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_Geant4 Version:_11.1.1
Operating System: Ubuntu (22.04.4) through WSL
_Compiler/Version:_gcc/11.4.0
_CMake Version:_3.22.1
Hello,
I have an interesting issue with some unexpected scoring mesh results.
I’ve modelled a situation in which a gamma radiation source (Co-60) is placed into a bag of sand with a volume of 1 cubic meter. A score mesh is created at a fixed distance outside the bag to track the surface flux of particles within specific energy ranges using a series of filters.
When running the simulation and visualising the score mesh results for the principal photon energies of Co60, I consistently end up with a central row of the of the mesh without any recorded entries. This central row does end up with additional recordings when filtering for lower energy gammas (I’ve assumed to be related to scattered photons). The sources are placed at different positions along the x-axis, with the score mesh orientated parallel to the yz plane and centred further along x-axis (in the positive direction). I’ve tried placing source both inside and outside of the sand bag volume.
The centre of the blank row is an area I would expect the highest numbers of recorded entries based on the proximity to the source/attenuation characteristics of the ray paths for the range of simulations I’ve tried. The expected trend holds above and below this row, but this particular row is always blank.
It might be difficult to see from the screenshots I’ve uploaded, but the blank row corresponds to a plane that sits just above the xy plane. I’ve tried simulating the source with a cos (2 pi) angle distribution and through modelling it as an isotropic source both with and without an angular bias, obtaining the same result each time.
The same row is also still blank when the sources are instead placed 10 cm below the xy plane, which in my head rules out angle biasing issues as this would alter the corresponding incident angles and potentially shift the blank row further in the -Z direction. I’ve tried this in interactive mode and batch mode.
Any thoughts or ideas as to why this might be occurring?
Related more fundamental questions I’m struggling to solve from the documentation:
When using scoring meshes, are the cell recordings equivalent to having a 100% efficient detector? I can see from the documentation that the dose scorer uses the material definition at the point in space but do the energy deposition and other scorers also do this? i.e. would placing a different material in the scorer region change the results of all these scorers?
I’m ideally trying to get a low resolution view of the gamma spectrum through each scoring cell, could the problem be to do with using the surface flux scorer and would one of the alternative scorers be better suited in this case? I’ve been using the method outlined in M.Asai’s SLAC presentation so far.
Below is an example of the scorer and general particle source source settings I’ve been using. I would be happy to provide more information about my source code if required.
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/score/create/boxMesh scoregrid
/score/mesh/boxSize 2.5 50.6 50.6 cm
/score/mesh/nBin 1 22 22
/score/mesh/translate/xyz 102.5 0. 0. cm
#There are several flux scorers like this up to the max gamma emission energy
#1.33 MeV. I haven’t included all of them for the sake of brevity.
/score/quantity/flatSurfaceFlux flux0300
/score/filter/particleWithKineticEnergy range800 0.8 0.899 MeV gamma
More scorers with different energy ranges in here for 400-500, 500-600 kev…
/score/quantity/flatSurfaceFlux flux1300
/score/filter/particleWithKineticEnergy range1300 1.3 1.399 MeV gamma
/score/close
set up the point source for 2 pi emission
/gps/particle gamma
/gps/pos/type Point
/gps/pos/centre 40. 0. 0. cm # 10 cm inside the sand volume
/gps/ang/type cos
/gps/ene/type Mono
/gps/ene/mono 1.3325 MeV
/gps/ang/rot1 0 0 1
/gps/ang/rot2 0 1 0
/run/beamOn 10000
/score/drawProjection scoregrid flux1300
‘’’
Examples for the isotropic situation:
‘’’
set up the point source for 2 pi emission through angular biasing of isotropic source
/gps/particle gamma
/gps/pos/type Point
/gps/pos/centre 40. 0. 0. cm # 10 cm inside the sand volume → For 10 cm outside volume 60. 0. 0. cm
/gps/ang/type iso
/gps/ene/type Mono
/gps/ene/mono 1.3325 MeV
/gps/ang/minphi 90 deg
/gps/ang/minphi 270 deg
/run/beamOn 10000
/score/drawProjection scoregrid flux1300
‘’’
