1-7MeV neutrons bombard polyethylene, and proton energy spectra are obtained by recoil proton method

_Geant4 Version:_11.1
_Operating System:_VScode

I rewrote B4a to simulate neutron bombardment of polyethylene to obtain recoil protons。I have a question that When I am using a 1MeV neutron source to draw an image, I cannot obtain a 1MeV recoil proton and can only obtain 0.96MeV,( 4MeV,obtain 3.96MeV)

I need help.