Alpha particles producing unexpected isotopes in 13C

I am simulating Am-241 alpha particles incident on C-13. I expect to see O-16 produced via the (alpha,n) reaction, but I am also seeing unexpected production of O-17 at the same order of magnitude as O-16. I get the same result using QGSP_BIC_HP and QGSP_BERT_HP physics lists.

Can anyone provide some insight into how Geant4 models alpha particle interactions? If this is purely a cross section issue, is it possible to view the tables that are being used? I wonder if it could be related to how the (alpha,n) reaction is modeled, and somehow an inelastic reaction occurs without the neutron being emitted.

I do not know exactly how nuclear reactions are modeled in Geant4. I wish only illustrate what can be learned from G4 public examples.

1- Energy spectrum of alphas from Am-241 decay chain is ~4 MeV → 9 MeV
See attached macro for example rdecay01.
rdecay01.mac.txt (213 Bytes)

2- (alpha, C-13) reaction. For alpha below or equal to 4 MeV, the only opened channel is O-17 production. For alpha = 9 MeV the main channel is O-16 production. The transition is in 5 MeV → 5.5 MeV. See macro for example Hadr03.
hadr03.mac.txt (305 Bytes)

3- in a realistic simulation, eg. with electromagnetic processes registered, alphas loose their energy by ionisation before to, eventually, interact.
Here a macro for example Hadr06, and the printout which confirms your observation.
hadr06.mac.txt (388 Bytes)
hadr06.out.txt (841 Bytes)

It would be good to print or plot the kinetic energy of alpha just before they interact. In SteppingAction, do something like :

if (process->GetProcessName() == “alphaInelastic”) {
G4double Ekin1 = step->GetPreStepPoint()->GetKineticEnergy();
G4double dEstep = step->GetTotalEnergyDeposit();
G4double Ekin2 = Ekin1 - dEstep;
print or plot Ekin2
}