By using the 1H(6Li,6Be)n charge-exchange reaction, continuum states in 6Be were populated up to E_t=16 MeV, E_t being the 6Be energy above its three-body decay threshold. In kinematically complete measurements performed by detecting alpha+p+p coincidences, an E_t spectrum of high statistics was obtained, containing approximately ~5x10^6 events. The spectrum provides detailed correlation information about the well-known 0^+ ground state of 6Be at E_t=1.37 MeV and its 2^+ state at E_t=3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by Delta L=1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the 6Li ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross section obtained in the measured energy range.Comment: 8 pages, 7 figure
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...
The electron-ion scattering experiment ELISe is part of the installations envisaged at the new experimental storage ring at the international Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany. It offers an unique opportunity to use electrons as probe in investigations of the structure of exotic nuclei. The conceptual design and the scientific challenges of ELISe are presented.
An unusually large value of the 22 C matter radius has recently been extracted from measured reaction cross sections. The giant size can be explained by a very loose binding that is, however, not known experimentally yet. Within the three-body cluster model we have explored the sensitivity of the s-motion-dominated 22 C geometry to the two-neutron separation energy. A low energy of a few tens of keV is required to reach the alleged experimental lower value of the matter radius, while the experimental mean radius requires an extremely tiny binding. The dependence of the 22 C charge radius on the two-neutron separation energy is also presented. The soft dipole mode in 22 C is shown to be strongly affected by the loose binding and should be studied in the process of Coulomb fragmentation.
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