Evolution of the GDR properties vs E Compilation

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

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 GGDR SGDR size and shape of nuclei (EGDR A-1/3 ) damping of the collective motions degree of collectivity of the excitation (sum rule 60 NZ/A)

What can we learn ? At low E* (E/A< 2 Me. V) 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 Me. V J. J. Gaardhoje et al PRL 53 (1984) 148 J. J. Gaardhoje et al PRL 56(1986)1783 Chakrabarty et al. PRC 36 (1987)1886 CASCADE input parameter: • EGDR = 15 Me. V (rather independent of E*) • Strenght = 100% EWSR (fully collective) • G increases with initial E* (but kept constant in each single calculation) Extracted a parametrization for G: G = 4. 8 + 0. 0026 E 1. 6 Interpretation: The width is increasing due to averaging of Chakrabarty et al. PRC 36(1987)1886 strength function over various shapes induced by spin and T

Low E* region - Width saturation Reaction: 40 Ar+70 Ge @ 10 Me. V/A Gaardhøje et al. (PRL 62(1989)2080) Hot nuclei (110 Sn) were populated at E* 230 Me. V Gamma-rays were measured in coincidence with Evaporation Residues The first evidence for width saturation at about 13 Me. V due to saturation of transferred angular momentum. Width saturation was also observed by Enders et al. PRL 69 (1992) 249 The broadening of the GDR lineshape is due to shape mixing associated with mixing of different deformed rotating nuclei. S=100% EWSR EGDR = 16 Me. V G = 13 Me. V a = A/8 Recently the reaction 18 O + 100 Mo was investigated 107 Me. V < E* < 140 Me. V 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 Me. V P. M. Kelly et al. PRL 82 (1999)3404 K. A. Snover NPA 687 (2001) 337 c 18 O + 100 Mo data Gaardhoje et al. Enders et al.

Quenching of the GDR in Hot nuclei All experiments show evidences for a saturation of the GDR gammaray yield at E* above 300 Me. V: Gaardhøje et al: 40 Ar+70 Ge @ 15 and 24 Me. V/A (PRL 59(1987)1409) Yoshida et al: 40 Ar+92 Mo @ 21 and 26 Me. V/A (PL 245 B(1990)7) Suomijärvi et al: 36 Ar+90 Zr @ 27 Me. V/A (PRC 53(1996)2258) P. Piattelli et al: 36 Ar+98 Mo @ 37 Me. V/A (NPA 649(1999)181 c) 40 Ar + 70 Ge E* = 500 Me. V 36 Ar + 98 Mo @ 37 Me. V/A E* = 500 Me. V E* = 430 Me. V Standard statistical model calculations (CASCADE) are not able to reproduce the data above E* 300 Me. V 36 Ar E* Zr @ Me. V + 90= 60027 Me. V/A Eg (Me. V)

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 Me. V Particle evaporation width Gev increases rapidly with E* leading to a suppression of the emission. Bortignon et al. PRL 67, 3360 (1991) a = A/8 a =A/10 a =A/12 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(-2 p. Gev/EGDR) is deduced. P. Chomaz NPA 569(1994) 203 c

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. 0026 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. Smerzi et al. PRC 44(1991)1713 – PLB 320(1994)216 Bonasera et al. NPA 569(1994)215 c T(Me. V) 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) a = A/8 a = A/10 Width (Me. V) Models: width increasing with T a = A/12 E* (Me. V) Chomaz NPA 569(1994)203 c

The saturation of the yield: experimental data 40 Ar+92 Mo @ 21 and 26 Me. V/A Ebeam 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. Vr/Vcm E* (Me. V Ares 21 Me. V/A 1 0. 7 0. 5 535 370 265 132 117 109 26 Me. V/A 1 0. 7 0. 5 610 470 360 132 117 110 Mg (25 -40 Me. V) x 10 -3 Mg (12 -20 Me. V) x 10 -3 T (Me. V) Reactions: J. Yoshida et al. PLB 245(1990)7 Mg (12 -20 Me. V) x 10 -3 The integrated g multiplicity in the region 12 – 20 Me. V saturates above E* >250 Me. V Smerzi et al. PLB 320(1994)216 Calculation with G(E*) E* (Me. V) 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* E*/A (Me. V) E*(Me. V) GG = 4. 8 + 0. 035 E* + 1. 6*10 -8 E*4 J. Kasagi et al. NPA 557(1993)221 c

Other evidences for the saturation of the yield Reaction: 36 Ar + 90 Zr @ 27 Me. V/A Hot nuclei detected in coincidence with g-rays Selection on residue velocities was applied E* > 300 Me. V from particle spectra J. H. Le Faou PRL 72(1994)3321 T. Suomijarvi et al. PRC 53, (1996)2258 Experimental data Standard CASCADE calculation CASCADE with a cut-off at E* = 250 Me. V CASCADE GDR parameters: S = 100% EWSR, G= 12 Me. V, EGDR = 76 / A 1/3, 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 with increasing width

Comparison with models Reaction: 36 Ar + 90 Zr @ 27 Me. V/A Experimental data CASCADE (Chomaz) CASCADE (Smerzi) CASCADE (Bortignon) CASCADE (Chakrabarty) 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 Me. V 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

Other evidences for the saturation of the yield Reaction 36 Ar + 98 Mo @ 37 Me. V/A Stronger gamma multiplicity suppression (a cut-off at about 200 Me. V 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: 36 Ar + 90 Zr @ 27 Me. V / A 36 Ar + 98 Mo @ 37 Me. V / A P. Piattelli et al. NPA 649 (1999)181 c

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

g spectra: comparison with CASCADE calculation CASCADE INPUT PARAMETERS E* = 430 Me. V, A = 111, G = 12 Me. V 108 E* = 350 Me. V, A = 108, G = 12 Me. V 430 Me. V E* = 200 Me. V, A = 127, G = 12 Me. V 106 E* = 160 Me. V, A = 127, G = 12 Me. V 350 Me. V X 104 g multiplicity (12 – 20 Me. V) New data 36 Ar +98 Mo @ 37 Me. V/A CASCADE new data CASCADE 36 Ar + 98 Mo 290 Me. V X 102 160 Me. V Mg exp/ Mg Cascade 200 Me. V Mg x 10 -3 X E* = 290 Me. V, A = 136, G = 12 Me. V E*/A (Me. V/A) From all experiments we have Evidences for a limiting temperature T 5 - 5. 5 Me. V for the excitation of the dipole vibration T (Me. V) X E*/A (Me. V/A) Natowitz et al. PRC 65 (2003) 034618

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 Me. V (K. Snover Ann. Rev. Nucl. Part. Sci 36, 545 (1986) Fusion reactions studies on 59 Cu show a smooth increase of the width from 6 Me. V up to 15 Me. V for E* = 100 Me. V. Centroid energy EGDR = 17 Me. V (Fornal et al. Z. Phys. A 340(1991)59) 40 Ca 48 Ca, 46 Ti Reaction + at 25 Me. V/A Hot nuclei populated with incomplete fusion reactions Sharp cutoff Smooth cutoff Observed GDR g-rays in coincidence with evaporation residues Rise time (arb. un. ) CASCADE INPUT E*=354 Me. V A=63 EGDR=16. 8 Me. V G = 15 Me. V • Saturation of g yield is observed • The g yield can be explained assuming a cutoff for GDR emission at E*/A = 4. 7 Me. V • Cascade calculation including smooth cutoff were performed: found a saturation energy E*/A = 5. 4 Me. V the increasing width is not able to reproduce the data S. Tudisco et al. Euro. Phys. Lett 58(2002)811 F. Amorini et al. PRC 69 (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 PRC 65(2002)034618

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]. Dissipative binary processes Fusion-Incomplete Fusion Extra yield at 10 Me. V 40 Ca+48 Ca @ 25 Me. V/A 40 Ca+ 46 Ti @ 25 Me. V/A M. Papa et al, PRC 68 (2003) 034606 F. Amorini et al. PRC 69 (2004) 014608 [1] D. Pierroutsakou et al. NPA 687 (2001) 245 c F. Amorini et al. PRC 69 (2004) 014608 [2] D. Pierroutsakou et al. , EPJA 17 (2003) 71

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 32 S + 100 Mo 36 S + 96 Mo Both studied at about 6 and 9 Me. V/A 36 Ar + 96 Zr 40 Ar + 92 Zr Studied at about 16 Me. V/A Eg (Me. V) 9 Me. V/A Eg (Me. V) 132 Ce was populated E* = 115, 173 and 305 Me. V Similar initial dipole moment difference 16 Me. V/A Eg (Me. V) The intensity of g – rays in the region 8 - 21 Me. V increase for the more charge asymmetric system by ~ 14% at 16 Me. V/A, by ~ 25 % at 9 Me. V/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

Conclusions Low E* (up to about 200 Me. V) is well understood: The width was found to increase up to about 15 Me. V. The GDR gamma multiplicity increases according to 100% EWSR Models are able to reproduce the observed trend High E* (above 300 Me. V): 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 Me. V 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.