The difference between Geant4 and Mcnp6 when using variance reduction of parallel geometry

when i do a simulation using variance reduction of parallel geometry, the case is like below:


the inner is helium-3, the outer is Borated Polyethylene.
radius of inner is 1.65 cm, radius of outer is 3,4,5,6,7,8 cm.
the neutron source of 2.53e-8 MeV is used, when using Geant4 to do the simulation,
the variance reduction of parallel geometry of importance is used, and for Mcnp6, the weight window is used, the results are below:

obviously, the result from Geat4 is wrong, some improvement should be done.

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