Physics Letters B 645 (2007) 128–132 www.elsevier.com/locate/physletb Spectroscopic factor of the 7 He ground state ✩ F. Beck a , D. Frekers b , P. von Neumann-Cosel a,∗ , A. Richter a , N. Ryezayeva a , I.J. Thompson c a Institut für Kernphysik, Technische Universität Darmstadt, D-64289 Darmstadt, Germany b Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany c Department of Physics, Surrey University, Guildford GU2 7XH, United Kingdom Received 28 September 2006; received in revised form 4 December 2006; accepted 18 December 2006 Available online 22 December 2006 Editor: V. Metag Abstract The neutron spectroscopic factor S n of the 7 He ground state is extracted from an R-matrix analysis of a recent measurement of the 7 Li(d, 2 He) 7 He reaction with good energy resolution. The width extracted from a deconvolution of the spectrum is Γ = 183(22) keV (full width at half maximum, FWHM). The result S n = 0.64(9) is slightly larger than predictions of recent ‘ab initio’ Green’s function Monte Carlo and fermionic molecular dynamics calculations.  2006 Elsevier B.V. All rights reserved. PACS: 21.10.Jx; 21.10.Pc; 25.45.Kk; 27.20.+n Keywords: Measured 7 Li(d, 2 He) 7 He reaction; Deduced g.s. resonance width; R-matrix analysis; Spectroscopic factor 1. Introduction The structure of light exotic nuclei is presently a subject of intense research. This is based on one hand on remarkable advances made in the theoretical description starting from ‘ab initio’ methods of nuclear structure calculations [1–3], and on the other hand on the rapid experimental progress in the pro- duction of radioactive beams and their use in reaction studies. As an example, thorough investigations of the neutron-rich He isotopes have demonstrated their dominant α-cluster structure, leading to a two-neutron halo in 6 He and a 4n + α structure in 8 He while the odd-mass isotopes 5,7 He are particle-unbound (see e.g. Ref. [4] and references therein). The present work, concerned with the structure of 7 He, is prompted by a recent study of the 7 Li(d, 2 He) 7 He reaction with good resolution [5] permitting a renewed extraction of the g.s. resonance parameters. In the frame of an R-matrix analysis, these data provide the neutron spectroscopic factor, which de- ✩ Work supported by the DFG under contract SFB 634. * Corresponding author. E-mail address: vnc@ikp.tu-darmstadt.de (P. von Neumann-Cosel). scribes to what extent the J π = 3/2 − g.s. of 7 He can be viewed as a 6 He ⊗ ν 1p 3/2 configuration. Spectroscopic factors are ba- sic quantities characterizing the single-particle (sp) nature of nuclear excitations and therefore serve as an important test of wave functions calculated with recently developed methods. In particular, predictions from Green’s function Monte Carlo (GFMC) calculations [6] are available [7] for the neutron spec- troscopic factors S n of the lowest J π = 3/2 − and 1/2 − states in 7 He, respectively: S n (3/2 − ) = 0.527(4) and S n (1/2 − ) = 0.873(6). The corresponding values from a shell-model calcu- lation with the Cohen and Kurath interaction [8] are 0.591 and 0.685. There exists [9] also a calculation for the g.s. within the fermionic molecular dynamics (FMD) model [10] using a re- alistic interaction derived with the unitary correlator operator method (UCOM) [3]. The predicted value is S n (3/2 − ) = 0.53. A spectroscopic factor of 0.37(7) has been extracted for the 7 He g.s. from a recent study of the 6 He(d, p) reaction in in- verse kinematics [11]. However, this value does not include the considerable theoretical uncertainty in dealing with transfer reactions to unbound states so that the authors of Ref. [11] con- cluded that their result was still in reasonable agreement with the GFMC prediction. 0370-2693/$ – see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2006.12.036