Frozen quantal fluctuation approach for modelling fission fragment charge distribution

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Frozen quantal fluctuation approach for modelling
fission fragmentcharge distribution
V.A. Rubchenya
Department of Physics, University of Jyväskylä,
Finland
ESNT Workshop, May 9 - 12, 2006
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?
p
n
d
Pre-compound stage
510-22
Pre-saddle evaporation
Saddle point
10-19
Descent from saddle
Charge distribution formation
1.110-19
Scission
Post-scission evaporation
10-8
Fission products
Y(A,Z), W(TF,Ekin), Mn(En,Tn), M?(E?,T?)
Time/ s
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The main dynamical effects

• Pre-compound particle emission Mnpre-eq,
  Mppre-eq, Epre-eqelapsed
• Role of the nuclear friction in the fission
• delay time for formation of fission degree of
  freedom
• modification of the fission width
• overdamped collective motion on the descent from
  saddle
• Mnpre-sc, Mppre-sc, Eelapsedpre-sc
• Charge polarization during the descent from
  saddle to scission charge distribution for
  isobaric chains Y(Z/A)
• Competion between different fission modes as
  function of composition and exitation energy of
  compound nuclei Y(A, Ecomp)
• Distribution of excitation energy between
  fragments Mn(A, Z, Ecomp)
• Shell structure for very deformed nuclei shell
  corrections, fission barriers, mass parameters,
  fission modes, level density

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• Frozen quantal fluctuatuations in the charge
  equlibration mode

Time evolution of isobaric width in the harmonic
approximation
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Primary isobaric charge distribution
parametriztion
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Configuration at the scission point
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?tip
p
p
n
n
AL, ZL, eL AH, ZH, eH
Strutinskys shell correction method was applied
using single particle spectra in deformed
Woods-Saxon potential.
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The approximation of the liquid drop charge
distribution parameters
Deviation from uniform distribution
The reverse stiffness parameter
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Excitation energy dependence
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Calculated neutron multiplicities in the thermal
induced fission
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Calculated fragment kinetic energies in the
thermal neutron induced fission
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Superasymmetric modes in the thermal neutron
induced fission
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Comparison between theortical calculations and
LOHENGRIN data
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A78
Y0.089 for the 242Pu(p,f) Ep55 MeV Y0.035
for the 242Pu(p,f) Ep13 MeV Y0.016 for the
242Am(nth, f)
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• The code can be used for calculation folloiwing
  output in the proton and neutron induced
  reactions at energy up to 100 MeV
• Evaporation residues cross sections.
• Pre-compound neutron and proton emission
  spectra.
• Pre-compound proton and neutron multiplicity.
• Pre-scission light charged particles (p, d, a)
  emission multiplicities.
• Pre-scission neutron spectrum.
• Post-scission light charged particle
  multiplicities.
• Post-scission neutron spectrum.
• Fragment kinetic energies.
• Pre-neutron emission fragment mass yields.
• Fission product mass yields.
• Fission product yields for isobaric chains.
• Fission product yields for elemental chains.

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The two-component exciton model was used for our
objectives an adequate description of the
initial excitation energy distribution and
composition of the compound nuclei in the neutron
and proton induced fission. For description of
exciton evolution process the folloing
transitions are taken into account- proton
particle-hole pair creation- neutron
particle-hole pair creation- conversion of a
proton particle-hole pair into a neutron
particle-hole pair- conversion of a neutron
particle-hole pair into a proton particle-hole
pair - the proton emission- the neutron
emission-after particle emission the exciton
evolution process may develop further
until reaching the criteria for transition to the
compound stage of the reactionThe exciton
transition cascade is ruptured at reaching one of
conditions1. exciton number reaches limited
value n nmax2. total life time of the
exciton stage exceeds limiting value Texc Tmax
which corresponds to statistical width decay3.
the number of emitted particles exceeds the
lemited value Mn Mnmax or Mn Mnmax .
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Pre-compound stage
We use the two-component exciton model for our
objective an adequate description of the initial
excitation energy distribution and composition
of the compound nuclei in the neutron and proton
induced fission at the incident energy from 10
to 100 MeV.
The lifetime of exciton state
The proton pre-compound single spectra from given
exciton state
The neutron pre-compound single spectra from
given exciton state
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Nuclear friction at pre-sadlle stage and
transition through fission barrier.
Modification of the statistical Bohr-Wheeler
fission width
are collective frequences at equilibrium and
saddle shapes
and
ß denotes the reduced dissipation coefficient
Competition between particle evaporation and
fission channels defines the fission chances
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Neutron and proton multiplicities as function of
neutron energy in 238U(n, f)
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Compompound nucleus mass and charge distributions
before scission
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Comparison with mass spectrometer experimental
data in 238U(p, f)
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Comparison with mass spectrometer experimental
data in 238U(p, f)
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Comparison with IGISOL-TRAP data in 238U(p, f) at
Ep25 MeV
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Saddle-to-scission descent stage

• saddle-to-scission time is altered by the
  nuclear dissipation

• Saddle and bifurcation points and valleys on the
  potential-energy surface of
• fissioning nucleus determine the properties of
  fission modes