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Introduzione GDR

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Compilation of models interpreting the evolution of GDR. properties with E ... of the study of Giant resonances on excited states was launched by Brink in 1955 ... – PowerPoint PPT presentation

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Title: Introduzione GDR


1
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 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
At low E (E/Alt 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
Sn isotopes were populated by fusion reactions up
to E 130 MeV
5
Hot nuclei (110Sn) were populated at E ? 230
MeV Gamma-rays were measured in coincidence with
Evaporation Residues
Reaction 40Ar70Ge _at_ 10 MeV/A
Gaardhøje et al. (PRL62(1989)2080)
S100 EWSR EGDR 16 MeV G 13 MeV a A/8
6
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7
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
Bortignon Chomaz
9
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 0.0026E1.6 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
The saturation of the yield experimental data
J.Yoshida et al. PLB245(1990)7
Reactions 40Ar92Mo _at_ 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
1 0.7 0.5
610 470 360
132 117 110
26 MeV/A
11
Other evidences for the saturation of the yield
Reaction 36Ar 90Zr _at_ 27 MeV/A
Hot nuclei detected in coincidence with
g-rays Selection on residue velocities was
applied E gt 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
Reaction 36Ar 90Zr _at_ 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 (350ltElt500 MeV)
13
Reaction
36Ar 98Mo _at_ 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 _at_ 27 MeV / A 36Ar
98Mo _at_ 37 MeV / A
P.Piattelli et al. NPA649 (1999)181c
14
Data are at too high energy to allow us to
extract the shape of the cutoff (more information
on the GDR yield suppression)
15
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16
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 E354 MeV A63 EGDR16.8 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
A saturation effect is also observed for mass
around 60 but at higher excitation energy
18
Pre-equilibrium dipole gray 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
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
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