Alpha spectra tails


I am trying to recreate an alpha spectrum from an experiment in Geant, however my simulation doesn’t seem to be producing the tail of the alpha peaks. I am using QGSP_BIC hadronic physics and EM standard option 4 Em physics list.


Attached is my spectrum, note I have applied a 20kev FWHM gaussian filter in post for energy resolution of the detector.

Any suggestions on how to reproduce the tailed distribution?