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None Isotropic angle distribution for point source  

Forum: Medical Applications
Date: 15 Jul, 2015
From: Ellis <Ellis >

Hi,

I'm doing some simulations (using the 'Brachytherapy example') and I started with a very simple situation: a point source in the middle of a phantom (box of water, 10x10x10 cm), with monoenergetic photons (356 keV) as the primary particles. So, I removed the geometry of the brachytherapy source that was defined in the example and I defined a point source using the general particle source. I defined an isotropic angle distribution of the primary particles in the macro file as follows: /gps/pos/type Point /gps/pos/centre 0. 0. 0. mm /gps/ang/type iso

I defined a scoring mesh with half dimensions 4.975 cm in each direction. And the number of bins was 199. So then the scoring voxels are 0.05x0.05x0.05 cm. Secondary particles were taken into account.

When I plot three cross-sections (through the center) of the energy deposition on a logarithmic scale (based on the percentage of the maximum energy deposition), it seems that the distribution of the energy deposition is asymmetrical along the x-direction. So, in the xy-plane and in the xz-plane the energy deposition was asymmetrical, see the left image and the middle image in the figure in the attachment. This seems strange, as I would expect a completely symmetric energy deposition.

Can anybody help me with this problem? Do I make a mistake in defining the isotropic distribution of the primary particles of the gps? Or is this a known phenomenon in using the 'iso' type angular distribution? Do more people come across this problem?

Thank you in advance.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2015/07/15/02.14-48275-sition_cross-sections.png

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