculate the thickness of the neutron halo of 6 He. For this quantity there are two contradicting experimental predictions, both of them are based on Glauber-type analyses of certain reaction cross section data. A simpler analysis gives ∼ 0.4 fm [8], while the other one, which comes from more realistic model assumptions, results in ∼ 0.9 fm [9]. The latter result is well reproduced in a relativistic mean field model [9]. As we shall see, our model strongly supports the second prediction, too.As the α particle is an inert cluster, it is natural to assume that an α + n + n model is a most adequate one for the description of 6 He. This nucleus is said to be borromean [10], which means that after the removal of any of the three clusters, the remaining nucleus decays into two fragments. This indicates that 6 He has a genuine three-body nature and explains why the macroscopic three-body models are so successful in describing its ground state structure and reactions [10]. Although several physical quantities have been calculated, and a good overall agreement with experiment has been reached in these models, it is neccessary to investigate the validity of their foundation starting from microscopic grounds, because nucleon exchange and cluster rearrangement effects are expected to be important in this mass range. II. MODELThe microscopic dynamical multiconfiguration three-cluster model starts from the following trial function for the six-body problem: Ψ = S,l1,l2,L Ψ α(nn) S,(l1l2)L + S,l1,l2,L Ψ n(αn) S,(l1l2)L =

Investigation ofHe6cluster structures

“…Among these Borromean nuclei, the case of the 6 He nucleus is of a special interest from both experimental and theoretical points of view to study the halo phenomenon and 3-body correlations, especially because the α core can be represented as structureless. The importance of the dineutron and cigarlike α+2n configurations predicted for the 6 He ground-state wave function [2] was investigated by means of a 2n transfer reaction or a ra-diative proton capture [3,4]. These two experiments give opposite conclusions concerning the relative importance of dineutron and cigarlike configurations.…”

“…Furthermore, the two neutrons halo systems, such * Present address: GSI, D-64291 Darmstadt, Germany; Electronic address: l.giot@gsi.de as 6 He, 11 Li and 14 Be, exhibit Borromean characteristics, whereby the two-body subsystems are unbound [2]. Among these Borromean nuclei, the case of the 6 He nucleus is of a special interest from both experimental and theoretical points of view to study the halo phenomenon and 3-body correlations, especially because the α core can be represented as structureless.…”

“…According to translational invariant shell model calculations [5], the 6 He nucleus is expected to have a large spectroscopic amplitude for the t +t configuration as well as for the α+2n configuration, like the 6 Li nucleus for the configurations α+d and 3 He+t . Microscopic multicluster calculations show also that the binding energy of 6 He is better reproduced by including some t +t clustering in the ground state wave function [6]. The spectroscopic factors predicted for the t +t configuration by the different models range from 0.44 [7] to 1.77 [5].…”

Three-triton states in9Li

“…This is one of the first example that completely different cluster configurations of triton-type (α + t + t) and α type (α+α+n+n) coexist in a single nucleus around the same energy region. A well known other example is the appearance (mixing) of 4 He+2n and t+t structures in 6 He [13][14][15].…”

Four-body DWBA calculations of the Coulomb breakup of 6He

“…Clearly, the interaction of valence neutrons is vital for the stability of the two-neutron halo nuclei. They provide interesting examples of the three-body problem in the limit of very weak binding [7], and the three-body methods (within both Faddeev and cluster type approaches) have been extensively applied to study them [3,[8][9][10][11][12][13][14][15][16][17][18][19]. Other approaches like non-relativistic [20] and relativistic [21,22] mean field models and many-body calculations [23] have also been used to investigate the structure of such nuclei.…”