AmBe NeutronSource spectrum issues

Dear All,

I’m simulating a Am241-Be9 neutron source, similar to the one in the extended/hadronic/NeutronSource example.

I have tried both the standard version as well as some modifications, but the output Neutron spectrum doesn’t match the reference one (see and others). For instance, I’m missing the 5.5, 8 and 10 MeV peaks. I have tried simulating the spectrum both inside and outside the steel casing. The AmBe ratio is about 10/90% Am/Be.
In paper “NUCLEAR INSTRUMENTS AND METHODS III (I973) 435–440; © NORTH-HOLLAND PUBLISHING CO. THE NEUTRON SPECTRUM OF Am-Be NEUTRON SOURCES”, they show the following, so I take there is a problem with the ground state of 12C, and possibly with the 1st excited state too.

What am I missing? I think there is an issue with simulating the C12 excited states, but I don’t really know for sure.

Thank you