Later in the paper it will become obvious that we chose to adopt BSS RF binning for spectral representation because it corresponds to the energy resolution of our instrument. Evidently, in the case of the bare 252Cf spectrum, ISO binning structure contains a much finer energy resolution over the entire spectrum, starting at approximately 30 keV and up to 20 MeV. In the case of the D2O moderated 252Cf spectrum, below 30 keV the ISO and BSS RF spectral resolutions are comparable. The thermal peak is added to the moderated spectra in order to account for 11.5% of thermal neutrons (ISO, 2001) that are otherwise captured by the cadmium shell, which is the part of ISO proposed D2O moderating sphere. As
described earlier, HPGNF physical moderating sphere does not have an outside Cd shell and therefore the portion of neutrons that are thermalized by heavy water can escape the sphere. Hence, we included additional low energy peak to the existing ISO spectrum, to account for the presence of those thermal neutrons. Moreover, the vicinity of scattering features such as walls, floor and ceiling contributes to the thermal peak, as well as to lower energy continuum.