Incorrect Am241 decay spectrum


I am trying to simulate alpha decay of Am241 with a silicon sensor but I am getting an absolutly wrong spectrum attached. Expected spectrum should have peaks at 5.48 and 5.44 MeV. But I see 5 different peaks which do not make any sense. My macro is below:

/gps/particle ion
/gps/ion 95 241
/gps/energy 0. eV 
/gps/pos/type Volume 
/gps/pos/shape Cylinder 
/gps/pos/centre 6.9575 6.9725 -7.35 mm 
#/gps/pos/rot1 0 1 0 
#/gps/pos/rot2 0 0 90 
/gps/pos/radius 3 mm
/gps/pos/halfz 1 um
/gps/ang/type iso 
/gps/number 1 
/grdm/applyARM false
/run/beamOn 1000000

Could you please recommend me a solution to this issue, I am out of ideas to resolve this.

Thanks very much!

simulation spectrum

I suspect that you’re getting the full, complete Am-241 decay chain with all of its daughters in your events. This is Geant4’s normal, default behaviour (each decay produces new tracks, which get processed, until there’s nothing left). If you look at the timestamps of your hits (assuming you record them), you can see this from the huge values some of them will have. There are two ways to avoid this.

  1. You can tell RadioactiveDecay to not follow the chain, by using /grdm/nucleusLimits 240 250 94 96 (or whatever limits you like; that’s what I use with Am-241).
  2. You can cut on the hit time and only record hits that are mostly syncrhonous with the initial Am-241 decay. This will involve your application code, not macro commands.

here a macro for example rdecay01 and the plots of alpha spectrum of Am241 alone and full decay chain.
sanchit.mac.txt (273 Bytes)