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

20 November 2003 Nens 03 8

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

20 November 2003 Nens 03 9

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

20 November 2003 Nens 03 10

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

20 November 2003 Nens 03 13

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

20 November 2003 Nens 03 14

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

20 November 2003 Nens 03 15

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.

20 November 2003 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.

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

20 November 2003 Nens 03 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.

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)

20 November 2003 Nens 03 25

6He excitations & resonances

Pronounced 2+ resonance No pronounced 1- resonance

20 November 2003 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?

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).