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The gamma decay of the GDR at finite temperature and of the pygmy resonance far from stability

The gamma decay of the GDR at finite temperature and of the pygmy resonance far from stability . G. Benzoni INFN sezione di Milano (Italy). Outline 1- Pygmy Dipole Resonance in 68 Ni background experiment results perspectives Preequilibrium Dipole Resonance Background Experiment

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The gamma decay of the GDR at finite temperature and of the pygmy resonance far from stability

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

  3. Pygmy Dipole Resonance

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

  5. 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. Beam energy selected in order to populate DIPOLE modes more effectively than other ones • 68Ni beam produced by fragmentation of 86Kr @ 900 MeV/u on thick Be target (4g/cm2): • 1010 ppspill 86Kr • Spill length 6s, period 10 s Two experiments performed: • 400 MeV/u 68Ni (2004) + 197Au • 600 MeV/u 68Ni (2005) + 197Au Higher statistics dataset 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. 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. Outgoing 68Ni E (CsI) 1.2 % 4.4 % DE (Si) ~35% Coulomb excitation of 68Ni @ 600 AMeV Incoming 68Ni beam 68Ni AoQ Z ~ 6 Days of effective beam time ~ 400 GB of data recorded ~ 3.107 ‘ good 68Ni events ‘ Incoming+Outgoing 68Ni

  10. Preliminary Preliminary Preliminary GEANT Simulations Coulomb excitation of 68Ni (600 MeV A) Pygmy Dipole Resonance A structure appears at 10-11 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. 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. t=0 fm/c CN fusion Dynamic Dipole GDR The Preequilibrium Dipole Response Charge NOT equilibrated All degree of freedom EQUILIBRATED Charge equilibration in N/Z asymmetric heavy-ion collisions

  17. A rapid re-arrangementof 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. O+Mo @ 4-8-14-20 MeV/u V. Baran et al . PRL87(2001)182501 • THEORY: • Preequilibrium dipole radiation present in all N/Z asymmetric reactions; • Dependence on charge asymmetry • mass asymmetry and • incident energy.

  19. Linearized spectra 40Ca + 100Mo 40Ca + 100Mo 36S + 104Pd 36S + 104Pd How to distinguish preequilibrium from thermalized emission? • 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 • 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 PPAC BaF2 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

  21. Reaction studied to populate 132Ce* CN 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) 012501

  22. 16O+116Sn @ 250 MeV Large pre-equilibrium component Counts [a.u.] Only statistical decay Ea [MeV] 64Ni+68Zn @ 500 MeV Determination of the preequilibrium component thanks to the measurement of the particles 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. 16O+116Sn@8 AMeV 16O+116Sn@15 AMeV Comparison with statistical model calculations to extract extra yield Extra gamma multiplicity 16O+116Sn @ 15 AMeV • Preequilibrium emission is enhanced at high beam energy • Same centroid as the GDR 16O+116Sn @ 8 AMeV

  24. 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) 182501-1

  25. 16 MeV/nucleon (E*=304 MeV) 6 MeV/nucleon (E*=117 MeV) 9 MeV/nucleon (E*=173.5 MeV) [1] [3] [2] S + Mo Ar + Zr S + Mo FGDR(Eg) (arb. units) Eg (MeV) Eg (MeV) Eg (MeV) D. Pierroutsakou. et al., PRC 71 (2005) 054605

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

  27. Exotic beams Increase further the N/Z asymmetry Perspectives • Stable beams: • measure angular distributions • Measure an asymmetric reaction WITHOUT dipole moment in entrance channel * C. Simenel et al PRL 86 (2001) 2971 The isotopes has been selected from the list of page II-8 of spiral documentation

  28. M. Di Toro, V. Baran, C. Rizzo 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

  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

  31. t  10-21 s GDR oscillation t  10-22s CN formation t=0 fm/c 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 16O(@8MeV/u)+116Sn

  32. This oscillation: -displays collective features like GDR -produces g emission Simulation code developed by Baran et al. based on BNV model applyed to 16O(@8MeV/u)+116Sn

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

  34. Pre-equilibrium Thermal Equilibrium 3.10-21 s The Dynamical Dipole Mode – Prompt dipole REARRANGMENT of p and n to establish the charge to mass equilibrium 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-arrangementof charge accompanied by emission of dipole radiation (timescale of the order of 10-21 s) • The intensity of dipole emission depends on the absolute value ofthe 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

  35. 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

  36. Nupecc long range plan 2004 Why Pygmy Resonance is important ? Pygmy Resonance has an important impact on the r-process nucleosynthesis 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

  37. 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 16O(@8MeV/u)+116Sn

  38. O+Mo @ 4-8-14-20 MeV/u Baran et al . PRL87(2001)182501 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 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 anisotropicg-emission • The higher N/Z asymmetry the stronger the effect

  39. 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

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