Radioactive Decay Only Yielding ~1 MeV Instead of Full Spectrum

[rraut25]

Hi All,
I am currently learning to use geant4 and am trying to simulate Bismuth‑207 decays using the standard Geant4 radioactive decay data (e.g., z83.a207 in RadioactiveDecay6.1.2). However, I only observe a mono‑energetic deposit around 1 MeV in each event, instead of the expected multi‑line spectrum.

I’ve confirmed that my geometry and histogram filling are correct, because if I bypass the decay module and manually sample Bi‑207’s lines in a custom primary generator, I see the correct multi‑peak distribution. I also verified my environment variables (G4RADIOACTIVEDATA and G4LEVELGAMMADATA) and the contents of z83.a207 look fine. Verbose output doesn’t clearly show any parsing error—just the final ~1 MeV deposited.

Here is my run.mac file

/run/initialize
#/process/had/rdm/setPhotoEvaporationFile 3
/process/had/rdm/applyARM true
#/process/had/rdm/analogueMC false
#/process/had/rdm/decayBiasProfile uniform
/process/had/rdm/hlThreshold 1e9 year
/run/beamOn 1000

setPhotoEvaporation is commented because it gives me following error: with the dreaded Z=3 A=0 outside allowed limits. If I disable certain /process/had/rdm commands (like setPhotoEvaporationFile 3, analogueMC false, or decayBiasProfile uniform), the simulation runs but still shows the same ~1 MeV deposit in all events.
I would really appreciate if anyone could point me towards the right direction. Please let me know if any further information about the issue is needed

PrimaryGeneratorAction.cc (4.6 KB)

[jrellin]

If you do not specify the energy of your particle in your primary generator or by macro, it will default to 1 MeV. And since Bi207 has a half life of >30 years, you will never see a decay to its daughter or any of its grand daughters.

particlesource->GetCurrentSource()->GetEneDist()->SetMonoEnergy(1.0 * MeV);

Particle gun is so much easier to use with simple setups.

[mkelsey]

I don’t think you’re generating your Bi-207 at rest. You never set the kinetic energy in GPS, so you’re using whatever was the default in the constructor. I’m pretty sure that default is 1*MeV, the same way it is for G4ParticleGun.

What’s going to happen in that case is that your primary will travel through your geometry, doing what nuclei do (make a Bragg peak!). Once the nucleus stops, then RDM will decay it into secondaries.

If you’re only seeing the 1*MeV Edep, and not the secondaries, then I suspect that you might have either a time cut or a track ID cut in your SD, scorer, whatever.