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 wayThe nucleus can vibrate and rotate as a liquid dropGamma Rays probes such collective excitations

Parameters governing the GDR:

EGDR 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.

0 5 10 15 20 25 30

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

a.u

.

Energy [MeV]

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*10-19s1*10-17s

"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 560/3-4 19737(2003)155

2

3

4

5

6

7

8

9

0 0.5 1 1.5 2 2.5Temperature

Wid

th [

MeV

}

No Shell CorrectionWith shell correctionPL B383(1996)252 This work

How GDR propertiesevolves with Rotation ?

As angular momentum increases the nucleus becomes more and more deformed

Nuc

lear

def

orm

atio

n

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)

1

*

dE

dST

a

EEET

gdrrot

*

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

EGDR

"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)022501-1

"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