Missing (alpha,n) reactions in Silicon-29

I have been trying to simulate (alpha,n) reactions in Silicon-29 and have found that there are no neutrons produced at E_alpha of <5.5 MeV. The threshold energy of the reaction is ~1.7 MeV, so I expected to see some neutrons at 3, 4, or 5 MeV, but have seen none. I have tried tallying the number of inelastic collisions in the silicon, and those seem to line up with about what I would expect.

I am sending mono-energetic alphas of a specified energy, through a vacuum, into a 1 um target of pure Si-29. I am using the following physics models, and have tried increasing the cross-section through cross-section biasing, but have still seen no neutrons:

  • G4HadronElasticPhysicsXS

  • G4HadronInelasticQBBC

  • G4EmLowEPPhysics

  • G4IonElasticPhysics

  • G4IonPhysicsXS

  • G4RadioactiveDecayPhysics

I have also tried disabling EM physics, to see if slowing is causing the discrepancy, but that did not produce neutrons.

There appear to be cross sections in the literature indicating cross-sections in the 0.001 - 0.1 barn range at around 5 MeV.

If anyone has had similar issues, or has ideas on what to do that would be appreciated!


_Geant4 Version:_11.1.2
_Operating System:_Ubuntu 22.04
_Compiler/Version:_gcc/g++ 12
_CMake Version:_3.22.1

If you have G4TENDL1.4 data set installed (see Geant4 web page), you can use G4IonPhysicsPHP()

arthur.mac.txt (309 Bytes)
arthur.out.txt (2.9 KB)


I do have the G4TENDL datasets downloaded but when I tried running Hadr03 with IonPhysicsPHP as the Ion Inelastic Physics list, I got a very strange set of output reactions attached below, while when I ran with IonPhysicsXS and IonPhysicsPHP I only saw the reaction alpha + Si29 → S33.

I have attached the relevant physics parameters below, as well as selected outputs and the physics list file I am using for Hadr03, as I suspect that is where the issue is, although I cannot find it myself.

Thank you for the help!
OutputRx.txt (21.6 KB)

PhysicsList.txt (3.6 KB)