1. Evolution of the GDR properties vs E*
Compilation in data and models
E* dependence:
Compilation of data for the mass region A ~ 110 – 120
Compilation of models interpreting the evolution of GDR
properties with E*
Comparison between data and models
Mass Dependence:
Data in the mass region A ~ 60 – 70
Isospin Dependence:
Pre-equilibrium GDR (short compilation of the results)
Y.Blumenfeld and D.Santonocito

2. A broad systematics exist on the GDR built on
The giant dipole resonance is well established as a general feature of all nuclei
A broad systematics exist on the GDR built on
ground state for almost all nuclei
The field of the study of Giant resonances on excited states was launched by Brink in 1955 who proposed that their properties should not depend significantly on the nuclear state.
Experimental investigation have focused on the GDR whose large excitation cross section and sizeble gamma decay branch of about 10-3 allowed a direct measurement of its properties.
Parameters governing the GDR:
EGDR size and shape of nuclei (EGDR  A-1/3 )
GGDR damping of the collective motions
SGDR degree of collectivity of the excitation
(sum rule  60 NZ/A)

3. What can we learn ?
At low E* (E/A< 2 MeV) the GDR properties are well understood and provided insights into shapes and fluctuations of hot nuclei.
At higher E* one might expect to probe the limits of existence of collective motion in nuclei and get informations on time scale for equilibration and decay of highly excited systems.
The disappearence of collective motion can add further information about phase transition in hot nuclear matter

4. Low E* region - First studies of the evolution of GDR properties vs E*
Sn isotopes were populated by fusion reactions up to E* = 130 MeV
J.J.Gaardhoje et al PRL56(1986)1783
J.J.Gaardhoje et al PRL53 (1984) 148
Chakrabarty et al. PRC36 (1987)1886
CASCADE input parameter:
EGDR = 15 MeV (rather independent of E*)
Strenght = 100% EWSR (fully collective)
G increases with initial E* (but kept constant in each single calculation)
Chakrabarty et al. PRC36(1987)1886
Interpretation: The width is increasing due to averaging of strength function over various shapes induced by spin and T
Extracted a parametrization for G: G = E1.6

5. Low E* region - Width saturation
Reaction: 10 MeV/A
Hot nuclei (110Sn) were populated at E*  230 MeV
Gamma-rays were measured in coincidence with Evaporation Residues
Gaardhøje et al. (PRL62(1989)2080)
The first evidence for width saturation at about 13 MeV due to saturation of transferred angular momentum.
Width saturation was also observed by Enders et al.
PRL69 (1992) 249
The broadening of the GDR line-shape is due to shape mixing associated with mixing of different deformed rotating nuclei.
S=100% EWSR
EGDR = 16 MeV
G = 13 MeV
a = A/8
P.M.Kelly et al. PRL82 (1999)3404
K.A.Snover NPA687 (2001) 337c
Recently the reaction 18O + 100Mo was investigated
107 MeV < E* < 140 MeV
Importance of pre-equilibrium emission in the determination of E*
Correction applied to GDR width for A ~ 120 systematics. Width increasing up to T = 3.2 MeV
Gaardhoje et al.
Enders et al.
18O + 100Mo data

6. Quenching of the GDR in Hot nuclei
All experiments show evidences for a saturation of the GDR gamma-ray yield at E* above 300 MeV:
Gaardhøje et al: 15 and 24 MeV/A (PRL59(1987)1409)
Standard statistical model calculations (CASCADE) are not able to reproduce
the data above E*  300 MeV
40Ar + 70Ge
E* = 600 MeV
Eg (MeV)
Quenching of the GDR in Hot nuclei
E* = 500 MeV
Yoshida et al: 21 and 26 MeV/A (PL245B(1990)7)
36Ar + 27 MeV/A
E* = 500 MeV
Suomijärvi et al: 27 MeV/A (PRC53(1996)2258)
E* = 430 MeV
36Ar + 37 MeV/A
P.Piattelli et al: 37 MeV/A (NPA649(1999)181c)

7. Where does the GDR yield saturation come from ?
Theoretical interpretations point to two main effects which can lead to a saturation of the GDR g multiplicity at high E*:
a suppression of the GDR at high T
a rapid increase of the width with T

8. Models: Yield suppression
Bortignon
Chomaz
P.F.Bortignon explains the effect taking into account the equilibration time of the GDR with the compound nucleus.
At high T the equilibration time is comparable or longer than particle evaporation time and this precludes the emission of the GDR gamma-rays during the first stages of the cascade.
The hindrance factor:
Gdown/(Gdown+Gev)
Gdown = 4.8 MeV
Particle evaporation width Gev increases rapidly with E* leading to a suppression of the emission.
Bortignon et al. PRL67, 3360 (1991)
a = A/8
a =A/10
a =A/12
P.Chomaz NPA569(1994) 203c
Chomaz proposes that the GDR quenching is due to the fact that strong fluctuation in the nuclear dipole moment are induced by the rapid sequential particle emission.
If time between emissions becomes shorter than characteristic GDR vibration time the motion is no longer characterized by the GDR frequency and the spectrum will be flat.
A suppression factor S= exp(-2pGev/EGDR) is deduced.

9. Models: width increasing with T
Prediction of a strong increase of the GDR spreading width with E* due to the damping through 2-body collisions which become increasingly important with increasing T.
The width is parametrized:
G = E*1.6
The saturation of the gamma multiplicity is mainly due to a large increase of the width of the GDR rather than to preequilibrium effects.
T(MeV)
Width (MeV)
Smerzi et al. PRC44(1991)1713 – PLB320(1994)216
Bonasera et al. NPA569(1994)215c
Chomaz NPA569(1994)203c
a = A/10
a = A/12
a = A/8
Photons are coming from transitions between two states of the CN with a finite lifetime: GGDR = G + 2 Gev
The effect is strongly dependent on E* of the system:
At low E* the lifetime is so long that the influence on GGDR is negligible (G >> Gev)
At high E* the lifetime becomes so small that the Gev >> G (dominant contribution)
Width (MeV)
E* (MeV)

10. Data were reproduced with a G strongly increasing with E*
The saturation of the yield: experimental data
J.Yoshida et al. PLB245(1990)7
Reactions: 21 and 26 MeV/A
Gamma-rays were measured in coincidence with heavy residues.
Selection in velocity of residues was applied for both reactions allowing to measure different E* with the same experiment.
No correction for pre-equilibrium emission was applied.
Ebeam Vr/Vcm E* (MeV Ares 1
0.7
0.5
535
370
265
132
117
109
21 MeV/A 1
0.7
0.5
610
470
360
132
117
110
26 MeV/A
E*(MeV)
Mg (12-20 MeV) x10-3
Mg (25-40 MeV) x10-3
Smerzi et al.
PLB320(1994)216
Calculation with G(E*)
The integrated g multiplicity in the region 12 – 20 MeV saturates above E* >250 MeV
Neutron multiplicity and T increase versus E* as expected from a decay from an equilibrated system.
Data were reproduced with a G strongly increasing with E*
GG = E* + 1.6*10-8E*4
E*/A (MeV)
T (MeV)
E* (MeV)
Mg (12-20 MeV) x10-3
J.Kasagi et al. NPA557(1993)221c

11. Other evidences for the saturation of the yield
Reaction:
36Ar + 90Zr @ 27 MeV/A
Hot nuclei detected in coincidence with g-rays
Selection on residue velocities was applied
E* > 300 MeV from particle spectra
J.H. Le Faou PRL 72(1994)3321
T.Suomijarvi et al. PRC53,(1996)2258
Experimental data
Standard CASCADE calculation
CASCADE with a cut-off at E* = 250 MeV
CASCADE GDR parameters: S = 100% EWSR, G= 12 MeV,
EGDR = 76 / A1/3, a= a(T)
The simplest way to reproduce the data is to introduce a cutoff in the calculation
Same cutoff value reproduces the data
Standard CASCADE
CASCADE with increasing width

12. Comparison with models Reaction: 36Ar + 90Zr @ 27 MeV/A
Experimental data
CASCADE (Chomaz)
CASCADE (Smerzi)
CASCADE (Bortignon)
CASCADE (Chakrabarty)
The increase of G induces a shift of the yield rather than a quenching
Comparison between data and calculations with models with increasing width fails in the high energy part of the spectrum
The g-ray multiplicity saturation is consistent with a disappearance of the GDR strength above E* = 250 MeV and not with an increase of the GDR width.
Data are reproduced with the same cut-off energy independently of the initial E* of the nucleus produced (350<E*<500 MeV)
Evidence for a limiting temperature T  5.5 MeV (A  110) for the excitation of the dipole vibration (J.H. Le Faou PRL72 (1994) 3321)

13. Other evidences for the saturation of the yield
Reaction
36Ar + 37 MeV/A
Stronger gamma multiplicity suppression (a cut-off at about 200 MeV is needed to reproduce the data)
The trend of g-multiplicity is decreasing with Ebeam
This suggests the occurrence of dynamical effects (BNV gives a qualitative explanation of these results the problem is still open for discussion)
MEDEA data:
36Ar + 90Zr @ 27 MeV / A
36Ar MeV / A
P.Piattelli et al. NPA649 (1999)181c

14. A.Smerzi et al. PLB320(1994)216 Shape of the cut-off
Data are at too high energy to allow us to extract the shape of the cutoff (more information on the GDR yield suppression)
Recently we investigated the GDR properties in the excitation energy region 160<E*<300 MeV to map the progressive disappearance of the GDR
The amount of reduction of the GDR strength strongly affects the resulting GDR yield in a region between 150 and 300 MeV.
A.Smerzi et al. PLB320(1994)216
P.F.Bortignon et al. PRL67(1991)3360
P. Chomaz NPA569(1994)203c
116Sn + 12C MeV/A
116Sn + 12C MeV/A
116Sn + 24Mg MeV/A
Ebeam E*(MeV) AR E*/AR
Reactions

15. g spectra: comparison with CASCADE calculation
E* = 290 MeV, A = 136, G = 12 MeV
E* = 200 MeV, A = 127, G = 12 MeV
E* = 160 MeV, A = 127, G = 12 MeV
160 MeV
200 MeV
290 MeV
X 102
X 104
CASCADE INPUT PARAMETERS
E* = 430 MeV, A = 111, G = 12 MeV
E* = 350 MeV, A = 108, G = 12 MeV
350 MeV
430 MeV
X 106
X 108
E*/A (MeV/A)
Mg x 10-3
36Ar 37 MeV/A
New data
CASCADE new data
CASCADE 36Ar + 98Mo
g multiplicity (12 – 20 MeV)
E*/A (MeV/A)
Mg exp/ Mg Cascade
From all experiments we have Evidences for a limiting temperature T  MeV for the excitation of the dipole vibration
Natowitz et al. PRC65 (2003)
E*/A (MeV/A)
T (MeV)

16. Studies of the evolution of the GDR properties on nuclei of mass A ~ 60 - 70
The Width of the GDR for cold nuclei is expected to be about G = 6 MeV
(K.Snover Ann. Rev. Nucl. Part. Sci 36, 545 (1986)
Fusion reactions studies on 59Cu show a smooth increase of the width from 6 MeV up to 15 MeV for E* = 100 MeV. Centroid energy EGDR = 17 MeV (Fornal et al. Z.Phys.A340(1991)59)
Sharp cutoff
Smooth cutoff
Reaction 40Ca + 48Ca,46Ti at 25 MeV/A
Hot nuclei populated with incomplete fusion reactions
Rise time (arb.un.)
Observed GDR g-rays in coincidence with evaporation residues
CASCADE INPUT
E*=354 MeV
A=63
EGDR=16.8 MeV
G = 15 MeV
Saturation of g yield is observed
The g yield can be explained assuming a cutoff for GDR emission at E*/A = 4.7 MeV
Cascade calculation including smooth cutoff were performed:
found a saturation energy E*/A = 5.4 MeV
the increasing width is not able to reproduce the data
S.Tudisco et al. EuroPhys. Lett 58(2002)811
F.Amorini et al. PRC69 (2004)

17. Saturation energies for both mass regions
A saturation effect is also observed for mass around 60 but at higher excitation energy
A similar mass dependence was found by Natowitz in the analysis of caloric curves PRC65(2002)034618

18. Entrance channel charge asymmetry
Pre-equilibrium dipole g–ray emission: dependence on the N/Z degree of freedom
Entrance channel charge asymmetry
Initial dipole moment
Rp, Rt: projectile and target radii, Zp, Zt: projectile and target atomic numbers, A: mass number of the composite system
Dipole g – ray emission increases with the entrance channel charge asymmetry for deep inelastic [1] and fusionlike heavy-ion reactions [2].
M.Papa et al, PRC68 (2003)
F. Amorini et al. PRC69 (2004)
[1] D. Pierroutsakou et al. NPA687 (2001) 245c
[2] D. Pierroutsakou et al., EPJA17 (2003) 71
25 MeV/A
40Ca+ 46Ti @ 25 MeV/A
Fusion-Incomplete Fusion
Dissipative binary processes
Extra yield at 10 MeV

19. Similar initial dipole moment
Pre-equilibrium dipole g-ray emission was studied in fusion reactions as a function of the incident energy
Reactions
D. Pierroutsakou et al., EPJ A (2003)423
32S + 100Mo
36S + 96Mo
Both studied at about 6 and 9 MeV/A
132Ce was populated
E* = 115, 173 and 305 MeV
36Ar + 96Zr
40Ar + 92Zr
Similar initial dipole moment
difference
Studied at about 16 MeV/A
 6 MeV/A
 9 MeV/A
 16 MeV/A
The intensity of g – rays in the region MeV increase for the more charge asymmetric system by ~ 14% at 16 MeV/A, by ~ 25 % at 9 MeV/A
The presence of pre-equilibrium dipole is an indication of the evolution of the system towards Fusion:
a tool to follow the dynamics of fusion
Eg (MeV)

20. Low E* (up to about 200 MeV) is well understood:
Conclusions
Low E* (up to about 200 MeV) is well understood:
The width was found to increase up to about 15 MeV.
The GDR gamma multiplicity increases according to 100% EWSR
Models are able to reproduce the observed trend
High E* (above 300 MeV):
A saturation in the gamma-ray multiplicity is observed by all the
experiment
The results indicate that collective motion disappears at E*/A  2.5 MeV
for A110 nuclei
The disappearance is a rather sharp effect
Indications of a mass dependence of the saturation energy have been
found
Similarities between the limiting temperatures for the GDR and the
critical temperatures observed in multifragmentation are intriguing.