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Neutron flux variation in k0-INAA, experiences
and solution in Kayzero for Windows

• R. van Sluijs, k0-ware, Heerlen, the Netherlands
• D. Bossus, J. Swagten, DSM Geleen, the
  Netherlands
• F. De Corte, A. De Wispelaere, RUG, Ghent, Belgium

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Pre-requisite for the standard k0-formula
Constant neutron fluence rate during irradiation
Fact neutron fluence rate is never perfectly
stable

• Un-expected and unwanted perturbation of the flux
• Typical variation well known for the irradiation
  facility
• slow build-up of neutron flux when starting the
  reactor or shooting the sample to the irradiation
  position
• Increase in flux during irradiation
• Extra-ordinary irradiation over several reactors
  stops
• (maximum possible activation or neutron
  dose fluence rate irradiation time)

Theoretically this problem was handled by DeCorte
(1987) and later by Lin Xilei (2001) and
Jacimovic (2003)
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k0-formula
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k0-formula and epithermal flux
With Comparator Factor Fc
Relation between Fc and neutron fluence rate
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Saturation correction factor (S in SDC-formula)

• S Saturation correction for decay during
  irradiation
• Conditions
• - neutron epi-thermal flux is constant during
  irradiation
• thermal to epi-thermal flux ratio (f) is
  constant during irradiation
• Normalized to 1 so Fc gives value of epi-thermal
  flux.

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Saturation correction factor (S in SDC-formula)
SDC for the simplest form of activation
For more complex activation-decay, for example in
case of mother-daughter relation, code 8 (one of
the least complex), SDC has to be replaced by
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Variability of the neutron fluence rate (DeCorte
1987)
F(t)1
(Jacimovic 2003)
F(t)1k.t
Jacimovic k ranging from 0.4 to 0.6/hr in
Triga Mark II DSM k approx 0.6/hr in
BR1
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Variability of the neutron fluence
ratesummation of independent short irradiations
As proposed by De Corte 1987 gt
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Variability in epi-thermal flux and f (and ?)

• Dividing an irradiation into short
  sub-irradiations allows
• Handling variations in
• epi-thermal neutron fluence rate
• thermal to epi-thermal neutron fluence rate
  ratio (f)
• even variations in ?

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Sub dividing an irradiation in separate short
irradiations

• Note
• Epi-thermal and thermal neutron fluence rate (and
  ?) have to be measured or
• The variation in time should be calculated
• For following trend of comparator factors the
  fluence rate measurement data need to be
  normalized to 1
• Numerical integration using Jacimovics formulae
• It is an extra correction

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Example 1 Measured fluxUn-expected and unwanted
perturbation of the flux
For stable reactors (e.g. BR1) this can be
noticed by a deviation of Fc from the typical
value. Fc ? ?e ? reactor
power Handled recorded flux file,
normalized using irradiation start-stop
time Perturbation at begin Fc2.11 at end Fc
2.34
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Example 2 Standardized flux variation Typical
variation well known for the irradiation facility
Slow build-up of neutron flux when starting the
reactor or shooting the sample to the irradiation
position, combined with constant de- or in-crease
in flux Handled - integration using
fixed start-up/cool down flux profile
- and using analytical formulae (Jacimovic,
2003)
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Example 3 (entering multiple irradiations)
Extra-ordinary irradiation over several reactors
stops
Maximum possible activation or neutron dose
fluence rate irradiation time 5x 7
hours Handled by giving an irradiation
flux profile or the measured flux Result
correct results even for the short living
radionuclides!
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Conclusion

• The k0-method can also be used in case of
  variations in neutron flux on the condition that
  variations are known.
• Variations in thermal to epi-thermal neutron flux
  ratio and even ? can be handled straight forward.
• Practical solutions are given and implemented in
  Kayzero for Windows for
• slow transport to the irradiation position or
  starting the reactor with loaded samples and
• a constant increase or decrease in neutron flux.
• Activation during several reactor cycles can also
  be handled without problems.

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Example 2 Standardized flux variation Typical
variation well known for the irradiation facility
Slow build-up of neutron flux when starting the
reactor or shooting the sample to the irradiation
position, combined with constant de- or in-crease
in flux Handled - integration using
fixed start-up/cool down flux profile
- and using analytical formulae (Jacimovic,
2003)
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Example 1 Measured flux (implementation)
Un-expected and unwanted perturbation of the flux

• Data to enter
• Start irradiation date and time
• Stop irradiation date and time
• File name of a tabseparated file containing
  measured flux as a function of time, see figure
  (irradiation.flx in measurement directory)
• (090000 1000000
• 090100 1002123
• 090200 1003453
• 090300 1012345 etc.)
• AM/PM allowed!
• Start and stop time are used for calcu-
• lation the normalization average of the
• neutron flux gauge value.
• By doing this the Fc will be giving the
• neutron flux. Fc ? ?e ? reactor power.

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Example 2 Standardized flux variation
(implementation)Typical variation well known for
the irradiation facility

• Data to enter
• Start irradiation date and time
• Stop irradiation date and time
• File name of a tabseparated file containing
  start-up and cool-down flux change as a function
  of time see figure (irr.flx in meas. directory)
• Slope -0.6 /hr
• -10 0
• -5 0.5
• -0 1
• 5 0.5
• 10 0
• Slope increase/decrease during
• given irradiation period.
• Start-up/Cool-down
• Time in minutes
• Flux 0-1, see figure