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Photonuclear attenuation magnified with cross section enhancement #956
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Add ausgab calls before and after a photonuclear interactions, in order to handle cross section enhancement (xcse) correctly. Without these calls, there is no compensation for the enhanced cross section, hence the photonuclear cross section is effectively increased. Also, update a couple loops that print out the ausgab flags (IAUSFL) values, to include the photonuclear attenuation input value.
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Wow @ftessier ! This is quite an effect! Good catch and great detective work! |
@ftessier Dumb question: To my knowledge |
Hi @ojalaj! Support for |
Thanks @ftessier for confirmation (& sorry for dumb questions)! |
No dumb question. It is very useful when users ask questions, especially regarding any code modification. Thanks! |
Add ausgab calls before and after a photonuclear interaction, in order to handle cross section enhancement (xcse) correctly. Without these calls, there is no compensation for the enhanced cross section, hence the photonuclear cross section is effectively increased by the xcse factor. Also add the photonuclear event trapping in Fano calculations, for consistency. However, the Fano calculation fails when photonuclear attenuation is turned on anyway, because the photon is discarded without depositing its energy. Also update a couple loops that print out the ausgab flags (IAUSFL) values, to include the photonuclear attenuation option value.
Description
xcse
) variance reduction technique is turned on, biasing calculated quantities.Expected behavior
xcse
is a true variance reduction technique, hence it should not alter calculated quantities.Impact
xcse
is used whilePhotonuclear attenuation = On
, and scales with the enhancement factor.cavrznrc
,cavsphnrc
,dosrznrc
, andegs_chamber
.Root cause
ausgab
calls forBeforePhotoNuc
(PHOTONUCAUSB
) norAfterPhotoNuc
(PHOTONUCAUSA
) events in application that implement cross section enhancement. Whenxcse
is turned, the total photon cross section is increased by the enhancement factor. However, there is no compensation for this biasing in theausgab
routine, for the photonuclear interaction (only).Examples
The dose to the air cavity of a Farmer ionization chamber surrounded by a tungsten buildup cap is decreased by about 2% for a bremsstralung beam emitted by 15 MeV electrons striking an aluminum target, and an
xcse
factor of 32 inside the tungsten cap. The discrepancy scales with thexcse
factor. This is how this bug was initially revealed.A more contrived example, although by no means far-fetched, is the dose to an air cavity inside a spherical shell of PMMA, 1 cm thick, irradiated by a uniform monoenergectic beam of 23 MeV photons, as depicted below. When
xcse
is turned on (enhancement of 1024), the dose is reduced by ~40% (input file foregs_chamber
):xcse
factorcs enhancement = 0
9.6485e-12 +/- 0.09%
cs enhancement = 1
5.5757e-12 +/- 0.33%
Relative photonuclear cross section for a few common materials, for reference:
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