New modes of halo excitation inthe6He nucleus

Abstract

Predictions are made for the structure of a second 2 ϩ resonance, the soft dipole mode and unnatural parity modes in the 6 He continuum. We use a structure model which describes the system as a three-body ␣ϩNϩN cluster structure, giving the experimentally known properties of 6 He and 6 Li, and use the distortedwave impulse approximation ͑DWIA͒ reaction theory appropriate for dilute matter. The presence of both resonant and nonresonant structures in the halo excitation continuum is shown to be manifest in chargeexchange reactions as well as inelastic scattering with single nucleons. ͓S0556-2813͑97͒50302-5͔PACS number͑s͒: 21.45.ϩv, 21.60.Gx, 24.30.Gd, 27.20.ϩn The known spectrum of 6 He contains only the 0 ϩ bound state and the well known 2 ϩ (E*ϭ1.8 MeV͒ three-body resonance, and then a desert in the three-body ␣ϩnϩn continuum up to the 3 H ϩ 3 H threshold at about 13 MeV ͓1͔. While for 11 Li a response ͑E1 strength͒ function has been reconstructed from exclusive experiments ͓2,3͔, such information is still lacking for 6 He. Except for momentum distributions from fragmentation experiments with 6 He beams ͓4-6͔, the only data are from charge-exchange reactions with 6 Li to the 6 He continuum, but with poor statistics and limited angles ͓7-9͔.The recent developments of radioactive nuclear beam techniques and of dynamic approaches to three-body continuum theory ͓10͔ make it possible to investigate to what extent our knowledge of the lightest Borromean halo nucleus 6 He is complete. What are the specific features of the continuum of a system with a halo ground state? Below we give predictions of a second 2 ϩ three-body resonance that may be accessible in experiment, and also ͑a much less pronounced͒ 1 ϩ resonance. The so-called ''soft dipole mode'' suggested in ͓11,12͔ still needs clarification ͓13͔. According to existing three-body models it is not a simple binary core -point dineutron resonance, neither in 11 Li nor probably in 6 He, but although this seems now widely accepted, further tests within these three-body models are desirable. It shows no three-body pole structure, as discussed, e.g., in ͓14͔, and therefore it is still an open question whether the ''soft dipole mode'' is just a dynamical enhancement arising from final state interactions in the direct excitation of the three-body continuum. It is now possible for experiments to tell whether the three-body frameworks are adequate, since these models are shown in the present paper to give rise to other soft modes of other multipolarities. Such modes were suggested in ͓15͔, but need both theoretical and experimental clarification. We believe that the predictions given below are reliable as guide for future experiments, and that the observation of the dipole and other modes predicted here would support the validity of three-body models and their representation of the ''soft dipole mode'' as not being a genuine three-body resonance.The nucleus 6 He has in past years been used as a reference case, with the most reliable information on the binary core-n interac...

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.…”

Comment on “New modes of halo excitations in the6Henucleus”

“…We perform the full basis calculation of the strength function with the same renormalized interaction as in [1] and show that the size of the basis, needed for converged calculations of the 6 He continuum spectrum, is much larger than that for the discrete spectrum. The renormalized interaction of [1] therefore cannot be used for the continuum spectrum calculations with the same basis as for the ground state.…”

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We try to explain the differences in the 6 He dipole strength function in [1] and [2]. We perform the full basis calculation of the strength function with the same renormalized interaction as in [1] and show that the size of the basis, needed for converged calculations of the 6 He continuum spectrum, is much larger than that for the discrete spectrum.

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“…However our approach has the following modifications [3]: i) We use Faddeev rather than the Schrödinger equation; ii) Prior to solving the hyper-radial equations we calculate the eigenvalues λ n , as functions of the hyper-radius ρ, of the angular part of the Faddeev operator. The quantities λ n (ρ)ρ −2 then serve as effective potentials in the radial equations; iii) We use analytic large-distance solutions of the Faddeev equations for short range potentials [4] which greatly enhances the method and allows fully converged calculations.The nα interaction, originally fit to the scattering data, was somewhat modified in [1,5,6] in order to compensate for the limited basis,HereP l is a projection operator on the corresponding orbital momentum state l and s is the neutron spin. All lengths are in fm and energies in MeV.…”

“…The nα interaction, originally fit to the scattering data, was somewhat modified in [1,5,6] in order to compensate for the limited basis,…”