Coulomb Excitation of Coulomb Excitation of Double Phonon Double Phonon

Giant ResonancesGiant Resonances

ProgrammeProgrammeMotivation

Introductory Remarks

Experimental Technique

ResultsCoulomb excitation of the DGDR

Hints for non-harmonic behaviour

Decay properties

Summary

A MotivationA Motivation

Coulomb Excitation of Relativistic HI-Projectiles

Very high excitation cross sections

Efficient detection in 4 Observation of rare processes

Investigation of radioactive isotopes

Multi-Phonon Giant Resonances:highly collective, (large) amplitude motions

New test field for microscopic theories

Doorway states to “exotic” decay processes?

The Virtual Photon FieldThe Virtual Photon Field

Equivalent photon spectrum defined for all multipolarities

(e.g. Bertulani, Baur; Phys. Rep. 163,5 (88))

Adiabatic Cut-off:

Preferable energy window

bE offcut

GRGRoffcutGRGR EEE 2

bv

zep

22

Coulomb Excitation in a Simplified ModelCoulomb Excitation in a Simplified Model

Method of virtual photons (Weizsäcker/Williams)equivalent (cross sections) to semi-classical treatment

Lorentz-contracted field acts for ashort interval t. Small momentum transfer

(independent of Strong transverse field

0 0

)(),(2R

CX EbEndEdbb

Full relativistic treatment (squares) by Matzdorf et al. Z. Phys. D6(87)5

GiantGiant ResonancesResonances(a reminder)(a reminder)

Small amplitude collective motion (shape/density, electric/magnetic)

Linear response a.f.o. relevant co-ordinate cross feature of all isotopes

Appears as a broad structure Strongly damped motion due to

a coupling of the coherent 1p-1h state to incoherent 2p-2h (doorway states) states

MicroscopicMacroscopic

G T m ode S J m ode

61

31

202.31 AAEGDR

f

Excitation and DissociationExcitation and Dissociation

Compound nucleus decay dominant Direct -decay: / tot 1.7 (0.9)

(Beene et al., PR C41 (90) 920)

Direct neutron decay: direct / tot few (van der Woude et al., NP A569 (94) 383c)

Multiple excitation of the GDRMultiple excitation of the GDR

N-phonon state of the GDR (assume harmonic oscillator)

excitation probability:Poisson distribution

Energy:

vvc:c: (GR) > 1 barn

(GR GR) > 100 mb

PNN eP

NP

!

1

GDRGDRN ENE

1AGeV on Pb

TheThe LAND approachLAND approach

Exclusive measurement of the projectile decay products using inverse kinematics !

Set-up:– Neutrons:Large Area Neutron Detector– Photons: Crystal ball, (BaF-array)– Projectile: Scintillators, MWPC,

Strip detectors, PIN Diodes

Relevant Observable Invariant MassInvariant Mass

Minv -MPR = Excitation En. PPRPiPn

i

Resolution for neutron detectionResolution for neutron detection

Momenta in LAB system

MeV3.0E

MeV1E

DGDR Excitation in relativisticDGDR Excitation in relativistic Coulomb Collisions Coulomb Collisions

• Corrected for background

• contributions from nuclear reactions 100 mb (scaled from 12C target)

E0

DGDR 28.30.7 MeV 6.31.6 MeV 21550 mb

DGDR/GDR 1.860.05 1.30.4 0.210.05

Response of the detection systemResponse of the detection system

• Resolution dominated by -detection

• Different response for 1n, 2n etc. channels unfolding introduces systematic error

Modelling the data Define differential excitation cross section

• semi-classical treatment of 1-phonon part• Gaussian for 2-phonon strength

Generate events• Statistical model • measured ,xn branching ratios

Digitise information (detector response) Analyse model spectra and compare

Excitation on different targetsExcitation on different targets (208Pb@640 A MeV)

Target systematic I: cross sectionTarget systematic I: cross section (208Pb@640 A MeV)

Harmonic oscillator (non-interacting phonons):

multi phonon ~ ZT n (2 - )

Reminder: CX ~ (1/Rmin , ZT2)

Experiment: = 0.41 (6) n = 1.8 (3)

Two-step excitation process proved !

Can the strength above the GDR be attributed to a two-phonon state ?

Target systematic II: harmonicityTarget systematic II: harmonicity (208Pb@640 A MeV)

Enhancement in the DGDR cross section:

2-Ph (exp) / 2-Ph. (harm) = 1.33 (16)

2-Phonon Excitation

Measured GDR cross section agrees with semi-classical, relativistic Coulomb

excitation calculation.

DGDR Resonance parameters for DGDR Resonance parameters for 208208PbPb (208Pb@640 A MeV)

apart from cross section no significant deviation from harmonicity

doubly magic 208Pb behaves like a “good vibrator”

E0

DGDR 26.01.4 MeV 12.74.2 MeV

DGDR/GDR 1.910.1 2.60.9

0.5 1.0 1.5

T2 / Tn

Cross sec.

Width

Energy E0

X / Xharm

Double Phonon Giant ResonanceDouble Phonon Giant ResonanceOverview over other experiments

Coulomb excitation at Coulomb excitation at relativistic energiesrelativistic energies

Nuclear scattering Nuclear scattering experimentsexperiments

Pion DCX reactionsPion DCX reactions

Similar structures found independently from particular excitation processes Nuclear Structure effect !Nuclear Structure effect !

DGDR ParametersDGDR Parameters

Indication for unharmonicity independent from reaction mechanism

50 100 150 200

0.5

1.0

1.5

2.0

2.5

3.0

3.5

<X> = 1.52(12)

FIT

Coulex

DCX

DG

DR/ G

DR

Mass number A

50 100 150 200

0.6

0.8

1.0

1.2

<X> = 0.954(14) FIT

Coulex

DCX

[ED

GD

R-E

GD

R]/E

GD

R

Mass number A

Decay propertiesDecay properties (208Pb@640 A MeV)

Combining results from (TAPS) and xn (LAND) measurem.:

BRGDR-n

= TGDR/ TGDR

n = 0.019 (2)

BRDGDR2-n

= TDGDR2

/ TDGDRn

= 4.5 (1.5) 10-4

BRDGDR2-n

/ BRDGDR,harm.2-n

= 1.25 (40)

(non-interacting bosons)(non-interacting bosons)

Conclusion: direct photons predominately from decay of a collective and not from compound

state.

Summary & OutlookSummary & Outlook

Excitation of relativistic projectiles is a promising tool for nuclear structure investigations

One-phonon GDR: in good agreement with semi-classical description.

Cross section observed in the DGDR region clearly from a two step excitation

Unharmonicity effects were found.In 208Pb (doubly magic) less pronounced than observed for 136Xe ( magic)

Scenario of non-interaction phonons supported by first direct extraction of branching ratios for the decay of the DGDR in the case of 208Pb.

New data for 238U, 136Xe and O-isotopes currently being analysed

StatusStatus

Excitation mechanism– Coupled-channel treatment does not account for higher

cross section of the DGDR (Bertulani et al. PR C53,334(96))

– Schematic model with small unharmonicity in the response (1% for Pb, 2% for Xe) explains cross section enhancement(Bortignon, Dasso PR C56,574(97))

Nuclear structure Enhancement of the B(E1,DGDRGDR) if expanded

in in a multi-phonon basis.(Soloviev et al. PR C97, R603(97))

– Background of 2p-2h states excited directly is smaller than 15% (Pb), see below.(Ponomarev, Bertulani PRL 79,3853(97)

The LAND CollaborationThe LAND Collaboration

R.Kulessa, E.Lubkiewicz, W.Walus, E.Wajda

(Univ. Cracow) B.Eberlein, R.Holzmann, H.Emling, Y.Leifels

(GSI, Darmstadt)

J.Cub, G.Schrieder, H.Simon

(TU Darmstadt)

J.Holeczek (Univ. Katovice)

K.Boretzky, Th.W.Elze, A.Grünschloß, H.Klingler, I.Kraus, A.Leistenschneider, I.Stamenko, K.Stelzer, J.Stroth

(Univ. Frankfurt) Th.Aumann, W. Dostal, B.Eberlein, J.V.Kratz

(Univ. Mainz)