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Evolution of the GDR properties vs E* Compilation in data and models

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.

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Evolution of the GDR properties vs E* Compilation in data and models

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  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. 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) GGDRdamping of the collective motions SGDRdegree of collectivity of the excitation (sum rule  60 NZ/A) 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

  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* J.J.Gaardhoje et al PRL53 (1984) 148 J.J.Gaardhoje et al PRL56(1986)1783 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) Extracted a parametrization for G: G = 4.8 + 0.0026 E1.6 Interpretation: The width is increasing due to averaging of strength function over various shapes induced by spin and T Chakrabarty et al. PRC36(1987)1886 Sn isotopes were populated by fusion reactions up to E* = 130 MeV

  5. Low E* region - Width saturation 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. Recently the reaction 18O + 100Mo was investigated 107 MeV < E* < 140 MeV 18O + 100Mo data 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. P.M.Kelly et al. PRL82 (1999)3404 K.A.Snover NPA687 (2001) 337c Hot nuclei (110Sn) were populated at E*  230 MeV Gamma-rays were measured in coincidence with Evaporation Residues Reaction: 40Ar+70Ge @ 10 MeV/A Gaardhøje et al. (PRL62(1989)2080) S=100% EWSR EGDR = 16 MeV G = 13 MeV a = A/8

  6. Quenching of the GDR in Hot nuclei E* = 500 MeV 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) 36Ar + 98Mo @ 37 MeV/A E* = 430 MeV Yoshida et al: 40Ar+92Mo @ 21 and 26 MeV/A(PL245B(1990)7) Suomijärvi et al:36Ar+90Zr @ 27 MeV/A (PRC53(1996)2258) 40Ar + 70Ge P.Piattelli et al: 36Ar+98Mo @ 37 MeV/A (NPA649(1999)181c) E* = 500 MeV Standard statistical model calculations (CASCADE) are not able to reproduce the data above E*  300 MeV E* = 600 MeV 36Ar + 90Zr @ 27 MeV/A Eg (MeV)

  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 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. 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(-2pGev/EGDR) is deduced. Bortignon et al. PRL67, 3360 (1991) P.Chomaz NPA569(1994) 203c Bortignon Chomaz

  9. Models: width increasing with T Width (MeV) T(MeV) • 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 a = A/12 Width (MeV) Chomaz NPA569(1994)203c E* (MeV) 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. Smerzi et al. PRC44(1991)1713 – PLB320(1994)216 Bonasera et al. NPA569(1994)215c

  10. Calculation with G(E*) The integrated g multiplicity in the region 12 – 20 MeV saturates above E* >250 MeV Mg (12-20 MeV) x10-3 Smerzi et al. PLB320(1994)216 Neutron multiplicity and T increase versus E* as expected from a decay from an equilibrated system. Mg (12-20 MeV) x10-3 T (MeV) Mg (25-40 MeV) x10-3 E* (MeV) Data were reproduced with a G strongly increasing with E* GG = 4.8 + 0.035E* + 1.6*10-8E*4 E*/A (MeV) E*(MeV) J.Kasagi et al. NPA557(1993)221c 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 535 370 265 132 117 109 1 0.7 0.5 21 MeV/A 132 117 110 1 0.7 0.5 610 470 360 26 MeV/A

  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 Experimental data CASCADE (Chomaz) CASCADE (Smerzi) CASCADE (Bortignon) CASCADE (Chakrabarty) The increase of G induces a shift of the yield rather than a quenching 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) Reaction: 36Ar + 90Zr @ 27 MeV/A 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)

  13. 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

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

  15. gspectra: comparison with CASCADE calculation CASCADE INPUT PARAMETERS E* = 290 MeV, A = 136, G = 12 MeV E* = 200 MeV, A = 127, G = 12 MeV E* = 160 MeV, A = 127, G = 12 MeV E* = 430 MeV, A = 111, G = 12 MeV E* = 350 MeV, A = 108, G = 12 MeV X 104 X 108 290 MeV 430 MeV X 102 X 106 200 MeV 350 MeV Mgx 10-3 160 MeV Mg exp/ Mg Cascade g multiplicity (12 – 20 MeV) New data E*/A (MeV/A) 36Ar +98Mo @ 37 MeV/A CASCADE new data CASCADE 36Ar + 98Mo T (MeV) E*/A (MeV/A) From all experiments we have Evidences for a limiting temperature T  5 - 5.5 MeV for the excitation of the dipole vibration E*/A (MeV/A) Natowitz et al. PRC65 (2003) 034618

  16. Studies of the evolution of the GDR properties on nuclei of mass A ~60 - 70 Rise time (arb.un.) S.Tudisco et al. EuroPhys. Lett 58(2002)811 F.Amorini et al. PRC69 (2004) 014608 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 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

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

  18. Dissipative binary processes Fusion-Incomplete Fusion Extra yield at 10 MeV 40Ca+48Ca @ 25 MeV/A 40Ca+ 46Ti @ 25 MeV/A Dipole g – ray emission increases with the entrance channel charge asymmetry for deep inelastic [1] and fusionlike heavy-ion reactions [2]. [1] D. Pierroutsakou et al. NPA687 (2001) 245c F. Amorini et al. PRC69 (2004) 014608 [2] D. Pierroutsakou et al., EPJA17 (2003) 71 M.Papa et al, PRC68 (2003) 034606 F. Amorini et al. PRC69 (2004) 014608 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

  19.  16 MeV/A  9 MeV/A  6 MeV/A Eg (MeV) Eg (MeV) Eg (MeV) 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 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

  20. 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.

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