A search for the Jπ = 1/2⁻ spin–orbit partner of the Jπ = 3/2⁻ ground state in ⁷He has been performed with the ⁷Li(d,²He) charge-exchange reaction. The results are incompatible with recent claims of such a state at very low excitation energy [Meister M et al 2002 Phys. Rev. Lett. 88 102501] but rather suggest a resonance with parameters Ex = (1.2⁺⁰.⁵₋₀.₄) MeV, Λ = (1.9⁺⁰.⁸₋₀.₄) MeV. GT strengths deduced for the transitions to the lowest states in 7He are in remarkable agreement with ab initio quantum Monte Carlo calculations

Berg, A. M. van den

Bäumer, C.

De Frenne, D.

Frekers, D.

Grewe, E. W.

Haefner, P.

Jacobs, E.

Johanson, H.

Kalmykov, Y.

Negret, A.

Neumann-Cosel, P. von

Popescu, L.

Rakers, S.

Richter, A.

Ryezayeva, N.

Schrieder, G.

Shevchenko, A.

Wörtche, H. J.

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Structure of 7He studied with the 7Li(d,2He) reactionP von Neumann-Cosel1, C Bäumer2, A M van den Berg3, D Frekers2,D De Frenne4, E-W Grewe2, P Haefner2, E Jacobs4, H Johanson5,Y Kalmykov1, A Negret4, L Popescu4, S Rakers2, A Richter1,N Ryezayeva1, G Schrieder1, A Shevchenko1 and H J Wörtche31 Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany2 Institut für Kernphysik, Universität Münster, D-48149 Münster, Germany3 Kernfysich Versneller Instituut, 9747 AA Groningen, The Netherlands4 Vakgroep Subatomaire en Stralingsfysica, Universiteit Gent, B-9000 Gent, Belgium5 Gesellschaft für Schwerionenforschung, D-64291 Darmstadt, GermanyE-mail: vnc@ikp.tu-darmstadt.deAbstract. A search for the Jπ = 1/2− spin-orbit partner of the Jπ = 3/2− ground statein 7He has been performed with the 7Li(d,2He) charge-exchange reaction. The results areincompatible with recent claims of such a state at very low excitation energy [Meister M et al2002 Phys. Rev. Lett. 88 102501] but rather suggest a resonance with parameters Ex = (1.2+0.5−0.4)MeV, Γ = (1.9+0.8−0.4) MeV. GT strengths deduced for the transitions to the lowest states in7Heare in remarkable agreement with ab initio Quantum Monte Carlo calculations.Information about the structure of the exotic, neutron-rich 7He is scarce. The g.s. of 7He islocated at an energy ER = 0.445 MeV above the neutron emission threshold with spin-parityJπ = 3/2− [1]. It is interpreted as the 1p3/2 single-particle state of the unpaired neutron relativeto a 6He core. Excited states at Ex = 2.9 MeV with likely Jπ = 5/2− [2] and at Ex = 5.8 MeV[3] have been established. Recently a controversy arose about the possible observation of the1/2− spin-orbit partner of the 7He ground state. An investigation of 6He + n correlations after8He breakup at GSI [4] indicates such a state in 7He at an unusually low excitation energy ofEx = 0.57(10) MeV with a small width Γ = 0.75(8) MeV. A study of isobaric analog states of7He in the 6He(p,n) compound nucleus reaction at Notre Dame [5] contradicts the finding ofRef. [4] suggesting that a possible 1/2− resonance should lie above Ex  2.2 MeV (the highestenergy accessible in their experiment). However, based on continuum shell-model calculationstheir conclusion have been questioned by Halderson [6].The present work, described in detail in [7], provides an alternative access to this importantquestion by utilizing the 7Li(d,2He)7He charge-exchange reaction at zero degrees, where GTtransitions are selectively excited. This reaction at intermediate energies has been developedrecently as a spectroscopic tool for the study of GT strength distributions [8]. GT transitionspopulating spin-orbit partners - like the 1p3/2 and 1p1/2 states - should have similar strengths.Since the g.s. is sufficiently populated in a charge-exchange reaction [9], a low-lying resonancein 7He with the parameters suggested by Ref. [4] should give a clear signal.Figure 1(a) displays the low-energy region of the 7He spectrum measured at 0◦. Adecomposition is performed including the experimentally established resonances (short-dashedlines) and a quasifree background (long-dashed line). The latter is based on the semi-Institute of Physics Publishing Journal of Physics: Conference Series 20 (2005) 183–184doi:10.1088/1742-6596/20/1/037 International Symposium on Correlation Dynamics in Nuclei183© 2005 IOP Publishing LtdLicense: CC BY-NC-ND 3.0 Unported - Creative Commons, Attribution, NonCommercial, NoDerivs https://creativecommons.org/licenses/by-nc-nd/3.0/( c )Figure 1. Decomposition of the low-energy region of the 7He excitation spectrum (a) withoutor (b) with an additional resonance (hatched area) assuming the parameters of Ref. [4], or (c)assuming parameters Ex = 1.2 MeV and Γ = 1.9 MeV (from [7]).phenomenological parameterization of Ref. [10]. Its magnitude is constrained by the very gooddescription of this model obtained for a measurement of the 6Li(d,2He)6He reaction under thesame kinematics at large scattering angles, where the quasifree cross section dominates. Theg.s. and the excitation region above 2.5 MeV are well described. However, in between the dataovershoot the fit, indicating the presence of a possible further resonance. Indeed, the necessityfor an additional state is not only evident at 0◦ but also in the other spectra at larger scatteringangles. Assuming an additional resonance at low Ex with the parameters of Ref. [4] (hatchedarea) leads to the poor fit shown in Fig. 1(b). Evidently, such a resonance should be clearlyvisible in the data. On the other hand, inclusion of an additional resonance with Ex = 1.2 MeVand Γ = 1.9 MeV provides an excellent description of the data, see Fig. 1(c).Because of the experimental uncertainties of the other resonances [2, 3] included in thedecomposition, the centroid energy of such an additional resonance cannot be determined veryprecisely [7]. The result is Ex = (1.2+0.5−0.4) MeV with a corresponding width Γ = (1.9+0.8−0.4)MeV. It is possible to extract the B(GT) strength of the lowest three states in 7He from theproportionality to the (d,2He) cross section at zero momentum transfer [11]. The resultingB(GT) values are in excellent agreement [7] with ab initio Quantum Monte Carlo calculations[12] using the methods described in [13]. This finding further supports the interpretation of thepossible new low-energy resonance as Jπ = 1/2− spin-orbit partner of the 7He ground state.This work has been supported by the DFG under contract SFB 634, the state of Nordrhein-Westfalen, the EU under contract TMR-LSF ERBIMGECT980125, the Fund for ScientificResearch-Flandres, and by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek aspart of the research program of the Stichting FOM.References[1] Tilley D R et al 2002 Nucl. Phys. A 708 3[2] Korsheninnikov A A et al 1999 Phys. Rev. Lett. 82 3581[3] Bohlen H G et al 2001 Phys. Rev. C 64 024312[4] Meister M et al 2002 Phys. Rev. Lett. 88 102501[5] Rogachev G V et al 2004 Phys. Rev. Lett. 92 232502[6] Halderson D 2004 Phys. Rev. C 70 041603(R)[7] Ryezayeva N et al 2005 Phys. Lett. B submitted[8] Frekers D 2004 Nucl. Phys. A 731 76[9] Stokes R H and Young P G 1967 Phys. Rev. Lett. 18 611[10] Erell A et al 1986 Phys. Rev. C 34 1822[11] Grewe E-W et al 2004 Phys. Rev. C 69 064325[12] Pieper S C and Wiringa R B 2001 Annu. Rev. Nucl. Part. Sci. 51 53[13] Schiavilla R and Wiringa R B 2002 Phys. Rev. C 65 054302184

Structure of ⁷He studied with the ⁷Li(d,²He) reaction

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Structure of ⁷He studied with the ⁷Li(d,²He) reaction

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