Threshold and Continuum Structures in Exotic Nuclei

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)

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

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.

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!!

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.

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.

7
Contributions from surface and beyond
Eikonal reaction theory for the breakup
12Be9Be ? 11Be(gs)X, 80A MeV
c
12Be
v
9Be
8
Momentum content p-shell
19F
No gamma detection
16O
14N
12C
11B
N14
N8
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
9
Knockout reactions
9Be(17C, 16C g)X (Ebeam60 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
10
N8 neutron shell closure in 12Be?
?0
C2S0.42
?1
C2S0.37
from A. Navin et al., PRL 85 (2000) 266
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
12
Deduced vs. shell model spectroscopic factors
Mostly weakly bound n-rich systems
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
13
Knockout Absolute spectroscopy
From B.A. Brown et al. PRC 65 (2002) 061601(R)
0.53(2)
Sp15.96
0.49(2)
Sn 18.72
Sp 12.13
0.68(4)
Sn 15.66
0.56(3)
14
Strongly bound hole states
Reduction Factor Rs
inclusive
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
15
Weakly bound states
Expts with good statistics
Reduction Factor Rs
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
16
Combining Knockout and other Tools
9Be(19C, 18C g)X (Ebeam60 MeV/A)
Maddalena et al., PRC63 024613 (2001)
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 A1 or the A2 nucleus?

18
1N stripping from 6He g.s.

• Calculate overlaps lt5He(Ea-n) 6He(gs)gt
  for a range of 5He(Ea-n)gt bin states,
• smooth histogram of Glauber bin cross sections.
• GSI data (H.Simon)

Theory sstr137 mb, sdiff38 mb Expt
sstr12714 mb, sdiff305 mb from T. Tarutina
thesis (Surrey)
Promising technique!
19
1N stripping from 14Be g.s.

• Calculate overlaps lt13Be(Ea-n)14Be(gs)gt
• 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.

Theory sstr109 mb, sdiff109 mb Expt
sstr12519 mb, sdiff5519 mb
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.

21
E1 E2 breakup of 8B

• One-proton bound state known
• 7Be?(0p3/20p1/2)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.

22
8B 208Pb ? 7Be parallel momentum distributions
44 MeV/A
Dot-dashed semiclassical Coul. Solid
Coulombnuclear DWBA Dashed CDCC coupled
channels - reduced
asymmetry
from Mortimer et al., Phys Rev C 65 (2002) 64619
23
3-body Borromean Nuclei

• 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.

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
25
6He excitations resonances
No pronounced 1- resonance
Pronounced 2 resonance
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?

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).