Energy spectrum from NaI detector

I’m a Geant4 beginner. I built a model according to the article and output the broadened energy spectrum, but it doesn’t seem accurate, and changing the distance of the radioactive source will not have any effect on the result.

  • First of all, which article did you use?
  • Second, is the volume between the radiation source and the NaI detector filled with something? If it is not (i.e. vacuum), the shape of the spectrum should not depend on distance, as the vacuum does not change the energy of the coming photons.