Threshold and Continuum Structures in Exotic Nuclei Ian

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) 20 November 2003 Nens 03 1

Topics to Discuss z. Few-body vs many-body behaviour z. Need for spectroscopy from breakup y. Knockout to bound states y 1 N stripping of Borromean halo nuclei z. Elastic breakup: y. E 1 and E 2 in 8 B breakup? y. Continuum states: energy correlations z. Conclusions 20 November 2003 Nens 03 2

Few- vs many-body dynamics z. Nuclei typically show few-body behaviour just near and above the cluster separation thresholds. z. Many exotic nuclei have just one or a few bound states, and hence show pronounced cluster dynamics even in their ground states. 20 November 2003 Nens 03 3

Role of the Continuum? z. The continuum appears in several ways: y. Part of expansion of bound states; xeg needed in RPA for weakly bound states y. Dominated by resonances; x. These ‘unbound states’ identified eg with shell model eigenstates above threshold y. In non-resonant continuum; xeg in breakup reactions. z. ALL important parts of nuclear structure!! 20 November 2003 Nens 03 4

Reactions to probe structure z. Structure may be probed by elastic scattering or cluster transfers, zbut breakup is typically the largest. z. Review: ythe structure information that is present in breakup amplitudes, ybound structure details that can be extracted from different classes of breakup reactions. 20 November 2003 Nens 03 5

Stripping Reactions z. Stripping = inelastic breakup, removes a surface nucleon by a high-energy interaction with a target. y. Can reveal the spectroscopic factors for a wide range of final states. y. These states may be distinguished by coincident -rays. z. Review the measurement of spin, parity, and absolute spectroscopic factors. 20 November 2003 Nens 03 6

Contributions from surface and beyond 12 Be+9 Be Eikonal reaction theory for the breakup 11 Be(gs)+X, 80 A Me. V c 12 Be v b 9 Be 20 November 2003 Nens 03 7

Momentum content: p-shell No gamma detection 19 F 16 O 14 N 12 C 11 B 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 20 November 2003 Nens 03 8

Knockout reactions )X (Ebeam=60 Me. V/A) 9 Be(17 C, 16 C (a) 8% s + 92% d (b) 26% s + 74% d (c) 100% d SM calculation predict no 16 C(0+) in the 17 C(g. s. ). Experiment measured a 20% branch into 16 C(0+). Higher order processes? Maddalena et al. , PRC 63(01)024613 20 November 2003 Nens 03 9

N=8 neutron shell closure in 12 Be? =0 C 2 S=0. 42 =1 C 2 S=0. 37 from A. Navin et al. , PRL 85 (2000) 266 20 November 2003 Nens 03 10

Ground state structure of 8 B p 3/2 137 ke. V p 3/2 566 ke. V Proton removal from 8 B measured at the GSI with gamma coincidences, sees a (15%) branch from an excited 7 Be(1/2 -) core component in the 8 B wave function. from D. Cortina-Gil et al. , Phys Lett B 529 (2002) 36, NPA 720 (2003) 3 20 November 2003 Nens 03 11

Deduced vs. shell model spectroscopic factors Can define reduction factor Mostly weakly bound n-rich systems th Shell model structure plus eikonal reaction More bound systems P. G. Hansen and J. A. Tostevin, ARNPS 53 (2003), 219 20 November 2003 Nens 03 12

Knockout: Absolute spectroscopy From B. A. Brown et al. PRC 65 (2002) 061601(R) Sp=15. 96 (e, e'p) 0. 53(2) 0. 51(3) Sn= 18. 72 0. 49(2) Sp= 12. 13 0. 68(4) 0. 67(5) Sn= 15. 66 0. 56(3) 20 November 2003 Nens 03 13

Reduction Factor Rs Strongly bound hole states • neutron n proton inclusive P. G. Hansen and J. A. Tostevin, ARNPS 53 (2003), 219 20 November 2003 Nens 03 14

Reduction Factor Rs Weakly bound states Expts with good statistics • neutron n proton P. G. Hansen and J. A. Tostevin, ARNPS 53 (2003), 219 20 November 2003 Nens 03 15

Combining Knockout and other Tools )X (Ebeam=60 Me. V/A) 9 Be(19 C, 18 C Sn=0. 8(0. 3) Me. V s-wave d-wave Maddalena et al. , PRC 63 024613 (2001) 20 November 2003 Nens 03 16

1 Neutron stripping from threebody Borromean Nuclei z. Removal of a neutron from 6 He, 11 Li, 14 Be, ypopulates states of 5 He, 10 Li or 13 Be. y. Experiments measure decay spectrum of 5 He = 4 He + n, 13 Be = 12 Be + n, etc z. Can we predict any energy and angular correlations by Glauber model? z. Can we relate these correlations to the structure of the A+1 or the A+2 nucleus? 20 November 2003 Nens 03 17

1 N stripping from 6 He g. s. z Calculate overlaps: <5 He(Eα-n) | 6 He(gs)> for a range of 5 He(E )> bin states, α-n z smooth histogram of Glauber bin cross sections. z GSI data (H. Simon) Promising technique! 20 November 2003 Theory: σstr=137 mb, σdiff=38 mb Expt: σstr=127± 14 mb, σdiff=30± 5 mb from T. Tarutina thesis (Surrey) Nens 03 18

1 N stripping from 14 Be g. s. z Calculate overlaps: <13 Be(Eα-n)|14 Be(gs)> z Inert-core 13, 14 Be wfs. z GSI data (H. Simon) y from T. Tarutina thesis (Surrey) z See softer data, and not pronounced virtual-s and resonant -d peaks. 20 November 2003 Theory: σstr=109 mb, σdiff=109 mb Expt: σstr=125± 19 mb, σdiff=55± 19 mb Nens 03 19

Elastic Breakup z. Elastic Breakup = Diffraction Dissociation: yall nuclear fragments survive along with the target in its ground state, yprobes continuum excited states of nucleus. y. For dripline nuclei , with few discrete states, these breakup reactions are the main probe of excited states z. Review correlations in the three-body continuum of Borromean nuclei. 20 November 2003 Nens 03 20

E 1 & E 2 breakup of 8 B z. One-proton bound state known: y 7 Be (0 p 3/2+0 p 1/2)|2+ at -0. 137 Me. V z. Need spectroscopy of non-resonant continuuum! y. B(E 1) & B(E 2) for transition p s, d need to be accurately known y. E 1 and E 2 amplitudes interfere in p||(7 Be) momentum distribution yso measure relative E 2/E 1 amplitudes from asymmetries. 20 November 2003 Nens 03 21

8 B + 208 Pb 7 Be parallel momentum distributions 44 Me. V/A Dot-dashed: semiclassical Coul. Solid: Coulomb+nuclear DWBA Dashed: CDCC coupled channels - reduced asymmetry CDCC calculations with scaled E 2 amplitudes - need to increase asymmetry again! from Mortimer et al. , Phys Rev C 65 (2002) 64619 20 November 2003 Nens 03 22

3 -body Borromean Nuclei z Ground state plot: 20 November 2003 z Continuum 3 -3 scattering states z Now average scattering wave functions over angles of knn and kcn-n z Obtain similar plots for continuum energies. Nens 03 23

Virtual states & Resonances from B. Danilin, I. Thompson, et al (in preparation) Virtual n-n pole 20 November 2003 Effect of n-n ‘resonance’ in E(c-n), E(cn-n) coordinates Nens 03 24

6 He excitations & resonances Pronounced 20 November 2003 2+ resonance Nens 03 No pronounced 1 resonance 25

Assorted Structure Challenges z. Light nuclear structure: y 6 Li quadrupole moment? y 8 B E 2 transitions ? y. Intruder states in 11 Be, 11 Li, 12 Be etc ? x. Can these be found in a model beginning with a NN force? x. Are tensor and/or 3 -body forces required? x. Core excitation in (near-) halo nuclei? y. Thresholds: are these fitted simultaneously? 20 November 2003 Nens 03 26

Conclusions z. Near-threshold states give rise to cluster dynamics and breakup z. Continuum states necessary for spectroscopic probes. z. Spectroscopy of states in the continuum is just as important as spectroscopy of discrete states (bound states or discrete resonances). 20 November 2003 Nens 03 27