Hello,
You could test if Ra226 is decaying properly by changing everything in your simulation to G4_Galactic (Vacuum) and making your detector detect Alphas. Then the alpha count should be equal to the primary count, since the daughter isotope will have a momentum and leave the simulation.
With 1e6 simulated events, you would expect around 400 neutrons, considering a neutron emmision ratio of ~1.5e4 neutrons/(s*mCi) (- this number comes from an old Amersham catalogue i am having around).
Ra-226 has a decay chain of 5 alpha decays until it is stable, so is this decay chain is not processing, then your neutron count is of by a factor of 5, which is still not enough to be close to the ‘theoretical’ value i would expect…
What physics list and cross section table are you using for the production of neutrons, i.e. the Be(a,xn)Y-Reaction?
We had a thread about the simulation of an AmBe-Neutron source and there is a paper linked at the end of a group who did this really intense, maybe you find something there:
AmBe Simulation - Different neutron counts for different primary particles - Physics Processes, Models and Cross Sections - Geant4 Forum
Regards
Max