Tracking only alpha particle after decay

Hello everyone. I am relatively new at using GEANT4 and therefore I apologize if the question is already answered somwhere else.

I am currently working on simulating the decay of 241Am and measuring the energy of the produced alpha particle. The Energy from just the alpha particles are perfectly in line with the expected values. Unfortunately I measure also much higehr energies on my detector. I invetiagted the problem and it seems like the reulting 237Np from the first decay intreacts with my detector as soon as it hits it and produces again some alpha particles which are then measured. This effect is not wanted, as in reality the decay will be inside a 241Am block and therefore the resulting ion should not fly around.

I got rid of this problem by only allowing the energy deposition of the initial alpha particle to be measured (if step->GetTrack()->GetParentID() == 1). This works great as long as the whole measurement is placed inside a vaccum. As soon as I add Air between the source and the detector teh alpha particle looses energy and so the parentID can be anything and is not suitable for any decision.

Is there a possibiliy to “kill” all secondary products of the decay which are not alpha particles? Or tto distnguish them at least?

Thanks in advance to anyone who might can help me!

I also tried these things which did not work out:

  • setting the energy for any non alpha particle to 0 by using


  • setting the momentum for any non alpha particle to (0, 0, 0) by using

G4ThreeVector mom(0.,0.,-1);

  • I also tried to just move the non alpha particles begind the detector, so that the won’t interact:

G4ThreeVector pos(0.,0.,2);

These were probably all bad ideas, but unfortunately I didn’t know how else to try.


you can limit the decay chain to Am241 :
/process/had/rdm/nucleusLimits 241 241 95 95

( In previous Geant4 versions, the command was called : /grdm/nucleusLimits )

Thank you @weller and @maire! Both solutions work perfectly!

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