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

A 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 Field Equivalent photon spectrum defined for all multipolarities (e.g. Bertulani, Baur; Phys. Rep. 163,5 (88)) éAdiabatic Cut-off: éPreferable energy window

Coulomb Excitation in a Simplified Model Method of virtual photons (Weizsäcker/Williams) equivalent (cross sections) to semi-classical treatment Lorentz-contracted field acts for a short interval  t.  Small momentum transfer (independent of   Strong transverse field Full relativistic treatment (squares) by Matzdorf et al. Z. Phys. D6(87)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 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

Excitation and Dissociation 8Compound 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 GDR N-phonon state of the GDR (assume harmonic oscillator)  excitation probability: Poisson distribution  Energy: v  c: v  c:  (GR) > 1 barn  (GR  GR) > 100 mb 1AGeV on Pb

TheLAND approach 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 Invariant Mass Relevant Observable  Invariant Mass M inv -M PR = Excitation En.   P PR  P  i  P n i 

Resolution for neutron detection Momenta in LAB system

DGDR Excitation in relativistic Coulomb Collisions Corrected for background contributions from nuclear reactions   100 mb (scaled from 12 C target)

Response 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 targets Excitation on different targets ( 208 A MeV)

Target systematic I: cross section Target systematic I: cross section ( 208 A MeV) Harmonic oscillator (non-interacting phonons):   multi phonon ~ Z T n (2 -  ) Reminder:  CX ~  (1/R min, Z T 2 ) 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: harmonicity Target systematic II: harmonicity ( 208 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 208 Pb DGDR Resonance parameters for 208 Pb ( 208 A MeV)  apart from cross section no significant deviation from harmonicity doubly magic 208 Pb behaves like a “good vibrator”

Double Phonon Giant Resonance Double Phonon Giant Resonance Overview over other experiments Coulomb excitation at relativistic energies Nuclear scattering experiments Pion DCX reactions Nuclear Structure effect ! Similar structures found independently from particular excitation processes  Nuclear Structure effect !

DGDR Parameters  Indication for unharmonicity independent from reaction mechanism

Decay properties Decay properties ( 208 A MeV) Combining results from  (TAPS) and xn (LAND) measurem.:  BR GDR  -n = T GDR   / T GDR n = (2)  BR DGDR 2  -n = T DGDR 2   / T DGDR n = 4.5 (1.5)  (non-interacting bosons)  BR DGDR 2  -n / BR DGDR,harm. 2  -n = 1.25 (40)  (non-interacting bosons) Conclusion: direct photons predominately from decay of a collective and not from compound state.

Summary & Outlook áExcitation of relativistic projectiles is a promising tool for nuclear structure investigations 4One-phonon GDR: in good agreement with semi-classical description. 4Cross section observed in the DGDR region clearly from a two step excitation 4Unharmonicity effects were found. In 208 Pb (doubly magic) less pronounced than observed for 136 Xe ( magic) 4Scenario of non-interaction phonons supported by first direct extraction of branching ratios for the decay of the DGDR in the case of 208 Pb. 8New data for 238 U, 136 Xe and O-isotopes currently being analysed

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)

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)