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Threshold and Continuum Structures in Exotic Nuclei

Threshold and Continuum Structures in Exotic Nuclei. Ian Thompson University of Surrey, Guildford, England with J. Tostevin, J. Mortimer, T. Tarutina (Surrey), B. Danilin (Surrey, Kurchatov). Topics to Discuss. Few-body vs many-body behaviour Need for spectroscopy from breakup

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Threshold and Continuum Structures in Exotic Nuclei

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  1. Threshold and Continuum Structures in Exotic Nuclei Ian Thompson University of Surrey, Guildford, England with J. Tostevin, J. Mortimer, T. Tarutina (Surrey), B. Danilin (Surrey, Kurchatov) Nens 03

  2. Topics to Discuss • Few-body vs many-body behaviour • Need for spectroscopy from breakup • Knockout to bound states • 1N stripping of Borromean halo nuclei • Elastic breakup: • E1 and E2 in 8B breakup? • Continuum states: energy correlations • Conclusions Nens 03

  3. Few- vs many-body dynamics • Nuclei typically show few-body behaviour just near and above the cluster separation thresholds. • Many exotic nuclei have just one or a few bound states, and hence show pronounced cluster dynamics even in their ground states. Nens 03

  4. Role of the Continuum? • The continuum appears in several ways: • Part of expansion of bound states; • eg needed in RPA for weakly bound states • Dominated by resonances; • These ‘unbound states’ identified eg with shell model eigenstates above threshold • In non-resonant continuum; • eg in breakup reactions. • ALL important parts of nuclear structure!! Nens 03

  5. Reactions to probe structure • Structure may be probed by elastic scattering or cluster transfers, • but breakup is typically the largest. • Review: • the structure information that is present in breakup amplitudes, • bound structure details that can be extracted from different classes of breakup reactions. Nens 03

  6. Stripping Reactions • Stripping = inelastic breakup, removes a surface nucleon by a high-energy interaction with a target. • Can reveal the spectroscopic factors for a wide range of final states. • These states may be distinguished by coincident -rays. • Review the measurement of spin, parity, and absolute spectroscopic factors. Nens 03

  7. b Contributions from surface and beyond Eikonal reaction theory for the breakup 12Be+9Be  11Be(gs)+X, 80A MeV c 12Be v 9Be Nens 03

  8. Momentum content: p-shell 19F No gamma detection 16O 14N 12C 11B N=14 N=8 distributions narrow (weak binding) or s-states as one crosses shell or sub-shell closures E.Sauvan et al., Phys Lett B 491 (2000) 1 Nens 03

  9. Knockout reactions 9Be(17C, 16C g)X (Ebeam=60 MeV/A) (a) 8% s + 92% d (b) 26% s + 74% d (c) 100% d SM calculation predict no 16C(0+) in the 17C(g.s.). Experiment measured a 20% branch into 16C(0+) . Higher order processes? Maddalena et al., PRC63(01)024613 Nens 03

  10. N=8 neutron shell closure in 12Be? =0 C2S=0.42 =1 C2S=0.37 from A. Navin et al., PRL 85 (2000) 266 Nens 03

  11. Ground state structure of 8B p3/2 137 keV p3/2 566 keV Proton removal from 8B measured at the GSI with gamma coincidences, sees a (15%) branch from an excited 7Be(1/2-) core component in the 8B wave function. from D.Cortina-Gil et al., Phys Lett B 529 (2002) 36, NPA 720 (2003) 3 Nens 03

  12. Can define reduction factor th Shell model structure plus eikonal reaction More bound systems Deduced vs. shell model spectroscopic factors Mostly weakly bound n-rich systems P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219 Nens 03

  13. (e,e'p) 0.51(3) 0.67(5) Knockout: Absolute spectroscopy From B.A. Brown et al. PRC 65 (2002) 061601(R) 0.53(2) Sp=15.96 0.49(2) Sn= 18.72 Sp= 12.13 0.68(4) Sn= 15.66 0.56(3) Nens 03

  14. neutron • proton Strongly bound hole states Reduction Factor Rs inclusive P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219 Nens 03

  15. neutron • proton Weakly bound states Expts with good statistics Reduction Factor Rs P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219 Nens 03

  16. Sn=0.8(0.3) MeV s-wave d-wave Combining Knockout and other Tools 9Be(19C, 18C g)X (Ebeam=60 MeV/A) Maddalena et al., PRC63 024613 (2001) Nens 03

  17. 1 Neutron stripping from three-body Borromean Nuclei • Removal of a neutron from 6He, 11Li, 14Be, • populates states of 5He, 10Li or 13Be. • Experiments measure decay spectrum of 5He = 4He + n, 13Be = 12Be + n, etc • Can we predict any energy and angular correlations by Glauber model? • Can we relate these correlations to the structure of the A+1 or the A+2 nucleus? Nens 03

  18. 1N stripping from 6He g.s. • Calculate overlaps: <5He(Eα-n) | 6He(gs)> for a range of 5He(Eα-n)> bin states, • smooth histogram of Glauber bin cross sections. • GSI data (H.Simon) Theory: σstr=137 mb, σdiff=38 mb Expt: σstr=127±14 mb, σdiff=30±5 mb from T. Tarutina thesis (Surrey) Promising technique! Nens 03

  19. Calculate overlaps: <13Be(Eα-n)|14Be(gs)> Inert-core 13,14Be wfs. GSI data (H.Simon) from T. Tarutina thesis (Surrey) See softer data, and not pronounced virtual-s and resonant-d peaks. 1N stripping from 14Be g.s. Theory: σstr=109 mb, σdiff=109 mb Expt: σstr=125±19 mb, σdiff=55±19 mb Nens 03

  20. Elastic Breakup • Elastic Breakup = Diffraction Dissociation: • all nuclear fragments survive along with the target in its ground state, • probes continuum excited states of nucleus. • For dripline nuclei , with few discrete states, these breakup reactions are the main probe of excited states • Review correlations in the three-body continuum of Borromean nuclei. Nens 03

  21. E1 & E2 breakup of 8B • One-proton bound state known: • 7Be(0p3/2+0p1/2)|2+ at -0.137 MeV • Need spectroscopy of non-resonant continuuum! • B(E1) & B(E2) for transition ps,d need to be accurately known • E1 and E2 amplitudes interfere in p||(7Be) momentum distribution • so measure relative E2/E1 amplitudes from asymmetries. Nens 03

  22. CDCC calculations with scaled E2 amplitudes - need to increase asymmetry again! 8B + 208Pb 7Be parallel momentum distributions 44 MeV/A Dot-dashed: semiclassical Coul. Solid: Coulomb+nuclear DWBA Dashed: CDCC coupled channels - reduced asymmetry from Mortimer et al., Phys Rev C 65 (2002) 64619 Nens 03

  23. Ground state plot: Continuum 3-3 scattering states Now average scattering wave functions over angles of knn and kcn-n Obtain similar plots for continuum energies. 3-body Borromean Nuclei Nens 03

  24. Virtual states & Resonances from B. Danilin, I. Thompson, et al (in preparation) Effect of n-n ‘resonance’ in E(c-n), E(cn-n) coordinates Virtual n-n pole Nens 03

  25. 6He excitations & resonances No pronounced 1- resonance Pronounced 2+ resonance Nens 03

  26. Assorted Structure Challenges • Light nuclear structure: • 6Li quadrupole moment? • 8B E2 transitions ? • Intruder states in 11Be, 11Li, 12Be etc ? • Can these be found in a model beginning with a NN force? • Are tensor and/or 3-body forces required? • Core excitation in (near-) halo nuclei? • Thresholds: are these fitted simultaneously? Nens 03

  27. Conclusions • Near-threshold states give rise to cluster dynamics and breakup • Continuum states necessary for spectroscopic probes. • Spectroscopy of states in the continuum is just as important as spectroscopy of discrete states (bound states or discrete resonances). Nens 03

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