Upload
shanon-ellis
View
214
Download
0
Tags:
Embed Size (px)
Citation preview
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)