Transcript Threshold and Continuum Structures in Exotic Nuclei Ian Thompson University of Surrey, Guildford, England with J.

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)
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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
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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.
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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!!
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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.
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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.
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Contributions
from surface and beyond
12Be+9Be
 11Be(gs)+X, 80A MeV
Eikonal reaction
theory for the
breakup
b  RC  RT
S (b)   | Sc (bc ) Sv (bv ) |  
c
12Be
v
b
9Be
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Momentum content: p-shell
No gamma
detection
19F
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
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Knockout reactions
)X
(Ebeam=60 MeV/A)
9Be(17C, 16C
(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
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N=8 neutron shell closure in
12Be?
=0
C2S=0.42
=1
C2S=0.37
from A. Navin et al., PRL 85 (2000) 266
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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
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Deduced vs. shell model
spectroscopic factors
Can define reduction factor
Mostly weakly
bound n-rich
systems
 exp t
Rs 
1
 th
th  Shell model structure
plus eikonal reaction
More bound systems
P.G. Hansen and J.A.Tostevin, ARNPS 53
(2003), 219
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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)
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Reduction Factor Rs
Strongly bound hole states
• neutron
 proton
inclusive
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
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Reduction Factor Rs
Weakly bound states
Expts
with
good
statistics
• neutron
 proton
P.G. Hansen and J.A.Tostevin, ARNPS 53 (2003), 219
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Combining Knockout and other Tools
)X
(Ebeam=60 MeV/A)
9Be(19C, 18C
Sn=0.8(0.3) MeV
s-wave
d-wave
Maddalena et al.,
PRC63 024613 (2001)
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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?
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1N stripping from 6He g.s.
Calculate overlaps:
<5He(Eα-n) | 6He(gs)>
for a range of
5He(E )> bin states,
α-n
smooth histogram of
Glauber bin cross
sections.
GSI data (H.Simon)
Promising technique!
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Theory: σstr=137 mb, σdiff=38 mb
Expt: σstr=127±14 mb, σdiff=30±5 mb
from T. Tarutina thesis (Surrey)
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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)
See softer data, and
not pronounced
virtual-s and
resonant-d peaks.
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Theory: σstr=109 mb, σdiff=109 mb
Expt: σstr=125±19 mb, σdiff=55±19 mb
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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.
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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.
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8B
+ 208Pb  7Be parallel
momentum distributions
44 MeV/A
Dot-dashed: semiclassical Coul.
Solid: Coulomb+nuclear DWBA
Dashed: CDCC coupled channels
- reduced asymmetry
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:
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Continuum 3-3
scattering states
Now average
scattering wave
functions over angles
of knn and kcn-n
Obtain similar plots
for continuum
energies.
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Virtual states & Resonances
from B. Danilin, I. Thompson, et al (in preparation)
Virtual n-n pole
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Effect of n-n ‘resonance’ in
E(c-n), E(cn-n) coordinates
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6He
excitations & resonances
Pronounced
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2+
resonance
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No pronounced 1resonance
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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?
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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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