Hello everyone.
I’m a beginner for geant4 and I’m trying to make a model to simulate the Interaction between cosmic rays and atmosphere and count the information of neutrons produced in this process.I just made a very simple model as the earth is a water ball and the the atmosphere is divided into ten layers.But this model is only about 20cm in size,as the radius of the real earth is about 6400km and i can’t run such a big model in my computer.So I wanna ask how to make a scaled model to simulate this progress,or i can change the minimum step length so i can run the big model?Do you have some suggestion to implement it, please?
Thanks in advance for your time.
Besr ragards.
Why not do plane layers, and shoot cosmic rays from above the top layer, using the appropriate cos^2 distribution?
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The thickness of the atmosphere is 100km ,its still a big model for me to run it. 
.Anyway,thanks for your reply,but I still wanna make a model as complete as possible and its necessary for me to make a scaled model or change the minimum steplength.
I’m not disagreeing with that 
But you can calculate the typical shower width (I think it’s something like 10 km or so) for cosmic rays incident at the top of the atmosphere. Geant4 will certainly let you make a geometry of say 10 km x 10 km x 200 km. You probably want to set the production cuts to something 1 m or so, to avoid tracking all the low-energy crud.