Coulomb Excitation of Double Phonon Giant Resonances

1
Coulomb Excitation of Double Phonon Giant
Resonances

• Programme
• Motivation
• Introductory Remarks
• Experimental Technique
• Results
• Coulomb excitation of the DGDR
• Hints for non-harmonic behaviour
• Decay properties
• Summary

2
A Motivation

• Coulomb Excitation of Relativistic
  HI-Projectiles
• Very high excitation cross sections
• Efficient detection in 4p
• Observation of rare processes
• Investigation of radioactive isotopes
• Multi-Phonon Giant Resonanceshighly collective,
  (large) amplitude motions
• New test field for microscopic theories
• Doorway states to exotic decay processes?

3
The 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

4
Coulomb Excitation in a Simplified Model

• Lorentz-contracted field acts for a
• short interval Dt.
• Small momentum transfer (independent of g)
• Strong transverse field

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

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

5
Giant Resonances(a reminder)

• Small amplitude collective motion
  (shape/density, electric/magnetic)
• Linear response a.f.o. relevant co-ordinate
• cross feature of all isotopes

Microscopic
Macroscopic

• 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

6
Excitation and Dissociation

• Compound nucleus decay dominant
• Direct g-decay Gg / Gtot 1.7
  (0.9)
• (Beene et al., PR C41 (90) 920)
• Direct neutron decay Gdirect / Gtot few
  
• (van der Woude et al., NP A569 (94) 383c)

7
Multiple excitation of the GDR

• N-phonon state of the GDR (assume harmonic
  oscillator)
• excitation probabilityPoisson distribution
• Energy

1AGeV on Pb

• v c s(GR) gt 1 barns(GR Ä GR) gt 100 mb

8
The LAND 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 F Invariant MassMinv
  -MPR Excitation En. PPR S Pgi S Pni

9
Resolution for neutron detection

• Momenta in LAB system

10
DGDR Excitation in relativistic Coulomb
Collisions

• Corrected for background
• contributions from nuclear reactions
• ? 100 mb (scaled from 12C target)

11
Response of the detection system

• Resolution dominated by g-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 g,xn branching ratios
• Digitise information (detector response)
• Analyse model spectra and compare

12
Excitation on different targets (208Pb_at_640 A
MeV)
13
Target systematic I cross section (208Pb_at_640
A MeV)
Can the strength above the GDR be attributed to
a two-phonon state ?

• Harmonic oscillator (non-interacting phonons)
• smulti phonon ZT n (2 - d)
• Reminder sCX (1/Rmin , ZT2)
• Experiment d 0.41 (6) n 1.8 (3)
• Two-step excitation process proved !

14
Target systematic II harmonicity (208Pb_at_640 A
MeV)
Measured GDR cross section agrees with
semi-classical, relativistic Coulomb excitation
calculation.
2-Phonon Excitation

• Enhancement in the DGDR cross section
• s2-Ph (exp) / s2-Ph. (harm) 1.33 (16)

15
DGDR Resonance parameters for 208Pb (208Pb_at_640 A
MeV)

• apart from cross section no significant deviation
  from harmonicity
• doubly magic 208Pb behaves like a good vibrator

16
Double Phonon Giant ResonanceOverview over other
experiments
Pion DCX reactions
Nuclear scattering experiments
Coulomb excitation at relativistic energies
Similar structures found independently from
particular excitation processes F Nuclear
Structure effect !
17
DGDR Parameters

• Indication for unharmonicity independent from
  reaction mechanism

18
Decay properties (208Pb_at_640 A MeV)

• Combining results from gg (TAPS) and xn (LAND)
  measurem.
• BRGDRg-n TGDRg / TGDRn 0.019 (2)
• BRDGDR2g-n TDGDR2g / TDGDRn 4.5 (1.5) 10-4
• BRDGDR2g-n / BRDGDR,harm.2g-n 1.25 (40) Þ
  (non-interacting bosons)
• Conclusion direct photons predominately from
  decay of a
• collective and not from compound state.

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Summary 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

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Status

• 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,DGDR?GDR) 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)

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The 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)