Hi Nadeera, so as you can see the strange oscillatory behaviour above 4 MeV has disappeared. As you stated before you are already using a routine which “smooths” the spectrum you obtain from FLUKA, I believe a similar (if not the same) procedure can be used to take into account the actual energy resolution of the detector you are trying to simulate. Of course when comparing this kind of detector response simulation to an experimentally obtained spectrum you need to make sure all relevant physical processes (e.g. for photons) and associated scorings are appropriate, also the detector itself needs to be accurately modeled to be able to make an accurate comparison. In case you would like more details on this looking at a similar problem please have a look at this interesting thread.
Hope this helps!
Andreas