Giant Resonances in Exotic NucleiExperimental Status and Perspectives

Thomas Aumann

Gesellschaft für Schwerionenforschung

INPC 2007

Tokyo, June 6th 2007

• Introduction

• The dipole response of neutron-rich nuclei

- Coulomb breakup of halo nuclei

- Giant and Pygmy collective excitations

- Asymmetry energy and neutron skin

• Future perspectives

The collective response of the nucleus: Giant Resonances

IsovectorIsoscalar

Monopole (GMR)

Dipole(GDR)

Quadrupole (GQR) B

erm

an a

nd

Fu

lz, R

ev. M

od

. Ph

ys. 4

7 (1

975)

47

208Pb

120Sn

65Cu

Photo-neutron cross sections

Electric giant resonances

The dipole response of neutron-rich nuclei

Neutron-Proton asymmetric nuclei: low-lying dipole strength

threshold strength

spectroscopic tool:

The one-neutron Halo 11Be

!non-resonant transitions

100% of the E1 strength absorbed into the Giant Dipole Resonance (GDR)

Stable nuclei:

120Sn

Two-neutron Halos: Correlations

T. Nakamura et al., Phys. Rev. Lett. 96 (2006) 252502.

n-n distance (fm) core-nn distance (fm)

|(6 H

e)|

2 RIKEN Data on 11Li

Calculation for 6He: Danilin

Non-energy weighted sum rule

→ T. Nakamura, F7-1

The dipole response of neutron-rich nuclei

Neutron-Proton asymmetric nuclei: low-lying dipole strength

new collective soft dipole mode

(Pygmy resonance)

Prediction: RMF (N. Paar et al.)

132Sn

? strong fragmentation

16O

20O

22O

! threshold strength

spectroscopic tool:

The one-neutron Halo 11Be

!non-resonant transitions

100% of the E1 strength absorbed into the Giant Dipole Resonance (GDR)

Stable nuclei:

120Sn

Experimental Tool: Electromagnetic excitation at high energies

High velocities v/c0.6-0.9 High-frequency Fourier components

E,max 25 MeV (@ 1 GeV/u)

b>RP+RT

Pb Absorption of

‘virtual Photons’

elm ~ Z2

Semi-classical theory:

delm / dE = N(E) (E)

Determination of ‘photon energy’ (excitation energy) via a kinematically complete

measurement of the momenta of all outgoing particles (invariant mass)

adiabatic cut-off:

Experimental Approach: Production of (fission-)fragment beams

B

Z

A

cm

e

u

Bρ – from position at middle focal plane of the FRS

β – from TOF

Z – from ΔELAND

Primary: 3*108 238U/spill @550MeV/u Secondary (mixed): 50 ions 132Sn/spill (~10/sec @500 MeV/u)

132Sn

Experimental Scheme: The LAND reaction setup @GSI

Excitation energy E* from kinematically complete measurement of all outgoing particles:

Neutrons

ToF, E

LANDtracking → BA/Q

Charged fragments

PhotonsALADINlarge-acceptance dipole

ToF, x, y, z

Crystal Ball and TargetBeam

projectiletracking

~12 m

Mixed beam

Dipole-strength distributions in neutron-rich Sn isotopes

Electromagnetic-excitation cross section

Photo-neutron cross section

P. Adrich et al., PRL 95 (2005) 132501

stable

radioactiveA

PDR GDR

Ecentr

[MeV]

sum

rule fraction

[%]

Ecentr

[MeV]

Γ

[MeV]

sum

rule fraction

[%]

124Sn - -15.3 4.8 116

130Sn10.1

(0.7)

7.0

(3.0)

15.9

(0.5)

4.8

(1.8)

145

(19)

132Sn9.8

(0.7)

4.0

(3.1)

16.1

(0.8)

4.7

(2.2)

125

(32)

PDR• located at 10 MeV• exhausts a few % TRK sum rule• in agreement with theory

GDR• no deviation from systematics

Low-lying strength in 132Sn mass neighborhood

odd nuclei allow extending (,n) measurements to lower excitation energies

→ comparison to (') data for stable isotopes

Stable nuclei, Photoabsorption, from:

A.Zilges et al., Phys.Lett. B 542,43 (2003)

S.Volz et al., Nucl.Phys. A 779, 1 (2006)

N. Ryezayeva et al., Phys.Rev.Lett. 89 (2002)

K. Govaert et al., Phys. Rev. C 57,2229 (1998)

5 MeV < E* < 9 MeV

A. Klimkiewicz et al, submitted to PRL

Symmetry energy S2(ρ) and neutron skin in 208Pb

• strong linear correlation between neutron skin thickness and parameters a4, p0

R.J.Furnstahl NPA 706(2002)85-110

Alex Brown, PRL 85 (2000) 5296

A

ZNΟSEE

,)()()0,(),( 42

2

...)(18

)(

),(

2

1)(

202

0

002

0

04

02

2

2 |

Kpa

ES

Theory: Precise knowledge of neutron-skin thickness could

constrain the density dependence of S()

Work Hypothesis: Pygmy-Strength (since related to skin)

should do the same job,

but, experimentally, is accessed much easier !

Inspired by recent article of Piekarewicz (Phys. Rev. C 73 , 044325 (2006))

Here:

Quantitative attempt by means of RHB + RQRPA,

(density-dependent meson-exchange DD-ME )

Paar, Vretenar, Ring et al. (Phys. Rev. C67, 34312 (2003))

Symmetry energy and neutron skin form dipole strength

RQRPA – DD-ME

N. Paar et al.

Result (averaged 130,132Sn) :

a4 = 32.0 ± 1.8 MeV

po = 2.3 ± 0.8 MeV/fm3

PDR strength versus a4, po

S() : moderate stiffness

Neutron skin thickness

δrRn-Rp

Rn – Rp :

130Sn: 0.23 ± 0.04 fm

132Sn: 0.24 ± 0.04 fm

LAND

Sn isotopes

A.Krasznahorkay et al.

PRL 82(1999)3216

A. Klimkiewicz, N. Paar, et al,

submitted to PRL

Euroball 15 Clusters

Located at 16.5°, 33°, 36° degreesEnergetic threshold ~ 100 keV

Hector BaF2

Located at 142° and 90° degreesEnergetic threshold ~ 1.5 MeV

Miniball segmented detectors

Located at 46°, 60°, 80°, 90° degrees Energetic threshold ~ 100 keV

Beam identification and tracking detectors

Before and after the target

Calorimeter Telescope

for beam identification(CATE)

RISING ARRAY @GSI

4 CsI9 Si

Coulomb excitation of 68Ni (600 MeV A)

A structure appears at 10-11 MeV in all detector types

8 10 12 140

10

20

30

40

50

60

5.0 7.5 10.0 12.50

5

10

15

20

25

Cou

nts

Energy [MeV]

68Ni

HPGe-Cluster

6 8 10 12 140

10

20

30

40

50

60

5.0 7.5 10.0 12.50

5

10

15

20

25

Counts

Energy [MeV]

68Ni

Baf2 Hector

6 8 10 12 140

10

20

30

40

50

60

5.0 7.5 10.0 12.5 15.00

5

10

15

20

25

Cou

nts

Energy [MeV]

68NiHPGe MiniBall

Preliminary

Preliminary

Preliminary

Preliminary

Preliminary

Preliminary

GEANT Simulations

F. Camera et al, see F8-2

(,n) data from LAND under analysis

The collective response of the nucleus: Giant Resonances

IsovectorIsoscalar

Monopole (GMR)

Dipole(GDR)

Quadrupole (GQR) B

erm

an a

nd

Fu

lz, R

ev. M

od

. Ph

ys. 4

7 (1

975)

47

208Pb

120Sn

65Cu

Photo-neutron cross sections

Electric giant resonances

Elastic (p,p) … Inelastic (p,p’), (,’) ... Charge exchange: (p,n), (3He,t) ... Quasifree (p,pn), (p,2p), (p, p) ...

In-Ring Experiments: Light-Hadron Scattering

Selective to SPIN-ISOSPIN (S,T) excitations :

- Giant resonances: Monopole.., GT... isoscalar / isovector

- Low-lying collective modes

- single-particle spectr. factors

- nucleon-nucleon correlations, clusters

Selective to SPIN-ISOSPIN (S,T) excitations :

- Giant resonances: Monopole.., GT... isoscalar / isovector

- Low-lying collective modes

- single-particle spectr. factors

- nucleon-nucleon correlations, clusters

Experimental challenge:

Formfactor (L) at low momentum transfer

in inverse kinematics Storage Ring

Target-Recoil and Gamma Detector

around internal target

Target-Recoil and Gamma Detector

around internal target

NESR

EXLExotic Nuclei Studied in Light-Ion Induced Reactions at NESR

Internal target

ELISeThe Electron-Ion (eA) Collider

Electronspectrometerp/p=10-4

gap 25 cmweight 90 t

Electronspectrometerp/p=10-4

gap 25 cmweight 90 t

RIKEN: Isoscalar excitations in 14O

RIKEN: 60 MeV/u 14O on liquid He target

Preliminary analysis: COMEX 2006, Nucl. Phys. A 788 (2007) 188c

26 cm

20 cm

28 cm

56Ni50 A.MeV104 pps

GANIL: GMR & GQR in the unstable 56Ni

56Ni(d,d’) @ GANILActive Target MAYA

C. Monrozeau et al., Nucl. Phys. A 788, 182c (2007)

Conclusion

• Low-lying dipole strength observed in light and medium-mass neutron-rich nuclei (→ D. Beaumel, F5-5)

• Threshold strength (halo nuclei) established as spectroscopic tool (→ T. Nakamura, F7-1)

• Peak-like structure below the GDR in 130,132Sn at about 10 MeV excitation energy exhausting about 5% of the energy-weighted sum rule

• Parameters of GDR in agreement with systematic trends derived from stable nuclei

• Symmetry energy and neutron-skin thickness from dipole strength: a first attempt

Outlook:

• Systematic measurements of dipole strength in neutron-proton asymmetric nuclei

• Theory+experiment: Relation of low-lying dipole strength to symmetry energy and neutron skin

• Decay characteristics (e.g., decay branch) (,') in 68Ni (RISING → F. Camera, F8-2), (,n) with LAND setup

• Monopole and quadrupole strength: active target (GANIL), liquid He (RIKEN, → H. Baba QW-022), internal gas target in a storage ring (GSI, FAIR)

The LAND/FRS collaboration S221

Uni Frankfurt

Th.W. Elze

R. Palit

Uni Krakow

P. Adrich

A. Klimkiewicz

R. Kulessa

G. Surówka

W. Walus

Uni Mainz

J.V. Kratz

C. Nociforo

GSI

T. Aumann

K. Boretzky

H. Emling

M. Fallot

H. Geissel

U. D. Pramanik

M. Hellström

K.L. Jones

Y. Leifels

H. Simon

K. Sümmerer

Santiago de Compostela

D. Cortina-Gil

208Pb analysis

Rn – Rp = 0.18 ± 0.035 fm

∑Bpdr(E1)=1.98 e2 fm2

from N.Ryezayeva et al., PRL 89(2002)272501

∑Bgdr(E1)=60.8 e2 fm2 from A.Veyssiere et al.,NPA 159(1970)561

RQRPA-

N.Paa

r

RQRPA-

N.Paar

RQRPA-

N.Paa

r

LAND

C.Satlos et al. NPA 719(2003)304

A.Krasznahorkay et al.NPA 567(1994)521

C.J.Batty et al.Adv.Nucl.Phys. (1989)1

B.C. Clark et al. PRC 67(2003)044306