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Neutron flux variation in k 0 -INAA, experiences and solution in Kayzero for Windows

Neutron flux variation in k 0 -INAA, experiences and solution in Kayzero for Windows. R. van Sluijs , k 0 -ware, Heerlen, the Netherlands D. Bossu s, J. Swagten , DSM Geleen, the Netherlands F. De Corte, A. De Wispelaere, RUG, Ghent, Belgium.

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Neutron flux variation in k 0 -INAA, experiences and solution in Kayzero for Windows

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  1. 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 MARC VII, April 3th- 7th 2005, Kona Hawai

  2. Pre-requisite for the standard k0-formula: Constant neutron fluence rate during irradiation • 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) Fact: neutron fluence rate is never perfectly stable Theoretically this problem was handled by DeCorte (1987) and later by Lin Xilei (2001) and Jaćimović (2003) MARC VII, April 3th- 7th 2006, Kona Hawai

  3. - k0-Nuclear Data (published in open literature) • Saturation, Decay and Counting correction - Reactor parameters: f and α (HØgdahl-convention, well described) - Detection efficiency accounting for absortion in the sample - Neutron self absortion in the sample - Result of the gamma spectrum measurement, dead-time, sample weight, counting time, coincidence correction etc. k0-formula MARC VII, April 3th- 7th 2006, Kona Hawai

  4. k0-formula and epithermal flux With Comparator Factor Fc: Relation between Fc and neutron fluence rate MARC VII, April 3th- 7th 2006, Kona Hawai

  5. 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. MARC VII, April 3th- 7th 2006, Kona Hawai

  6. 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 : MARC VII, April 3th- 7th 2006, Kona Hawai

  7. Variability of the neutron fluence rate (DeCorte 1987) F(t)=1 : (Jaćimović 2003) F(t)=1+k.t : Jaćimović : k ranging from –0.4 to –0.6%/hr in Triga Mark II DSM : k approx +0.6%/hr in BR1 MARC VII, April 3th- 7th 2006, Kona Hawai

  8. Variability of the neutron fluence rate:summation of independent short irradiations 1.2 1 0.8 td' tirr' Normalized Epi-Thermal Flux: F(t) 0.6 0.4 0.2 0 8:00 9:00 10:00 11:00 12:00 13:00 14:00 time As proposed by De Corte 1987 => MARC VII, April 3th- 7th 2006, Kona Hawai

  9. 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  MARC VII, April 3th- 7th 2006, Kona Hawai

  10. k F t 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 Jaćimović’s formulae • It is an extra correction MARC VII, April 3th- 7th 2006, Kona Hawai

  11. 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 Fc=+2.11% at end Fc +2.34% MARC VII, April 3th- 7th 2006, Kona Hawai

  12. Example 2: Standardized flux variationTypical 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 (Jaćimović, 2003) MARC VII, April 3th- 7th 2006, Kona Hawai

  13. 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! MARC VII, April 3th- 7th 2006, Kona Hawai

  14. 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. MARC VII, April 3th- 7th 2006, Kona Hawai

  15. MARC VII, April 3th- 7th 2006, Kona Hawai

  16. Example 2: Standardized flux variationTypical 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 (Jaćimović, 2003) MARC VII, April 3th- 7th 2006, Kona Hawai

  17. 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) • (09:00:00 1000000 • 09:01:00 1002123 • 09:02:00 1003453 • 09:03:00 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. MARC VII, April 3th- 7th 2006, Kona Hawai

  18. 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 MARC VII, April 3th- 7th 2006, Kona Hawai

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