1. INFN sezione di Milano (Italy)
The gamma decay of the GDR at finite temperature and of the pygmy resonance far from stability
G. Benzoni
INFN sezione di Milano (Italy)

2. G.AR.F.I.E.L.D. Outline 1- Pygmy Dipole Resonance in 68Ni background
experiment
results
perspectives
Preequilibrium Dipole Resonance
Background
Experiment
Results
G.AR.F.I.E.L.D.

3. Pygmy Dipole Resonance

4. Giant Dipole Resonance Pygmy Dipole Resonance
Collective oscillation of neutrons against protons P N
Average Transition charge densities
Pygmy Dipole Resonance
Collective oscillation of neutron skin against the core N
Average Transition charge densities

5. How dipole response changes moving towards neutron rich nuclei ???
Physics Case:
How dipole response changes moving towards neutron rich nuclei ???
Dipole strength shifts at low energy
Collective or non-collective
nature of the transitions?
T. Hartmann PRL85(2000)274
48Ca
0.29% EWSR
40Ca
0.025% EWSR
How to excite this mode??
Stable nuclei  photoabsorption
132Sn
4% EWSR
Exotic nuclei
Virtual photon breakup
LAND experiment
Virtual photon scattering
RISING experiment
124Sn
Adrich et al. PRL 95(2005)132501

6. 400 MeV/u 68Ni (2004) + 197Au 600 MeV/u 68Ni (2005) + 197Au
Two experiments performed:
400 MeV/u 68Ni (2004) + 197Au
600 MeV/u 68Ni (2005) + 197Au
Beam energy selected in order to populate DIPOLE modes more effectively than other ones
Higher statistics dataset
68Ni beam produced by fragmentation of 900 MeV/u on thick Be target (4g/cm2):
1010 ppspill 86Kr
Spill length 6s, period 10 s
T.Aumann et al EPJ 26(2005)441

7. High resolution g-spectroscopy at the FRS
FRS provides secondary radioactive ion beams:
fragmentation and fission of primary beams
high secondary beam energies: 100 – 700 MeV/u
fully stripped ions
FRS
2g/cm2 Au
RISING

8. for beam identification
RISING ARRAY
Euroball 15 Clusters
Located at 16.5°, 33°, 36° degrees
Energetic threshold ~ 100 keV
Hector 8 BaF2
Located at 142° and 90° degrees
Energetic threshold ~ 1.5 MeV
Miniball 7 HPGe segmented detectors
Located at 46°, 60°, 80°, 90° degrees
Energetic threshold ~ 100 keV
Beam identification and tracking detectors
Before and after the target
Calorimeter
Telescope
for beam identification
CATE
4 CsI
9 Si

9. Coulomb excitation of 68Ni @ 600 AMeV
Outgoing 68Ni
DE (Si)
E (CsI)
1.2 %
4.4 %
Incoming 68Ni beam
AoQ
68Ni Z
~ 6 Days of effective beam time
~ 400 GB of data recorded
~ ‘ good 68Ni events ‘
~ 35%
Incoming+Outgoing 68Ni

10. A structure appears at 10-11 MeV in all detectors
Coulomb excitation of 68Ni (600 MeV A)
Pygmy Dipole Resonance
Preliminary
GEANT Simulations
A structure appears at MeV in all detectors

11. Coulomb excitation of 67Ni (600 MeV A)
The 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 g source
Resonance width G < 1 MeV
Preliminary

12. Predictions are available only for 68Ni
RPA
RMF
Preliminary
68Ni
68Ni
~10
MeV
~10 MeV
G. Colo’ private communication
D. Vretnar et al. NPA 692(2001)496
Both RPA and RMF predict for 68Ni Pygmy strength at approximately 10 MeV for 68Ni. The degree of collectivity is still debated
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
Eb (68Ni)  7.8 MeV Eb (67Ni)  5.8 MeV

13. Conclusions
Measurement of high energy g-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 (GPDR < 1 MeV)
Low Energy Dipole strength has also been observed in 67Ni and 69Ni

14. RISING Collaboration
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. Styczen
Niewodnicaznski 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.Ward
University 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

15. Preequilibrium Dipole Resonance

16. All degree of freedom EQUILIBRATED
The Preequilibrium Dipole Response
Charge NOT equilibrated
All degree of freedom EQUILIBRATED
GDR
t=0 fm/c
fusion CN
Dynamic Dipole
Charge equilibration in N/Z asymmetric heavy-ion collisions

17. 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)
information on the charge equilibration in relation to the reaction mechanism;
information on the damping of the dipole mode;
information on the symmetry energy of the nuclear matter at lower densities than the saturation one

18. Dependence on charge asymmetry mass asymmetry and incident energy.
THEORY:
Preequilibrium dipole radiation present in all N/Z asymmetric reactions;
Dependence on charge asymmetry
mass asymmetry and
incident energy.
MeV/u
V. Baran et al . PRL87(2001)182501

19. How to distinguish preequilibrium from thermalized emission?
40Ca + 100Mo
36S + 104Pd
reaction CN
Proj N/Z
Target N/Z
CN N/Z
40Ca + 100Mo
140Sm* 1
1.38
1.258
36S + 104Pd
1.25
1.26
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
Linearized spectra
40Ca + 100Mo
36S + 104Pd
The intensity is never higher than 25% of the total yield with stable beams
The measurements need to be extremely clean and exclusive
Flibotte et al. PRL77(1996)1448

20. Garfield + Hector @ Laboratori Nazionali di Legnaro
g + 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
BaF2
Garfield
PPAC

21. Reaction studied to populate 132Ce* CN
Ebeam (MeV/u)
E* (MeV)
D (fm)
16O+116Sn
8.1
100
8.6
64Ni+68Zn
4.7
1.2
15.6
200
7.8
The analysis of the N/Z symmetric reaction lead to important results on Temperature dependence of different contributions to GDR width
O.Wieland et al. PRL 97 (2006)

22. Large pre-equilibrium component
250 MeV
500 MeV
Large pre-equilibrium component
Counts [a.u.]
Ea [MeV]
Only statistical decay
Determination of the preequilibrium component thanks to the measurement of the particles
reaction
Ebeam (MeV/u)
E* (MeV)
ECN (MeV)
16O+116Sn
8.1
100 95
15.6
200
165
Di che natura è la componente di preequilibrio????
Comparison with statistical model using the GDR parameters:
EGDR= 14 MeV
GGDR= 7.3 (8 MeV/u)
12.9 (15 MeV/u)
S. Barlini et al., to be published in PRC

23. Comparison with statistical model calculations to extract extra yield
AMeV
AMeV
Extra gamma multiplicity
15 AMeV
Preequilibrium emission is enhanced at high beam energy
Same centroid as the GDR
8 AMeV

24. Excitation of collective dipole mode ~85 fm/c
Simulation of reaction dynamics allows the evaluation of the DIPOLE MOMENT during the preequilibrium phase (dinuclear system before CN equilibration)
Excitation of collective dipole mode ~85 fm/c
16O+116Sn 8 MeV/u
b=0 fm
CN formation (equilibration)
Bremsstrahlung formula gives directly the g emission of the dynamical dipole
M. Di Toro, V. Baran, C. Rizzo
V.Baran et al., PRL87 (2001)

25. 6 MeV/nucleon (E*=117 MeV) 9 MeV/nucleon (E*=173.5 MeV) 16 MeV/nucleon
[1]
[2]
[3]
S + Mo
S + Mo
Ar + Zr
FGDR(Eg) (arb. units)
Eg (MeV)
Eg (MeV)
Eg (MeV)
D. Pierroutsakou . et al., PRC 71 (2005)

26. experiment simulation 16O+116Sn Centroid energy different from exp.
8 AMeV
15 AMeV
experiment
16O+116Sn
15 MeV/u
8 MeV/u
simulation
Centroid energy different from exp.
Yield increasing with energy even if absolute value is not well reproduced

27. Perspectives Stable 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*
sfusion mb
Spin
D(T=0) fm
D (mass)
P.D. Yield*
132Sn
4.9
40Ca
172Yb
98.22
855 77 38
0.18 14
77Ni
3.9
95Mo
100
728 88 35
0.03 12
140Xe
5.4
32S
1016
74.8 29
0.22 9
94Kr
4.2
78Se
672 23 5
22Ne
6.4
150Nd
1443
71.51 10
0.29 2
48Ca
124Sn
99.8
857.8
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

28. Garfield + Hector collaboration
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.Nannini
INFN, Sezione di Firenze, Firenze, Italy
A.Ordine
INFN sez di Napoli, Napoli
M. Di Toro, V. Baran, C. Rizzo

29. Conclusions
Measurement of high energy g-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 (GPDR < 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.

30. Thank you

32. If (N/Z)proj≠(N/Z)targ
t=0 fm/c
t  10-22s
CN formation
t  s
GDR oscillation
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

33. -displays collective features like GDR
This oscillation:
-displays collective features like GDR
-produces g emission
Simulation code developed by Baran et al. based on BNV model applyed to

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

35. The Dynamical Dipole Mode – Prompt dipole
REARRANGMENT of p and n to establish the charge to mass equilibrium
Pre-equilibrium
Thermal Equilibrium s
Density plots, projected on the reaction plane, for central collisions
Time evolution of the dipole moment parallel to the deformation axis for b=0.
Simenel et al. PRL86(2001)2971
A rapid re-arrangement of charge accompanied by emission of dipole radiation (timescale of the order of s)
The intensity of dipole emission depends on the absolute value of the charge that must be shifted to restore the mass balance
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
Ganil October 2005

36. Virtual photon scattering technique first experiment with a relativistic beam
GDR - PYGMY Excitation
400 MeV/u 68Ni (2004) + 197Au
600 MeV/u 68Ni (2005) + 197Au
T.Aumann et al EPJ 26(2005)441

37. Why Pygmy Resonance is important ?
Pygmy Resonance has an important
impact on the r-process
nucleosynthesis
Nupecc long range plan 2004
Goriely et al PLB436(1998)10
Different mean field approaches give different predictions in terms of collectivity, strength and line-shape of the pygmy resonance
Isovector properties of nuclear force

38. If (N/Z)proj≠(N/Z)targthe system displays dipole oscillation due to charge equilibration
displays collective features like GDR
-produces g emission
Simulation code developed by Baran et al. based on BNV model applyed to

39. Experimental problems:
few combination proj-target can produce the same CN
fusion reaction selection
spin selection
No contaminations from target impurities
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
MeV/u
Baran et al . PRL87(2001)182501
Open questions:
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 g-emission
The higher N/Z asymmetry the stronger the effect

40. Future plans
????????

41. Conclusions
We have measured high energy g-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 (GPDR < 1 MeV)
Low Energy Dipole strength has also been observed in 67Ni and 69Ni

42. Coulomb excitation of 68Ni (600 MeV A) Pygmy Dipole Resonance
Eg [MeV]
BaF2 at 142°
BaF2 at 90°
Preliminary
Preliminary
The structure does not appear at 142° because of the much higher background
Calls for further investigation in view of new campaign