20 November 2003 Nens 03 1
Threshold and Continuum Structures in Exotic Nuclei
Ian ThompsonUniversity of Surrey,Guildford, EnglandwithJ. Tostevin, J. Mortimer, T. Tarutina (Surrey),B. Danilin (Surrey, Kurchatov)
20 November 2003 Nens 03 2
Topics to Discuss
Few-body vs many-body behaviourNeed 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
20 November 2003 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.
20 November 2003 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!!
20 November 2003 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.
20 November 2003 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.
20 November 2003 Nens 03 7
Contributions from surface and beyond
12Be+9Be 11Be(gs)+X, 80A MeV12Be+9Be 11Be(gs)+X, 80A MeV
TC RRb
| )(bS )(bS |(b)S vvcc
Eikonal reaction
theory for the
breakup
9Be
c
12Bebv
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19F
16O
14N
12C
11B
N=8
N=14
Momentum content: p-shell
E.Sauvan et al., Phys Lett B 491 (2000) 1
distributions narrow (weak binding) or s-states as one crosses shell or sub-shell closures
No gammadetection
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SM calculation predict no 16C(0+) in the 17C(g.s.). Experiment measured a 20% branch into 16C(0+) .Higher order processes?
9Be(17C, 16C )X(Ebeam=60 MeV/A)
Maddalena et al., PRC63(01)024613
Knockout reactions
(a) 8% s + 92% d (b) 26% s + 74% d(c) 100% d
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N=8 neutron shell closure in 12Be?
C2S=0.42
C2S=0.37
=0
=1
from A. Navin et al., PRL 85 (2000) 266
20 November 2003 Nens 03 11
Ground state structure of 8B
from D.Cortina-Gil et al., Phys Lett B 529 (2002) 36, NPA 720 (2003) 3
Proton removal from 8Bmeasured at the GSI withgamma coincidences, sees a (15%) branch from an excited 7Be(1/2-) core component in the 8B wave function.
p3/2
137 keV
p3/2
566 keV
20 November 2003 Nens 03 12
Deduced vs. shell model spectroscopic factors
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
More bound systems
Can define reduction factor
th Shell model structure plus eikonal reaction
1 th
texp
sRMostly weaklybound n-richsystems
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Knockout: Absolute spectroscopy
0.53(2)
0.49(2)
(e,e'p)
0.51(3)
0.67(5)0.68(4)
0.56(3)
From B.A. Brown et al. PRC 65 (2002) 061601(R)
Sp=15.96
Sn= 18.72
Sp= 12.13
Sn= 15.66
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Strongly bound hole states
• neutron proton
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
inclusive
Red
ucti
on F
acto
r R
s
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Weakly bound states
• neutron proton
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
Red
ucti
on F
acto
r R
s
Exptswith good statistics
20 November 2003 Nens 03 16
Combining Knockout and other Tools 9Be(19C, 18C )X(Ebeam=60 MeV/A)
d-wave
s-wave
Sn=0.8(0.3) MeV
Maddalena et al., PRC63 024613 (2001)
20 November 2003 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, etcCan 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?
20 November 2003 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 mbExpt: σstr=127±14 mb, σdiff=30±5 mbfrom T. Tarutina thesis (Surrey)Promising technique!
20 November 2003 Nens 03 19
1N stripping from 14Be g.s.
Calculate overlaps: <13Be(Eα-n)|14Be(gs)>
Inert-core 13,14Be wfs. GSI data (H.Simon)
from T. Tarutina thesis (Surrey)
Theory: σstr=109 mb, σdiff=109 mbExpt: σstr=125±19 mb, σdiff=55±19 mb
See softer data, and not pronounced virtual-s and resonant-d peaks.
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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.
20 November 2003 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 ps,d need to be
accurately known E1 and E2 amplitudes interfere in p||(7Be) momentum
distribution so measure relative E2/E1 amplitudes from
asymmetries.
20 November 2003 Nens 03 22
8B + 208Pb 7Be parallel momentum distributions
Dot-dashed: semiclassical Coul.Solid: Coulomb+nuclear DWBADashed: CDCC coupled channels - reduced asymmetry
44 MeV/A
CDCC calculations with scaled E2 amplitudes - need to increase asymmetry again!
from Mortimer et al., Phys Rev C 65 (2002) 64619
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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.
20 November 2003 Nens 03 24
Virtual states & Resonances
Virtual n-n poleEffect of n-n ‘resonance’ in E(c-n), E(cn-n) coordinates
from B. Danilin, I. Thompson, et al (in preparation)
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6He excitations & resonances
Pronounced 2+ resonance No pronounced 1- resonance
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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?
20 November 2003 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).
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