Modelling 12C(n,a)9Be not generating alphas

Im firing 10 MeV neutrons into a 10cm x 10cm x 10cm graphite cube (density 2.25 g/cm^3). Im expecting an interaction of the 12C + n → a +9Be. the energy to be distributed between the alpha and 9Be is about 4.3 Mev. however all of this 4.3 Mev goes to the Be9.

This means that I’m not seeing any alphas in my simulation and seems to break the conservation of energy as this is a two body problem and i expect energy to be given to the alpha as well.

I’m using the Shielding physics list. I have tried QGSP_BIC_HP and QGSP_BIC for neutrons of this energy and that hasn’t made a difference either.

I have attached the summed energy deposited in the graphite, broken down by the secondary particles we see.

10MevNeutron.pdf (48.4 KB)



please, try out QBBC Physics List.


I ran Hadr03 using the attached macro script. The version that I used is the unmodified distribution example from Geant4 11.0.2. It uses G4HadronPhysicsQGSP_BIC_HP. The run yields 3188 alphas for 10000 primary 10MeV neutrons. Based on the measured Q-value, they are from the 12C(n,a)9Be reaction to mainly the ground state and less frequently the 1st excited state of 9Be. How are you trying to detect the alphas in your simulation?

C12_n_inelastic.mac.txt (484 Bytes)

To underline John answer, a detailed verbose of 1 event :
available energy : 10 MeV + Q = 10 - 6.1703 = 3.82 MeV
gamma = 3.048 MeV
Be9 = 534.7 keV
alpha = 239.5 keV

Indeed, all physics constructors which include ParticleHP give same results, as it must be.
All other constructors miss the channel (n,alpha)
dasari.mac.txt (488 Bytes)
dasari.out.txt (7.1 KB)

Im tracking the energy deposited in my graphite block by pdgid