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Halo Structure of 14Be VOLUME 86, NUMBER 4 PHYSICAL REVIEW LETTERS 22JANUARY 2001 Halo Structure of 14Be M. Labiche,1,* N. A. Orr,1,† F. M. Marqués,1 J. C. Angélique,1 L. Axelsson,2 B. Benoit,3 U. C. Bergmann,4 M. J. G. Borge,5 W. N. Catford,6 S. P.G. Chappell,7 N. M. Clarke,8 G. Costa,9 N. Curtis,6,8 A. D’Arrigo,3 E. de Góes Brennand,3 O. Dorvaux,9 G. Fazio,10 M. Freer,1,8 B. R. Fulton,8 G. Giardina,10 S. Grévy,11,1 D. Guillemaud-Mueller,11 F. Hanappe,3 B. Heusch,9 K. L. Jones,6 B. Jonson,2 C. Le Brun,1 S. Leenhardt,11 M. Lewitowicz,12 M. J. Lopez,12 K. Markenroth,2 A. C. Mueller,11 T. Nilsson,2,‡ A. Ninane,1,§ G. Nyman,2 F. de Oliveira,12 I. Piqueras,5 K. Riisager,4 M. G. Saint Laurent,12 F. Sarazin,12,k S. M. Singer,8 O. Sorlin,11 and L. Stuttgé9 1Laboratoire de Physique Corpusculaire, ISMRA et Université de Caen, IN2P3-CNRS, F-14050 Caen Cedex, France 2Experimentall Fysik, Chalmers Tekniska Högskola, S-412 96 Göteborg, Sweden 3Université Libre de Bruxelles, PNTPM, CP 226, B-1050 Bruxelles, Belgium 4Institut for Fysik og Astronomi, Aarhus Universitet, DK-8000 Aarhus C, Denmark 5Instituto de Estructura de la Materia, CSIC, E-28006 Madrid, Spain 6Department of Physics, University of Surrey, Guildford, GU2 7XH, United Kingdom 7Nuclear and Astrophysics Laboratory, University of Oxford, Oxford OX1 3RH, United Kingdom 8School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom 9Institut de Recherches Subatomique, IN2P3-CNRS, Université Louis Pasteur, F-67037 Strasbourg Cedex, France 10Dipartimento di Fisica, Università di Messina, I-98166 Messina, Italy 11Institut de Physique Nucléaire, IN2P3-CNRS, F-91406 Orsay Cedex, France 12GANIL (CEA/DSM-CNRS/IN2P3), BP 55027, F-14076 Caen Cedex, France (Received 6 June 2000) The two-neutron halo nucleus 14Be has been investigated in a kinematically complete measurement of the fragments (12Be and neutrons) produced in dissociation at 35 MeV͞nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were mea- sured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with 2 three-body model calculations suggests that the halo wave function contains a large n͑2s1͞2͒ admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonreso- nant soft-dipole excitation. DOI: 10.1103/PhysRevLett.86.600 PACS numbers: 27.20.+n, 24.30.Gd, 25.60.Dz, 25.60.Gc The size and distribution of matter in the nucleus have ture of 14Be beyond the matter radius [11–14]. Compared long played a central role in nuclear physics. In stable nu- to the other halo systems, the comparatively strong binding 14 clei the neutron and proton distributions exhibit essentially of the valence neutrons in Be (S2n ෇ 1.34 6 0.11 MeV 2 identical radii. In contrast, for some light nuclei far from [15,16]) combined with the n͑d5͞2͒ component may pro- stability, which combine a large neutron excess with very vide a new window on continuum excitations, including weak binding, large differences have been found. Such the long sought after soft-dipole resonance [17,18]. “halo” systems are well described by a core, resembling a The goal of the present study was thus to explore normal nucleus, surrounded by an extended valence neu- the halo structure and continuum excitations of the two- tron density distribution [1]. neutron halo nucleus 14Be. The tool chosen was a kine- In general terms the halo may be regarded as a threshold matically complete measurement of the fragments (12Be phenomenon whereby the loosely bound valence neutrons and two neutrons) from the dissociation of a 14Be beam on tunnel with significant probability into the region outside C and Pb targets. Such a measurement allowed the two- the core potential. Within a simple quasideuteron descrip- neutron removal cross sections, neutron angular distri- tion, the extent of the halo is governed by the separation butions, and invariant mass spectra to be extracted (an energy and reduced mass of the system [2]. Under more analysis of the neutron-neutron correlations has been realistic considerations the development of the halo is also presented elsewhere [19]). The use of C and Pb targets influenced by the centrifugal barrier [3]. In the cases of permitted the electromagnetic component of the dissocia- 6,8He, 11Be, and 11Li, which have been investigated ex- tion (EMD) to be deduced. perimentally in considerable detail, the valence neutrons The 14Be beam (E ෇ 35 MeV͞nucleon, ϳ130 pps) 18 occupy the 2s1͞2 and/or 1p3͞2,1͞2 single-particle orbitals. was prepared from a 63 MeV͞nucleon O primary beam In 14Be the configuration of the halo neutrons would, in a using the LISE3 spectrometer at GANIL. The energy 2 naive shell model prescription, be n͑d5͞2͒ . More sophisti- spread in the beam was 10% and was compensated 2 cated models suggest, however, that a n͑s1͞2͒ admixture is for by a time-of-flight measurement over a 24 m flight also present [4–7]. Unfortunately, a paucity of experimen- path between a parallel-plate avalanche counter (PPAC) tal data [8–10] has precluded the elucidation of the struc- located at the first focus of the spectrometer and the beam 600 0031-9007͞01͞86(4)͞600(4)$15.00 © 2001 The American Physical Society VOLUME 86, NUMBER 4 PHYSICAL REVIEW LETTERS 22JANUARY 2001 identification Si detector (300 mm) located just up- 40 stream of the targets. The beam particles were 35 tracked onto the breakup targets (C 275 mg͞cm2,Pb ͞ 2 30 (a) 570 mg cm ) using two position sensitive PPAC’s (reso- [b/sr] lution FWHM ഠ 1 2 mm). The charged fragments from Ω 25 2 breakup were identified using a large area (5 3 5 cm ) po- /d sition sensitive (FWHM ഠ 0.5 mm) Si-CsI telescope (Si σ 20 500 mm, CsI 2.5 cm) centered at zero degrees and located d 15 11.4 cm downstream of the target. The energy response of 10 the telescope (FWHM ෇ 1.5%) was calibrated using vari- ous mixed secondary beams containing 12Be with energies 5 straddling that expected for 12Be fragments from the 0 14 dissociation of Be. In order to account for events arising 30 from reactions in the telescope, data were also acquired without a reaction target and the beam energy reduced 25 by an amount corresponding to the energy loss in the (b) target. 20 The neutrons emitted at forward angles were detected using the 99 elements of the DEMON array [20]. The array 15 covered angles between 113± and 240± in the horizontal plane and 614± in the vertical with the modules arranged 10 in a staggered configuration at distances between 2.5 and 5 6.5 m from the target [20]. Such a geometry provided for a relatively high two-neutron detection efficiency (1.5%) 0 while reducing the rate of cross talk —both intrinsically 0 5 10 15 20 25 30 35 40 and via the use of an off-line rejection algorithm—to neg- θ [deg] ligible levels [20,21]. A threshold of 15 MeV on the neu- n tron energy was applied in the off-line analysis to eliminate FIG. 1. (a) Single-neutron angular distributions for dissocia- the small number of evaporation neutrons arising from the tion on C (open symbols) and Pb (solid points). The results for target [21]. C have been scaled by a factor of 1.8 so as to represent the nu- The results obtained for the two-neutron removal cross clear contribution to dissociation on Pb (see text). (b) Deduced 12 EMD single-neutron angular distribution for reactions on Pb. sections, s22n ( Be identified in the telescope), the single-neutron angular distributions, ds͞dV (12Be and neutron), and the associated angle integrated (0± 40±) tion [22] the average neutron multiplicity should be 1.5, in accordance with that measured here. cross sections, sn, are displayed in Table I and Fig. 1a. In addition, the average neutron multiplicities have been Turning to the Pb target, where nuclear and Coulomb breakup is present, the enhanced cross section is indica- derived (mn ෇ sn͞s22n) and are also listed. The single- neutron angular distributions were well characterized by tive of a substantial EMD contribution. Assuming that a Lorentzian line shape [8,21] and the corresponding the nuclear-Coulomb interference is small, the C target data (which arises essentially from nuclear induced reac- momentum width parameters, Gn, have been tabulated. In terms of the mechanisms leading to dissociation, tions) may be scaled to estimate the nuclear contribution to unless the halo neutrons are highly spatially correlated, breakup on Pb [18,21]. Given a root-mean-square radius of 14 nucl͑ ͒ ෇ nuclear breakup will proceed via single-neutron removal 3.2 fm for Be [13,14], s22n Pb 0.85 6 0.07 b and, EMD͑ ͒ ෇ (absorption or diffraction) followed by the in-flight de- consequently, s22n Pb 1.45 6 0.40 b. The latter can cay of 13Be [22]. This process is reflected in the narrow be compared to the value of 0.47 6 0.15 b measured at single-neutron angular distribution for the C target which 800 MeV͞nucleon [18]. Importantly, for halo nuclei, the is well reproduced assuming passage via a low-lying reso- EMD cross section is dominated by the E1 component nance in 13Be [21,23]. Furthermore, as approximately [24]. An enhancement with decreasing beam energy is equal contributions are expected for absorption and diffrac- thus expected, owing to the large amount of dipole strength TABLE I.
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