Neutron energy spectrum

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_Geant4 Version:_11.1.2
_Operating System:Mac
CMake Version:

Dear All,
I am trying to plot the neutron spectrum produced from the Cf252 source.
therefore, I used the NeutronSource example.
I reduced the absorber dimension for almost point source, and replaced the Absorber material to vacuum, so the neutrons do not lose its energy.
What kind of material I should use around the source to register the neutron energy (without doing any kind of interaction)
Any suggestions please

If you are not doing any interaction, then the material does not matter. you can record the neutron energy at the surface of the material, and stop tracking there.

Yes I managed to call the Kinetic energy.