Energy distribution of final neutron steps in paraffin


For shielding purpose we’ve been simulating the stopping power of lithiated paraffin by simulating a mono-energetic 1.8MeV uniform neutron point source inside a 1m radius sphere of the lithiated paraffin. No neutrons made it out of the sphere.
Physics list used was “QGSP_BIG_HP”

Since we wanted to track the path of the neutrons, we tracked all the steps with a SteppingAction.
We were interested in the final energy of the neutron, so we took the post_step energy of the last step of each neutron track.
From this analysis, we got some interesting results, shown below:

The final energy of 10^6 neutrons seems to be uniformly distributed between 0 and 10KeV, with 10KeV as a upper boundary.
Does anyone know why this upper boundary for the post step energy of the last step is set at 10KeV?

Thanks in advance!