Deposited Energy Higher Than Monoenergetic Source

I built a couple applications for ERDA, this one is for source to target simulations, it only detects Alphas, using the following lines inside the G4VUserDetectorConstruction class (amongst other lines):

    G4SDParticleFilter* alphaFilter = new G4SDParticleFilter("AlphaFilter", "alpha");

Checked this, using a Proton only source, which resulted in zero detection.

The thing is, I’m using a mono energetic GPS source, at 5.45MeV (Am-241), but the “Energy Deposition” detector (G4_Si) registers one or two events at higher energies ~6MeV to ~8MeV every few billion simulated events. As far as I know, Alphas do not decay in such a way that it would deposit energy, nor Geant4 generates two events to be registered at the same time. Am I missing something in this reaction, He-Si, something exoenergetic?

I can provide more of the code or the geometry, if needed.

Appreciate every help.

_Geant4 Version: 11.1
_Operating System: Windows10

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Dear Michelaf,

Welcome to the Geant4 forum

Alpha capture may be possible in silicon. Particularly, Si29(alpha,n) reaction has a threshold of 1.7MeV, and Si30(alpha,n) of 3.9 MeV. These neutrons may induce subsequent reactions that lead to higher total energy than initial energy of the alpha.

However, there was a post few months ago pointing to the possibility that Geant4 did not include the Si29(a,n) reaction.

To investigate it further, one may save the events where total energy is higher than expected, or check the production of secondary neutrons via alpha-n reaction in Si29/30.

I am not expert on the subject, so I would be glad to learn from others thanks to this question :slight_smile:


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Thank you very much for the quick reply, Alvaro.

I’ll investigate that reaction. Can this reaction trigger another alpha or the reaction itself counts for energy deposition in Geant4? Sadly, I think it’s impractical to reproduce it with verbose 2 or something of that sort, as it happens once every 1bi+ simulated alphas or so.

It doesn’t seem to affect the simulations in any practical way, but the fact that I am using an alpha detection only code and some other reaction was being registered, made me question if there was something wrong with the code.

PS: Sorry for the delay, holiday weekend down here.

Michel França


I knew about this business because a similar anecdote but from the experimental physics point of view.

You may want to record the track information during the steps of the event, and display it at the end of the event only if the total energy deposited is higher than expected. Here they explained how to record such information.

If you find out something interesting (or a bug) please let us know :slight_smile: