Diapositive 1

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Workshop on the Theories of Fission and Related
Phenomena ESNT Workshop May 9- 12, 2006
A new statistical scission-point model fed with
microscopic ingredients
Sophie Heinrich CEA/DAM-Dif/DPTA/Service de
Physique Nucléaire
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A new statistical scission-point model Preamble
Our goal To reconsider the original Wilkins
scission-point model (1976) in order to provide
some fission fragments properties, sustaining it
with microscopic ingredients, and avoiding ad hoc
parameters. (Thesis work)
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Fundamental Hypothesis

• Observations
• Wide amplitude process
• Dramatic importance of shell effects of the
  fission fragments.
• Reorganization of the nucleus internal structure
• Systems history is simulated by a statistical
  equilibrium at the scission point.

Static Approach
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Why a renewal of the static approach ?

• Many reachable observables

• We have
• Experimental data with new insight (exotic
  nuclei, super heavy nuclei) and wide energy range
  (SPIRAL , GSI, )
• A need of predictions for nuclear data
• (used directly or as a guide for reaction models)

• We should provide
• Fission fragments distributions (mass, energy )
• Fragments properties (deformation, isospin )
• Without phenomenological ajustement.

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Why a renewal of the static approach ?

• Possible improvements to scission-point model

• Huge increase in computer processing power
  (CCRT, Ter_at_tec)
• Very good microscopic description for individual
  (deformed) energy levels
• New level densities
• Better comprehension of scission-point features

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Scission-point Model

• Energy at the scission point
• Individual microscopic energy (HF Gogny force)
  for each fragments
• Nuclear interaction between the 2 fragments
• Coulombian interaction between the 2
  fragments
• Available Energy of the system
• E( compound system before fission ) E(
  scission )

• Fission fragments distributions are entirely
  determined at Scission Point by the energy
  available in the system of the complementary
  fragment pairs...

• If scission point can be precisely
  characterized, there is no more ajustable
  parameter !

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Basic equation
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Scission-point Model

• If we can precisely evaluate D at the scission
  point, there is no more adjustable parameters.

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Tips-Distance Effect
Evolution of 228Th distributions D 1fm to
10fm
How to choose D ???
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Fission Path
Potential barrier

• Exit
• Point

S P
D
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Correlation between Scission-point and
Exit-point

• Selection of the Exit Points
• Strong modification of total binding energy
• Hexadecapolar moment drop
• Nucleon density at the neck lt 0.01 nuc.fm-3

Fit on 2 ellipsoids
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Mass and charge distributions
228 Thorium D 5fm Excitation energy of
fissioning nucleus 10MeV
CEA DAM/DPTA/SPN/MED/Sophie Heinrich
ESNT
Workshop May 9-12 2006
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222-gt228Thorium
N132
Competition between the symmetric and the
asymmetric fission for isotopes from A222 to
A228.
N134
N136
N138
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(No Transcript)
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Total Kinetic Energy ltVcoul Vnucgt
Z
5U
A
A
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Average TKE of a specific fragment pair
We can pick a specific fragment pair
and calculate the TKE of the system
distribution.
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Average deformation and excitation energies
Z
Average deformation of one fragment
A
A
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• Spherical/deformed and proton/neutron shell
  effects are well reproduce (Gogny force works
  fine).

• Most probable configurations reliable
  predictions for nuclear data with a parameter
  coming from microscopic calculation.

• Many observables are available mean TKE, mean
  excitation energy,... and can be used for further
  evaluations (number of emitted neutron, )

• We still suffer from a lack of description about
  what happens before the scission-point
  (prescission energy, emission of particules,).

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• Try out microscopic level densities.

• Include temperature microscopic calculation
  (no more shell effect).

• Rethink the whole definition of scission point.

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• J.L. Sida (PhD Director)
• H. Goutte
• J.F. Berger
• M. Girod
• S. Hilaire
• P. Romain
• B. Morillon
• P.Morel
• M. Dupuis
• F. Chappert

Thanks ...
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• Generalized Superfluid Model Critical energy,
  critical temperature, etc, corresponding to
  phase transition between normal and superfluid
  phase.
• Level Density

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• First interpretation liquid drop fission
• Equal mass fragments

• Actually, we often observe a heavy and a light
  fragment
• We need to consider quantum effects

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Time dependant Potential Energy Surface
Héloïse Goutte CEA/DAM Theoretical Nuclear
Structure Lab.

• Multi-valley
• symmetric valley
• asymmetric valley

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1 Nucleus N nucleons with strong interaction