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

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)

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

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 = 4.8 + 0.0026 E1.6

Low E* region - Width saturation Reaction: 40Ar+70Ge @ 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

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: 40Ar+70Ge @ 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: 40Ar+92Mo @ 21 and 26 MeV/A (PL245B(1990)7) 36Ar + 90Zr @ 27 MeV/A E* = 500 MeV Suomijärvi et al: 36Ar+90Zr @ 27 MeV/A (PRC53(1996)2258) E* = 430 MeV 36Ar + 98Mo @ 37 MeV/A P.Piattelli et al: 36Ar+98Mo @ 37 MeV/A (NPA649(1999)181c)

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

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.

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 = 4.8 + 0.0026E*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)

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: 40Ar+92Mo @ 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 = 4.8 + 0.035E* + 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

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

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)

Other evidences for the saturation of the yield Reaction 36Ar + 98Mo @ 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 + 98Mo @ 37 MeV / A P.Piattelli et al. NPA649 (1999)181c

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 17 MeV/A 160 127 1.26 116Sn + 12C 23 MeV/A 200 127 1.58 116Sn + 24Mg 17 MeV/A 290 136 2.13 Ebeam E*(MeV) AR E*/AR Reactions

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 +98Mo @ 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  5 - 5.5 MeV for the excitation of the dipole vibration Natowitz et al. PRC65 (2003) 034618 E*/A (MeV/A) T (MeV)

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) 014608

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

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) 034606 F. Amorini et al. PRC69 (2004) 014608 [1] D. Pierroutsakou et al. NPA687 (2001) 245c [2] D. Pierroutsakou et al., EPJA17 (2003) 71 40Ca+48Ca @ 25 MeV/A 40Ca+ 46Ti @ 25 MeV/A Fusion-Incomplete Fusion Dissipative binary processes Extra yield at 10 MeV

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 8 - 21 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)

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.