The gamma decay of the GDR at finite temperature and of the pygmy resonance far

from stability

G. BenzoniINFN sezione di Milano (Italy)

Outline1- Pygmy Dipole Resonance in 68Ni

• background• experiment• results• perspectives

2. Preequilibrium Dipole Resonance• Background• Experiment• Results• perspectives

G.AR.F.I.E.L.D.

Pygmy Dipole Resonance

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Pygmy Dipole ResonanceCollective oscillation of neutron skin against the

core

Giant Dipole ResonanceCollective oscillation of neutrons against protons

Dipole strength shifts at low energy

Collective or non-collective nature of the transitions?

Physics Case:How dipole response changes moving towards neutron rich

nuclei ??? T. Hartmann PRL85(2000)274

40Ca 0.025% EWSR

48Ca 0.29% EWSR

Adrich et al. PRL 95(2005)132501

132Sn 4% EWSR124Sn Exotic nuclei

Virtual photon breakup

LAND experiment

Virtual photon scattering

RISING experiment

How to excite this mode??Stable nuclei photoabsorption

Two experiments performed:

• 400 MeV/u 68Ni (2004) + 197Au

• 600 MeV/u 68Ni (2005) + 197Au

)2()( GDRT.Aumann et al EPJ 26(2005)441

Higher statistics dataset

68Ni beam produced by fragmentation of 86Kr @ 900 MeV/u on thick Be target (4g/cm2):

• 1010 ppspill 86Kr •Spill length 6s, period 10 s

Beam energy selected in order to populate DIPOLE modes more effectively

than other ones

High resolution -spectroscopy at the FRS

RISING

FRS

FRS provides secondary radioactive ion beams:• fragmentation and fission of primary beams • high secondary beam energies: 100 – 700 MeV/u• fully stripped ions

2g/cm2 Au

Euroball 15 ClustersLocated at 16.5°, 33°, 36° degreesEnergetic threshold ~ 100 keV

Hector 8 BaF2

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

Miniball 7 HPGe segmented detectorsLocated at 46°, 60°, 80°, 90°

degrees Energetic threshold ~ 100 keV

Beam identification and tracking detectors

Before and after the targetCalorimeter Telescopefor beam

identificationCATE

RISING ARRAY

4 CsI9 Si

Coulomb excitation of 68Ni @ 600 AMeV

68Ni

Z

AoQ

Incoming 68Ni beam Outgoing 68Ni

DE (Si)

E (C

sI)

1.2 %

4.4 %

~ 6 Days of effective beam time~ 400 GB of data recorded

~ 3.107 ‘ good 68Ni events ‘

Incoming+Outgoing 68Ni0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

66Co 67Ni 68Ni 69Ni 70Cu

~ 35%

Coulomb excitation of 68Ni (600 MeV A)Pygmy Dipole Resonance

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68Ni

HPGe-Cluster

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Baf2 Hector

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68NiHPGe MiniBall

Prelim

inary

Prelim

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Prelim

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Prelim

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Prelim

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Prelim

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GEANT Simulations

A structure appears at 10-11 MeV in all detectors

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67Ni

HPGe Cluster

Coulomb excitation of 67Ni (600 MeV A)Pr

elim

inar

yThe peak structure is

roughly 2 MeV lower than in 68Ni

There is indication from a more fragmented

structure

In all cases the measured width is consistent with

that extracted from GEANT simulations with a monochromatic source

Resonance width < 1 MeV

Both RPA and RMF predict for 68Ni Pygmy strength at approximately 10 MeV for 68Ni. The degree of collectivity is

still debated

D. Vretnar et al. NPA 692(2001)496

RMF

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0 5 10 15 20 25 30 35 40Energy (MeV)

mb

RPA

G. Colo’ private communication

68Ni 68Ni

Eb (68Ni) 7.8 MeV Eb (67Ni) 5.8 MeV

Predictions are available only for 68Ni

In the case of 67Ni as it is a vibration of the neutron skin the value of the neutron binding energy is important. As a simple rule the localization in

energy of the strength should be linearly correlated to the neutron binding energy

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~10 MeV~10 MeVPre

liminary

Prelim

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Conclusions

Measurement of high energy -rays from Coulex of 68Ni at 600 MeV/u.

First experiment of this type ever performed Strength at 10.5 MeV has been observed in all three kind of detectors Peaks line-shape is consistent with GEANT simulations (PDR < 1 MeV) Low Energy Dipole strength has also been observed in 67Ni and 69Ni

A.Bracco, G. Benzoni, N. Blasi, S.Brambilla, F. Camera, F.Crespi, S. Leoni, B. Million, M. Pignanelli, O. Wieland, P.F.Bortignon, G.Colo’

University of Milano, and INFN section of Milano, Italy

A.Maj, P.Bednarczyk, J.Greboz, M. Kmiecik, W. Meczynski, J. StyczenNiewodnicaznski institute of Nuclear Physics, Kracow, Poland

T. Aumann, A.Banu, T.Beck, F.Becker, A.Burger, L.Cacieras, P.Doornenbal, H. Emling, J. Gerl, M.Gorska, J.Grebozs, O.Kavatsyuk, M.Kavatsyuk, I. Kojouharov, N. Kurtz, R.Lozeva, N.Saito,

T.Saito, H.Shaffner, H. Wollersheim and FRS collaboration

GSI

J.Jolie, P. Reiter, N.WardUniversity of Koeln, Germany

G. de Angelis, A. Gadea, D. Napoli, National Laboratory of Legnaro, INFN, Italy

S. Lenzi, F. Della Vedova, E. Farnea, S. Lunardi, University of Padova and INFN section of Padova, Italy

D.Balabanski, G. Lo Bianco, C. Petrache, A.Saltarelli, University of Camerino, Italy

M. Castoldi and A. Zucchiatti, University of Genova, Italy

G. La Rana, University of Napoli, Italy

J.Walker,University of Surrey

RISING Collaboration

Preequilibrium Dipole Resonance

The Preequilibrium Dipole Response

GDR

t=0 fm/c

fusion CN

Dynamic Dipole

Charge NOT equilibrated All degree of freedom

EQUILIBRATED

Charge equilibration in N/Z asymmetric heavy-ion collisions

A rapid re-arrangement of charge accompanied by emission of dipole radiation

The intensity of dipole emission depends on the absolute value of the charge that must be shifted to restore the mass balance (Dipole moment)

o information on the charge equilibration in relation to the reaction mechanism;

o information on the damping of the dipole mode;

o information on the symmetry energy of the nuclear matter at lower densities than the saturation one

132Ce

N/Z=1 N/Z=1.38

+

N/Z=1.25 N/Z=1.29

Z

N Z

N/Z=1.28

N/Z=1.28

N

+

132Ce

96Mo

100Mo32S

36S

D(t=0)=18.2 fm

D(t=0)=1.7 fmtp

tptp

NZ ZN

ZNZZ

ARR

tRtRANZtD

))0()0(()0(

O+Mo @ 4-8-14-20 MeV/u

V. Baran et al . PRL87(2001)182501

THEORY: Preequilibrium dipole radiation present in all N/Z asymmetric

reactions; Dependence on charge asymmetrycharge asymmetry

mass asymmetrymass asymmetry and incident energyincident energy.

Compare the high energy gamma-ray emission from a N/Z SYMMETRIC and a N/Z ASYMMETRIC reaction.

Evidence of a prompt dipole emission: EXCESS of the high energy gamma-ray yield in the asymmetric reaction

reaction CN Proj N/Z Target N/Z CN N/Z40Ca + 100Mo 140Sm* 1 1.38 1.258

36S + 104Pd 140Sm* 1.25 1.26 1.258

Flibotte et al. PRL77(1996)1448

How to distinguish preequilibrium from thermalized emission?

40Ca + 100Mo36S + 104Pd

•The intensity is never higher than 25% of the total yield with stable beams • The measurements need to be extremely clean and exclusive

Linearized spectra40Ca + 100Mo

36S + 104Pd

Garfield + Hector @ Laboratori Nazionali di Legnaro

BaFBaF22

GarfieldGarfield PPACPPAC

• + LCP + residues• comparison btw. N/Z Asymmetric and Symmetric reactions leading to same CN at same E* and I • two different bombarding energies

8.1 AMeV and 15.6 AMEV

reaction Ebeam (MeV/u)

E* (MeV) D (fm)

16O+116Sn 8.1 100 8.664Ni+68Zn 4.7 100 1.216O+116Sn 15.6 200 8.664Ni+68Zn 7.8 200 1.2

Reaction studied to populate 132Ce* CN

O.Wieland et al. PRL 97 (2006) 012501

from 16O induced reaction will be discussed in a following paper [ref]

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64Ni+68Zn statistical

Model

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Model

E*=100MeV

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ield

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64Ni+68Zn statistical

Model

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64Ni+68Zn statistical

Model

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ield

[a.u

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64Ni+68Zn statistical

Model

The analysis of the N/Z symmetric reaction lead to important results on

Temperature Temperature dependence of dependence of

different different contributions to GDR contributions to GDR

widthwidth

1

tZN

32.1

pZN

16O+116Sn @ 250 MeV

64Ni+68Zn @ 500 MeV

Large pre-equilibrium component

Cou

nts

[a.u

.]

E [MeV]

Only statistical decay

reaction Ebeam (MeV/u)

E* (MeV)

ECN (MeV)

16O+116Sn 8.1 100 95

16O+116Sn 15.6 200 165

Determination of the preequilibrium

component thanks to the measurement of

the particles

S. Barlini et al., to be published in PRC

Comparison with statistical model using the GDR parameters:EGDR= 14 MeVGDR= 7.3 (8 MeV/u) 12.9 (15 MeV/u)

16O+116Sn@8 AMeV 16O+116Sn@15 AMeV

0,0E+00

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1,0E-04

1,5E-04

2,0E-04

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m

16O+116Sn @ 8 AMeV

16O+116Sn @ 15 AMeV

Comparison with statistical model calculations to extract extra yield

•Preequilibrium emission is enhanced at high beam

energy•Same centroid as the GDR

Extra gamma multiplicity

Simulation of reaction dynamics allows the evaluation of the DIPOLE MOMENT during the preequilibrium phase

(dinuclear system before CN equilibration)

2

3

2

)(3

2

DEc

edEdP

Bremsstrahlung formula gives directly the

emission of the dynamical dipole

M. Di Toro, V. Baran, C. Rizzo V.Baran et al., PRL87 (2001) 182501-1

16O+116Sn 8 MeV/ub=0 fm

Excitation of collective dipole mode ~85 fm/c

CN formation (equilibration)

D. Pierroutsakou . et al., PRC 71 (2005) 054605

E (MeV)

6 MeV/nucleon(E*=117 MeV)

9 MeV/nucleon

(E*=173.5 MeV)

16 MeV/nucleon(E*=304 MeV)

Ar + Zr[1] [3]

E (MeV)

S + Mo

[2]S + Mo

F GD

R(E

) (a

rb.

unit

s)

E (MeV)

Centroid energy different from exp.

Yield increasing with energy even if absolute

value is not well reproduced

15 MeV/u8 MeV/u

simulati

on

0,0E+00

5,0E-05

1,0E-04

1,5E-04

2,0E-04

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m

8 AMeV

15 AMeVexperim

ent16O+116Sn

PerspectivesStable beams:

measure angular distributions Measure an asymmetric reaction WITHOUT dipole moment in entrance channel

Exotic beams Increase further the N/Z asymmetry

Proj. Energy MeV/u

Ta CN E* fusion mb

Spin D(T=0) fm

(mass)

P.D. Yield*

132Sn 4.9 40Ca 172Yb 98.22

855 77 38 0.18 14

77Ni 3.9 95Mo 172Yb 100 728 88 35 0.03 12140Xe 5.4 32S 172Yb 100 1016 74.8 29 0.22 994Kr 4.2 78Se 172Yb 100 672 88 23 0.03 522Ne 6.4 150Nd 172Yb 100 1443 71.51 10 0.29 248Ca 5 124Sn 172Yb 99.8 857.8 88 5 0.14 1* C. Simenel et al PRL 86 (2001) 2971

The isotopes has been selected from the list of page II-8 of spiral documentation

A.Corsi, O.Wieland, A.Bracco, F.Camera, S.Brambilla, G.Benzoni, M.Casanova, F.Crespi, S.Leoni,

A.Giussani, P.Mason, B.Million, D.Montanari, A.Moroni, N.Blasi, Dipartimento di Fisica, Universitá di Milano and I.N.F.N. Section of

Milano, Milano Italy

A.Maj, M.Kmiecik,M.Brekiesz, W.Meczynski, J.Styczen, M.Zieblinski, K.Zuber

Niewodniczanski Institute of Nuclear Physics Krackow Poland

F.Gramegna, V.L.Kravchuk S.Barlini, A.Lanchais, P.F.Mastinu and L.Vannucci

INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy

G.Casini, M.Chiari, and A.NanniniINFN, Sezione di Firenze, Firenze, Italy

A.OrdineINFN sez di Napoli, Napoli

Garfield + Hector collaboration

M. Di Toro, V. Baran, C. Rizzo

Conclusions Measurement of high energy -rays from Coulex of 68Ni at 600 MeV/u.

First experiment of this type ever performed Strength at 10.5 MeV has been observed in all three kind of detectors Peaks line-shape is consistent with GEANT simulations (PDR < 1 MeV) Low Energy Dipole strength has also been observed in 67Ni and 69Ni

Dynamical dipole mode measured in N/Z asymmetric reaction 16O+116Sn Complete treatment of preequilibrium particle emission Extra yield increasing with energy (8 MeV/u 15 MeV/u)Accordance with simulation based on bremsstrahlung emission

G.AR.F.I.E.L.D.

Thank you

t=0 fm/ct 10-22s

CN formationt 10-21 s

GDR oscillation

NCMZCM RRANZD ,,

If (N/Z)proj≠(N/Z)targ

the system displays dipole oscillation due to charge equilibration

Simulation code developed by Baran et al. based on BNV model applyed to 16O(@8MeV/u)+116Sn

This oscillation: -displays collective features like GDR-produces emission

Simulation code developed by Baran et al. based on BNV model applyed to 16O(@8MeV/u)+116Sn

This emission can be calculated with the bremsstrahlung formula once known D:

2''3

2

)(3

2

DEc

edEdP

From Centroid energy E0 one can estimate the value of

• bsym vs nuclear density• bsym vs neutron number

From the FWHM extract information on the damping mechanism• Time needed to damp the dipole oscillation• Understand how nucleons move in the fusion process

symbE 0

The Dynamical Dipole Mode – Prompt dipole

Ganil October 2005

Density plots, projected on the reaction plane, for central collisions

Simenel et al. PRL86(2001)2971

REARRANGMENT of p and n to establish the charge to mass equilibrium

A rapid re-arrangement of charge accompanied by emission of dipole radiation (timescale of the order of 10-21 s)

The intensity of dipole emission depends on the absolute value of the charge that must be shifted to restore the mass balance

Time evolution of the dipole moment parallel to the deformation axis for

b=0.

Pre-equilibriumThermal Equilibrium

3.10-21 s

• 400 MeV/u 68Ni (2004) + 197Au

• 600 MeV/u 68Ni (2005) + 197Au

)2()( GDR

PbO 20816

T.Aumann et al EPJ 26(2005)441

GDR - PYGMY Excitation

Virtual photon scattering technique first experiment with a relativistic beam

Why Pygmy Resonance is important ?

Pygmy Resonance has an important

impact on the r-process nucleosynthesis

Different mean field approaches give different predictions in terms of collectivity, strength and line-shape of the pygmy resonance

Isovector properties of nuclear force

Goriely et al PLB436(1998)10Nupecc long range plan 2004

NCMZCM RRANZD ,,

If (N/Z)proj≠(N/Z)targthe system displays dipole oscillation due to charge equilibrationdisplays collective features like GDR-produces emission

Simulation code developed by Baran et al. based on BNV model applyed to 16O(@8MeV/u)+116Sn

2''3

2

)(3

2

DEc

edEdP

• Centroid is at lower energies not yet seen exp.

• Still to study angular distribution check if pure dipole• The dinamical contribution will show a clear anisotropic emission• The higher N/Z asymmetry the stronger the effect

Experimental problems:

• the CN has to be produced at the same E* and I• The intensity is never higher than 25% of the total yield with stable beams • The measurements need to be extremely clean and exclusive

• few combination proj-target can produce the same CN• fusion reaction selection • spin selection • No contaminations from target impurities

Open questions:O+Mo @ 4-8-14-20 MeV/u

Baran et al . PRL87(2001)182501

Conclusions

We have measured high energy -rays from Coulex of 68Ni at 600 MeV/u.

First experiment of this type ever performed Strength at 10.5 MeV has been observed in all three kind of detectors

Peaks line-shape is consistent with GEANT simulations (PDR < 1 MeV)

Low Energy Dipole strength has also been observed in 67Ni and 69Ni