Beam_size parameter in DepositionGeant4 module

Dear colleagues,
I have a minor question about simulation configuration. The DepositionGeant4 module has a parameter “beam_size”, which refers to the width of Gaussian beam profile mentioned in user manual. But I’m a little confused about description.
For simple 2-D Gaussian distribution, the pdf is

assuming sigma_x = sigma_y = sigma, the pdf in polar coordinate is

What is the relationship between sigma and “width” in manual?
Wei Tong


Hello @WeiTong ,

using the parameter beam_size we directly set the source position for Geant4 via the command G4SPSPosDistribution::SetBeamSigmaInR (command see here) . In Geant4, this command again is used as the width of the two gaussian distributions (see around line 285 here). I would assume that this directly refers to the sigma in your equation. If in doubt, please refer to the documentation of the G4RandGauss class.

Does this answer your question?


Hi @pschutze,

I got it, thanks for your reply!

All the best
Wei Tong