Photonuclear attenuation magnified with cross section enhancement

[ftessier]

Description

• The photonuclear cross section is magnified when the cross section enhancement (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

• The bug arises when xcse is used while Photonuclear attenuation = On, and scales with the enhancement factor.
• The most severe bias occurs when the relative photonuclear cross section is largest, as shown further down.
• It affects the following EGSnrc applications: cavrznrc, cavsphnrc, dosrznrc, and egs_chamber.
• It was introduced when photonuclear attenuation was added to EGSnrc, around 2012.

Root cause

• There are no ausgab calls for BeforePhotoNuc (PHOTONUCAUSB) nor AfterPhotoNuc (PHOTONUCAUSA) events in application that implement cross section enhancement. When xcse is turned, the total photon cross section is increased by the enhancement factor. However, there is no compensation for this biasing in the ausgab 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 the xcse 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 for egs_chamber):

  	xcse factor	Dose per incident fluence
  cs enhancement = 0	—	9.6485e-12 +/- 0.09%
  cs enhancement = 1	1024	5.5757e-12 +/- 0.33%

  

  Relative photonuclear cross section for a few common materials, for reference:

[ftessier]

beamnrc is also affected.

[mainegra]

Wow @ftessier ! This is quite an effect! Good catch and great detective work!

[ojalaj]

@ftessier Dumb question: To my knowledge xcse feature is not used/available in beamnrc/cavrznrc/cavsphnrc/dosrznrc so how it can affect their results? Or with those applications, do you mean BCSE or any such feature? Is dosxyznrc affected? And if e.g. with beamnrc BCSE is not used, there is no problem even if Photonuclear attenuation = On?

[ftessier]

Hi @ojalaj! Support for xcse was added in beamnrc in 2005, but the option is not available in the gui (but it can be added in the input file), as far as I can tell. So by and large there is no issue if you have not entered CSE IN CMS in the input file. It is turned off by default. There is no issue with bcse. The dosxyznrc application is not affected. See #958 to see what files are affected and how.

[ojalaj]

Thanks @ftessier for confirmation (& sorry for dumb questions)!

[ftessier]

No dumb question. It is very useful when users ask questions, especially regarding any code modification. Thanks!