Length scales of simulation

Recording, Visualizing and Persisting Data
1

If I have a geometry where the scattering events are occurring at Km length scales but my detector size is only few mm, is that going to be challenging to simulate?
The reason is I am unable to visualize this geometry in my 3D viewer and I thinking the reason is the grid or mesh for the entire problem is not fine enough (km size box in which I have a small mm size detector). But is this just a visualization issue or will it also affect the simulation - (for example simulating scattering of photons off particles in atmosphere into my detector?).
How should I be modeling this? Is there a special way?
Thanks

2

Could you be a little more specific? km to mm shouldn’t be a problem.

If you /vis/drawVolume, the G4 vis system will define the scene from the visible components of the geometry. Normally we recommend that the world volume is marked invisible (you can do that in your detector construction
world_log -> SetVisAttributes (G4VisAttributes::GetInvisible());
or by a UI command, such as
/vis/geometry/set/visibility World 0 false

You can also be specific:
/vis/drawVolume <sub-detector-name>

Or (with Geant4 10.6):
/vis/viewer/centreAndZoomInOn <sub-detector-name>

Have a look at “Debugging geometry with vis” in the Application Developers Guide (http://geant4-userdoc.web.cern.ch/geant4-userdoc/UsersGuides/ForApplicationDeveloper/html/Visualization/debugginggeometrywithvis.html)

3

Thanks Allison
Yes I am launching photons into a long pipe filled with scattering medium and looking at returns onto a detector which is only 10x10 mm^2.
When I constructed the volume, I can see the pipe but could not see the detector. When I increased the detector to a sizeable volume I could then see it. So my thought was that the reason I couldn’t see the detector earlier was because only few pixels (<1) covered it because drawing a volume must take a certain number of pixels. Even after zooming I couldn’t quite see it. I did not get any errors except that I have not launched any photons yet.

4

The visualization is limited by your display – for your scale difference of 10^6, you’re quite right that drawing the full geometry will make your little detector invisible.

The tracking doesn’t work that way. It “voxelizes” the geometry intelligently, with voxels of different sizes limited only by floating point. So your little detector will be fully resolved by tracking and interactions, though you may need to run a very large number of events before you get hits :slight_smile: