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Question looking for a Variance Reduction techique to use with Pencil Beam simulations 

Forum: Biasing and Scoring
Date: 24 Oct, 2016
From: Maura E.M. <Maura E.M.>

 I need to find an adequate VR technique to decrease the CPU time of Pencil Beam simulations. 
The Pencil Beam model couldn't be simpler. It consist of a 6MeV mono-energetic photon point source placed on the top surface of a water cylinder. The goal is to score dose2water in the water cylinder.

I tried to use Topas VR technique named "Secondary Biasing. It expects, among the other parameters,the maximum energy, and the name for each process generating the particles to be split.

I wish to split ONLY "primary" 6MeV photons into 100 fictitious photons. Therefore I set the MaximumEnergy to 6.18 MeV (nomimal Linac energy). Unluckily this VR technique only filters by an upper energy bound. Thus all photons whose energy is lower than the chosen MaximumEnergy will be split producing replicas of low energy photons which we do not care to track.

Furthermore, this VR technique expects the names of the processes that generate the particles to be split (gammas). The problem is to single out the true "primary" photons. In fact the PencilBeam model does NOT include an electron source generating the photons. Geant names the generating process for such 6MeV photons "Primary". To prove it I have added to the PencilBeam model an N-Tuple scorer recording all gammas that hit the water cylinder upper surface, just below the photon source. I am attaching the .header and the .phsp files where you can see (use Excel or MatLab) that most photons are regarded as "Primary" even if their energy is by far lower than 6MeV. The reason is that Geant4 considers photons produced by Compton events as "Primary" regardless of the number of Compton interactions they have had.

Is there an appropriate VR technique for the PencilBeam simulations ? In general, is there a VR technique to use whenever the particle source generates gammas rather than electrons ?

Any suggestion / comment is greatly appreciated. Thank you in advance. Best regards, Mauede


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