Protons Neutrons Nuclear structure at finite temperature probed by the Giant Dipole Resonance G. Benzoni, O. Wieland, A. Bracco, N. Blasi, F. Camera, F. Crespi, S. Leoni, B. Million Dipartimento di Fisica, Università di Milano INFN – Sezione di Milano The protons and the neutrons inside an atomic nucleus behave in a collective way The nucleus can vibrate and rotate as a liquid drop Gamma Rays probes such collective excitations Parameters governing the GDR: E GDR Nuclear Shape  GDR Lifetime and Damping of GDR  GDR How many protons and neutrons vibrates In deformed nuclei the vibrations along the three axes have different resonance frequencies. In spherical nuclei, vibrations along the three axes are identical and correspond to a single resonance frequency. The Giant Dipole Resonance (GDR) is a collective coherent vibration of ALL the protons against ALL the neutrons inside the nucleus Gamma rays energy measures directly the resonance frequency Hector Coupled to the RISING array at GSI The nuclear structure group of Milano has developed and built the HECTOR ARRAY to measure the  -decay of the GDR How GDR properties evolves with nuclear temperature ? Gamma_Lifetime 5* s 1* s "Search for Exotic Shapes of Hot Nuclei at Critical Angular Momenta“ Nucl.Phys.A687(2001)192 "Radiative Fusion from very symmetric reactions: The Giant Dipole Resonance in 179Au nucleus" PLB 560/ (2003)155 How GDR properties evolves with Rotation ? As angular momentum increases the nucleus becomes more and more deformed Nuclear deformation Nuclei can be heated using heavy ion fusion reaction, namely accelerating a projectile against a target. In certain conditions, the two nuclei fuse, and a hot thermalized system, which does not remember how it was formed, is created (compound nucleus) Hot thermalized nuclei cools down ‘evaporating’ particles and gammas which are measured by detectors. Experimental measurements has shown that: The dipole vibration is superimposed on the nucleus whatever is its state (ground state, single particle excitation, compound) The main damping width does not change whatever is its nuclear temperature or angular momentum The measured FWHM of the GDR increases due to the increasing deformation induced by a softening of the nucleus because of temperature At very high temperature compound intrinsic lifetime contributes significantly to the damping width At low temperature shell effects strongly affects GDR width  GDR E GDR "Probing Nuclear Shapes Close to the Fission Limit with GDR in 216Rn“ Physics Review C 70(2004)64317 "Evidence for the Jacobi shape transition in hot 46Ti“ Nucl.Phys.A731(2004)319 "Double Giant Dipole Resonance in Hot Nuclei“ Nucl.Phys.A731(2004)202 "Compound and Rotational Damping in Warm Deformed Rare-Earth Nuclei“ Physical Review Letters 93,2(2004) "Effect of E1 decay in the population of superdeformed structures" Phys.Lett.B 540(2002)199 "The gamma-decay of the GDR in highly excited Ce nuclei" JoP G 31(2005)S1973